class notes 45.1: electromagnetic compatibility · classification notes no. 45.1 det norske veritas...

CLASSIFICATION NOTESNo. 45.1

DET NORSKE VERITAS CLASSIFICATION ASVeritasveien 1, N-1322 Høvik, Norway Tel.: +47 67 57 99 00 Fax: +47 67 57 99 11

ELECTROMAGNETIC COMPATIBILITY

DECEMBER 1995

© Det Norske Veritas 1995Data processed and typeset by Division Ship and Offshore, Det Norske Veritas Classification AS02-08-20 09:41 - CN45-1.doc

Printed in Norway by Det Norske Veritas12.95.2000ERROR! AUTOTEXT ENTRY NOT DEFINED.

FOREWORDDET NORSKE VERITAS (DNV) is an autonomous and independent Foundation with the object of safeguarding life, propertyand the environment at sea and ashore.

DET NORSKE VERITAS CLASSIFICATION AS (DNVC), a fully owned subsidiary Society of the Foundation, undertakesclassification and certification and ensures the quality of ships, mobile offshore units, fixed offshore structures, facilities andsystems, and carries out research in connection with these functions. The Society operates a world-wide network of surveystations and is authorised by more than 120 national administrations to carry out surveys and, in most cases, issue certificateson their behalf.

Classification NotesClassification Notes are publications which give practical information on classification of ships and other objects. Examples ofdesign solutions, calculation methods, specifications of test procedures, as well as acceptable repair methods for somecomponents are given as interpretations of the more general rule requirements.An updated list of Classification Notes is available on request. The list is also given in the latest edition of the Introduction-booklets to the"Rules for Classification of Ships", the"Rules for Classification of Mobile Offshore Units" and the"Rules forClassification of High Speed and Light Craft".In "Rules for Classification of Fixed Offshore Installations", only those Classification Notes which are relevant for this type ofstructure have been listed.

DET NORSKE VERITAS

CONTENTS

1. Introduction .....................................................................41.1 Scope ...............................................................................41.2 The EM environment .......................................................41.3 The typical noise path ......................................................52. General .............................................................................62.1 Shielding..........................................................................73. Component design and selection.....................................83.1 General ............................................................................83.2 Capacitors ........................................................................83.3 Inductors ..........................................................................83.4 Electromechanical devices...............................................83.5 Ferrit components ............................................................83.6 EMI gaskets .....................................................................93.7 Cabling and connectors....................................................94. Installation........................................................................9

4.1 General ............................................................................ 94.2 Circuits and components ............................................... 104.3 Filtering......................................................................... 104.4 Screens and shields ....................................................... 114.5 Wiring ........................................................................... 124.6 Grounding ..................................................................... 145. Testing for EMC............................................................ 215.1 General .......................................................................... 216. Rules and regulations.................................................... 226.1 EEC and the EMC Directive ......................................... 226.2 A comparison between EU Directive 89/336/EEC and therequirements of DNVC ....................................................... 227. EMC management......................................................... 267.1 General .......................................................................... 26

4 Classification Notes- No. 45.1

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1. Introduction

1.1 ScopeThe scope of this paper is to describe the means andmeasures to avoid electromagnetic interference (EMI)problems. Different phases of the development of a systemhave been addressed such as planning, designing, installingand testing.

1.2 The EM environmentIn the environment the equipment is exposed toelectromagnetic interference (EMI) originated by physicalphenomena or generated by various equipment. EMI issomewhat arbitrarily defined to cover the frequency spectrumfrom about 10 Hz to 100 GHz. For radiated emissions alower frequency limit of 10 kHz is often used, although EMIcan exist in many equipment and systems below thisfrequency. Except for electrostatic discharge (ESD) thererarely exists a pure DC EMI problem.

The EM environment will be variable from place to place,ship to ship and between locations. Estimation of EMIenvironment in any situation is required before adequateprotection methods can be selected which will enableequipment to operate without error in all environments. Forexample, if control valve operation is initiated automaticallyby a micro computer during cargo discharge, the equipmentmust be capable of continuous operation in harbourelectromagnetic environment.

Depending on the different environments, a wide variety ofinterference sources can be encountered. Power convertors,switch gears, contactors, relays, welders, radio and televisiontransmitters and mobile radios, are among the mostconspicuous EMI sources. Transient disturbances, whichoccur most frequently, usually for short random periods oftime and mostly result from interferences caused bylightning, earth-faults or switching of inductive circuits.These disturbances can have a frequency range from 50 Hzup to a few hundred megahertz with time duration includingtransients ranging from less than 10 nanoseconds to a fewseconds.

Some typical values for electrical field strength of radiatednoise to be anticipated on board a medium size cargo ship areshown in Table 1-1.

Location Value

Wheel housetop

5 to 80 V/m 10 cm above the steel deck120 to > 200 V/m at 2 m above the steel deck4 V/m at the window0,5 to 1 V/m at the centre console

Bridge wings 3 to 13 V/m 10 cm above the steel deck50 to 200 V/m 2 m above the steel deck

Open deckunder theantenna onforepeak

0 to 30 V/m; 480 kHz0 to 7 V/m; 4,18 MHz200 V/m 2 m above the steel deck

Accommodation < 0,1 V/m near hand rail on stairsMachineryspaces

< 0,1 V/m 1 V/m near a running alternator

Table 1-1 Typical EMI values on board cargoship � Ref. 2 �

Interference can be defined as the undesirable effect of noise.If noise voltage causes unsatisfactory operation of a circuit, itis interference. Usually noise cannot be eliminated but onlyreduced in magnitude until it no longer causes interference.

Susceptibility is the characteristic of electronic equipmentthat results in undesirable responses when subjected toelectromagnetic energy. The susceptibility level of a circuitor device is the noise environment in which the equipmentcan operate satisfactorily.

Electromagnetic compatibility (EMC) is the ability to eitherequipment or systems to function as designed withoutdegradation or malfunction in their intended operationalelectromagnetic environment. Further, the equipment orsystem should not adversely affect the operation of, or beadversely affected by any other equipment or system.

Electrostatic discharge (ESD) is a phenomenon that isbecoming an increasingly important concern with today'sintegrated circuit technology. The basic phenomenon is thebuild-up of static charge on a person's body or furniture withsubsequent discharge to the product when the person orfurniture touches the product. In dry atmosphere andespecially where carpets are used in a computer room, theoperator can be charged to high voltage. When the dischargeoccurs, relatively large currents momentarily course throughthe product. These currents can cause IC memories to clear,machines to reset, etc. If a computer unit is touched by such acharged operator a discharge spark can occur and result inmalfunctioning. In unfavourable condition, the staticdischarge can approach 25 kV in magnitude, but normallynot more than 6 kV by contact and 8 kV in air.

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Harmonic distortion is a phenomenon when non-linearloads, e.g. static power converters, arc discharge devices,change the sinusoidal nature of the AC power therebyresulting in the flow of harmonic currents in the AC system.The degree to which harmonics can be tolerated isdetermined by the susceptibility of the load (or power source)to them. The least susceptible type of equipment is that inwhich the main function is in heating. The most susceptibletype of equipment is that whose design or constructionassumes a (nearly) perfect sinusoidal fundamental input e.g.communication or data processing equipment. Other types ofelectronic equipment can be affected by transmission of ACsupply harmonic through the equipment power supply or bymagnetic coupling of harmonics into equipment components.Computers and similar equipment such as programmablecontrollers frequently require AC sources that have no morethan 5 % harmonic voltage distortion factor with the largestsingle harmonic being no more than 3 % of the fundamentalvoltage.

1.3 The typical noise pathThe systems boundaries for penetration of interference maybe power feed lines, input signal lines, output signal lines orequipment enclosure. With only a few exceptions, EMIbegins as a desirable signal current flowing along an inducedpath. The signal current becomes a source of interferencewhen it is diverted to one or more unintentional paths thatlead, ultimately to a victim. Some circuit elements maygenerate new voltages, currents, or fields. Typical transitionpoints include the generation of voltages by ground currentsflowing through the distributed impedance of ground, thegeneration of fields by currents flowing along conductors,and the leakage of currents to nearby circuit elements throughstray capacitance. It is important to identify the transitionpoints along the coupling paths to a victim because thesepoints make the mostly effective locations for EMI fixes.

Navigation receiver

Generator & regulatorPower supply

Autopilot controlservos

Output displaydevices

Other On-boardequipment

Surface controls

Antenna

Outside world(exclude)

Ground

1

2

3

4

56

1212 12

910

1

8

7 7

8

12 1

3

1 Power cable conducted emission 4 Interconnecting cable conductedsusceptibility

7 Common ground impedanceemission coupling

10 E-field radiation

2 Power cable conductedsusceptibility

5 Antenna lead conducted emission 8 Common ground impedancesusceptibility coupling

11 H-field susceptibility

3 Interconnecting cable conductedemission

6 Antenna lead conductedsusceptibility

9 H-field radiation 12 E-field susceptibility

Figure 1-1 EMI coupling mechanisms.


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A typical noise path is shown in Figure 1-2. As can be seen,three elements are necessary to produce a noise problem, i.e.there must be a noise source, a receiver circuit that issusceptible to the noise, and finally a coupling channel totransmit the noise from the source to the receiver. It followsthat there are three ways to break the noise path: the noisecan be suppressed at the source, the receiver can be madeinsensitive to the noise, or the transmission through thecoupling channel can be minimised. In some cases, noisesuppression techniques must be applied to two or to all threeparts of the noise path.

Noiseemitter

Couplingchannel Receiver

Figure 1-2 Typical noise path

2. GeneralIn the region around an electric lead that carries analternating current, an electromagnetic field is set up. Thefield changes in strength and direction in phase with thealternating current. The field propagates away from the leadas electromagnetic waves with the speed of light.

All macroscopic electromagnetic phenomena may beexpressed mathematically (Maxwell's equations). Theequations describe the distributed-parameter nature ofelectromagnetic fields, i.e. the electromagnetic quantities,distributed throughout space, e.g. a set of partial differentialequations being functions of spatial parameters x,y,z in three-dimensional space as well as time. From a mathematicalstandpoint, these equations are difficult, although they arequite easy to describe in conceptual terms. Where appropriateone uses approximations, and the governing equationsbecome ordinary differential equations where the variablesare functions of only one parameter i.e. time.

An approximation of the field intensity and radiated powerfrom antenna in free space:

The power density S at a point due to the power PT radiatedby an isotropic radiator is given as follows:

S P rT� �/ 4 2� and S E�

2 120/ � where S = powerdensity [W/m2]; r = distance [m]; PT = transmitted power[W]. The field intensity of an isotropic radiator in free space

is: Er

PT�

5 5,

For a half-wave dipole in the direction of maximum

radiation: S Pr

T�

�

�

1 644 2

,�

and Er

PT�

7 01, where 1,64

= maximum gain of a half-wave dipole.

The efficiency of an antenna is the ratio of the radiationresistance to the total resistance of the system. The totalresistance includes radiation resistance, resistance inconductors and dielectric, including the resistance of loadingcoils if used and the resistance of the earthing system.

For portable transceivers, walkie-talkies and mobiletelephone sets with power ratings ranging from 0,5W to 12Wthe statistical average of the electric field strength can be

expressed as: E kr

PT� where PT= manufacturer's

advertised rating of the transceiver in watts. The factor k is acoefficient established by experiments. The coefficients areranging from k = 0,45 to k = 3,35 with a mean of k = 1,6.

The use of transceivers of which the antenna is too close toelectronic equipment is a matter of great importance. Aseparation distance of 2 m between the antenna and theequipment is highly recommended. In addition, operation atreduced power ratings will materially reduce the influence ofradiated interference resulting from the use of portabletransceivers.

The ratio of the electric field E to the magnetic field H is thewave impedance. In the far field this ratio E/H equals thecharacteristic impedance of the medium e.g. E/H=Zo�377 �for air or free space. In the near field the ratio is determinedby the characteristics of the source and distance from thesource to where the field is observed. If the source has highcurrent and low voltage (E/H<377 �) the near field ispredominantly magnetic. Conversely, if the source has lowcurrent and high voltage (E/H>377 �) the near field ispredominantly electric.

For a rod or straight wire antenna, the source impedance ishigh. The wave impedance near the antenna - predominantlyan electric field is also high. As distance is increased, theelectric field loses some of its intensity as it generates acomplementary magnetic field. In the near field, the electricfield attenuates at a rate of 1/r3 whereas the magnetic fieldattenuates at a rate 1/r2. Thus the wave impedance from astraight wire antenna decreases with distance andasymptotically approaches the impedance of the free space inthe far field.

For a predominately magnetic field- such as produced by aloop antenna- the wave impedance near the antenna is low.As the distance from the source increases, the magnetic fieldattenuates at a rate 1/r3 and the electric field attenuates at arate of 1/r2. The wave impedance therefore increases withdistance and approaches that of free source at a distance of�/2�. In the far field, both the electric and magnetic fieldsattenuate at a rate of 1/r.


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At frequencies less than 1 MHz, most coupling withinelectronic equipment is due to the near field, since the nearfield at these frequencies extends out to 50 metres or more.At 30 kHz, the near field extends out to 1,59 km. Therefore,interference problems within any given equipment should beassumed to be the near field problems unless it is clear thatthey are far field problems.

In the near field the electric and magnetic fields must beconsidered separately, since the ratio of the two is notconstant. In the far field, however, they combine to form aplane wave having an impedance of 377 �. Therefore, whenplane waves are discussed, they are assumed to be in the farfield. When individual electric and magnetic fields arediscussed they are assumed to be in the near field.

2.1 ShieldingThe term 'shield' usually refers to a metallic enclosure thatcompletely encloses an electronic product or portion of thatproduct. The perfect shield is a barrier to transmission ofelectromagnetic fields. The effectiveness of a shield is theratio of the magnitude of electric (magnetic) field that isincident on the barrier to the magnitude of the electric(magnetic) field that is transmitted through the barrier. Ashield effectiveness of 100 dB means that the incident fieldhas been reduced by a factor 100.000 as it exits the shield.

Magnetic fields form around electrical conductors (accordingto Ampere's Law). Coupling between a magnetic field and anadjacent electrical conductor will occur unless the adjacentconductor has a shield preferably of high permeabilityferrous materials (iron, mumetal, etc.) or is physicallyseparated. A shield with a high magnetic permeability is thebest solution for magnetic field shielding, if groundingtechniques cannot be properly employed. A copper-braidedshield that would serve well as a shield against electric field,the coupling is however, not as effective for magnetic fieldshielding.

Electric fields are much easier to guard against than magneticfields. EMI resulting from electric field coupling becomes agreat concern as frequency increases. Thus cables operatingat frequencies of 1 MHz or more, are prone to emit this typeof EMI, resulting in 'crosstalk' when it occurs betweenadjacent cables.

For both magnetic and electric field shielding, effectivenessdepends upon achieving the highest degree of EMI lossthrough absorption and reflection. There are several ways tocharacterise the effectiveness of a shield. First the sum ofabsorption and reflection loss will yield the total loss i.e. totalEMI reduction for a shield. A second means of measuringshielding effectiveness is to measure the fraction of theelectric or magnetic field that reaches the other side of thebarrier (shield). Total shielding effectiveness may be givenby the following equation:

SE = A + R + B

A = Absorption lossR = Reflection lossB = Secondary reflection loss

Absorption loss is generally the greatest contributor toshielding effectiveness because of the large amount of EMIthat shields can conduct away. The magnitude of absorptionloss varies directly with the thickness of shielding barrier, theelectrical conductivity of the barrier or the magneticpermeability of the shielding material. The thicker the shieldand the higher its electrical conductivity or magneticpermeability, the better the shielding effectiveness. Mostshielding materials with high electrical conductivity oftenhave a low magnetic permeability. The electrical conductivityof copper is higher than of any other commercial metal -fivetimes more electrically conductive than steel, for example.But the magnetic permeability of iron is 1000 times that ofcopper. A high electrical conductivity is the most importantquality that a shield for high-frequency cable should have.

Magnetic fields are more difficult to shield, since thereflection loss may approach zero for certain combinations ofmaterial and frequency. With decreasing frequency, themagnetic field reflection and absorption losses of non-magnetic materials such as aluminium, decrease.Consequently, it is difficult to shield against magnetic fieldsusing non magnetic materials. At high frequencies theshielding efficiency is good due to both reflection andabsorption losses, so that the choice of materials becomesless important. The use of non-magnetic shields aroundconductors provides nil magnetic shielding. Conductingmaterial can provide magnetic shielding, i.e. the incidentmagnetic field induces currents in the conductor producingan opposite field to cancel the incident field in the regionenclosed by the shield.


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Material Frequency Absorption Reflection loss

(kHz) loss1) all fields Magnetic field2) Electric field Plane wave

Magnetic�r=1000

<11-10

10-100>100

0-30dB30-90dB>90dB>90dB

0-10dB0-30dB

10-30dB10-60dB

>90dB>90dB>90dB

60-90dB

>90dB>90dB

60-90dB30-90dB

Non magnetic�r=1

<11-10

10-100>100

0-10dB0-10dB

10-30dB30-90dB

10-30dB30-60dB30-60dB60-90dB

>90dB>90dB>90dB>90dB

>90dB>90dB>90dB>90dB

1) Absorption loss for 0,8mm thick shield.2) Magnetic field reflection loss for a source distance of 1m. (Shielding is less if distance is less than 1m and more if distance is greater

than 1m). Explanation: 0-10dB=Bad; 10-30dB=Poor; 30-60dB=Average; 60-90dB=Good; >90dB= Excellent

Table 2-1 Shielding effectiveness of magnetic and non-magnetic materials

3. Component design and selection

3.1 GeneralThe primary objective of EMC should be given to the task ofminimising the amount of noise generated by the equipment,since the noise may interfere with other equipment. It isalways desirable to control as much noise at the source aspossible, since that approach can avoid an interferenceproblem for countless number of receiver circuits. Byselecting the proper noise reduction method and components,EMI can in many instances by reduced considerably.

3.2 CapacitorsCapacitors are generally effective noise decouplers. Use ofparallel capacitors in low-impedance circuits is usuallyinsufficient. They are most effective with high-impedanceloads. Whenever a parallel suppression component is used,the impedance levels of not only the element but also theparallel path should be computed or estimated at the desiredfrequency.

3.3 InductorsWhereas capacitors are used to divert noise currents,inductors are placed in series with wire to block noisecurrents. This will be effective if the impedance of theinductor at the frequency of the noise current is larger than anoriginal series impedance seen looking into the wire. Seriesinductors are most effective in low-impedance circuits.

3.4 Electromechanical devicesA number of electric products as typewriters, printers androbotic devices use small electromechanical devices such asDC motors, stepper motors AC motors and solenoids totranslate electrical energy into mechanical motion. Thesedevices can create significant EMC problems. DC motorscreate high-frequency spectra due to arcing at the brushes aswell as providing paths for common-mode currents throughtheir frames. The spectral content tends to create radiatedemission problems in the radiated emission regulatory limitfrequency range between 200MHz and 1GHz, depending onthe motor type. In order to suppress this arcing resistors orcapacitors may be placed across the commutator segments.

For AC motors, the rotor and stator consist of closely spacedinductors, the problem of large parasitic capacitance betweenthe rotor and stator exits. If high-frequency noise is presenton the AC-wave form feeding these motors then it is likelythat this noise will be coupled to the chassis or to the ACpower cord, where its potential for radiated or conductedemissions will usually be enhanced.

3.5 Ferrit componentsFerrite materials are basically non conductive ceramicmaterials, consisting mainly of iron oxide that is blended withother metallic oxides, calcined, then sintered, resulting in apolycrystalline, spinel structured ceramic. The materialdiffers from other ferromagnetic materials such as iron in thatthey have low eddy-current losses at frequencies up tohundreds of MHz. Thus they can be used to provide selectiveattenuation of high-frequency signals that we may wish tosuppress from the standpoint of EMC and not effect the moreimportant lower-frequency components of the functionalsignal.


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3.6 EMI gasketsEMI gaskets are employed for either temporary or semi-permanent sealing applications between joints or structures inorder to reinstate loss of shielding integrity at seams andjoints where other than permanent fastenings methods arepermitted, e.g.:

� Securing access doors to enclosures, cabinets, orequipment

� Mounting cover plates or removal panels for equipmentmaintenance, alignment, or other purposes

3.7 Cabling and connectorsCables and connectors should be designed to achieve asystem's specified levels of emission suppression andresistance to outside EMI. Cable assemblies should alsodeliver the required undistorted signals while achievingproper mechanical performance. It is necessary to givespecial attention to the cable-connector interface and theenvironment in which the cable assembly must perform.

Cable shielding is an effective means of controlling orlimiting radiated EMI. Conducted EMI can be difficult toovercome without the use of filters, but experts often suggesttrying proper shield grounding techniques before filters areintroduced. Radiated EMI, however, can be controlled withshielding. Cable shields must provide protection against bothmagnetic fields and electric fields. Each requiring a differentshielding mechanism.

Twisted pairs can accomplish some magnetic field shieldingbecause the twisting provides equal and opposite inducedvoltage. A short pair pitch pair (number of twists per unitlength) will provide a greater degree of magnetic fieldrejection.

Note:A first step in designing a system that will meet the EMCrequirements, is to separate power cables and data cables.

This will limit the opportunity for magnetic field interferenceof non-power cables by the magnetic field generated by theload cables. If sufficient separation is not possible, the non-power cable shield must provide shielding against bothmagnetic fields and electric fields.

Power cables should have a magnetic field shield. This canbe done by locating the cables near a part of the cabinetframe which can act as a high permeability shield and groundplane. The four most common possibilities of shielding are:foil laminates, braided shields, optimized braids andcombination shields.

The effectiveness of these approaches can be adjusted bychanging the coverage of a braided shield of the amount ofthe overlap in foil laminate shield. Varying the coverage (asolid tube equals 100 percent coverage) can lead to greatershielding effectiveness. Likewise, increasing the thickness,overlap or electrical conductivity will improve theperformance of the shield.

Because braided shields introduce apertures, 100 percentcoverage is theoretically impossible. Therefore these shieldsrepresent a departure from the solid 'tube' approach toshielding, and are thus less effective at achieving reflectionloss than are foil laminates in some cases. But, because theygenerally contain a large mass of metal per unit of cablelength, characterizing a low impedance path, they canaccomplish more absorption loss. Braided shields are mosteffective at shielding against radiation frequencyinterference.

Optical coverage indicates the amount of the surface of aninsulated conductor that is covered by shielding, as viewedwith the eye perpendicularly. Greater optical coverage doesnot necessarily mean greater shielding effectiveness withrespect to reflective and absorptive loss. But, braid angle andindividual wire diameter have a significant effect on shieldeffectiveness.

There are several approaches to terminating a cable shield atthe connector. Proper consideration must be given to whetherthe shields are to be grounded through a connector backsideat one or both ends of the circuit and that the cable shield atbackshell offer equivalent shielding quality as both separatecomponents and an assembled part.

4. Installation

4.1 GeneralThe exposure of an item of equipment to interference can berelated to the electrical environment in which it is situated.The degree of interference is related to the characteristics ofthe source, the nature of coupling impedances, the sensitivityof electronic equipment and the quality of the earthing andprotective measures utilised at the installation site.

In the installation of electrical and electronic systems, anumber of options are available as to how to earth the signalcircuits, to choose cable shields and earthing of the shield,each of which can contribute to the reduction of interference.In addition, the treatment of signal lines and power cableswith respect to the cable routing and cable separation, the useof filters and enclosures, bounding practices, etc. are ways bywhich coupling of interference to sensitive circuits can bereduced. Figure 4-1 gives an overview of available options toachieve EMC control.


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ArcSuppresionInduction &solid state

FiltersClamps

LP, BP, HP & BR Filters

GroundingWiringShielding

Housing

Low Level

FilteringCircuits &Components

RotatingDevices

Relays &Solenoids

ElectronicCicuits

Chassis & CabinetsRoomsMaterialsThickness

GasketsSealsApertures

PackagingConnectors

ShieldedFilter Type

Cabling

GroupingTypesGroundLoopsShielding

BuildingsRoomsCabinetsChassisCircuit/Cable

Structures

TypesSurfacesCorrosion

Bonds

PowerMains

FiltersBeads/RodsLossy LineConnectorsIsolationtransformers

EMC Control

Figure 4-1 An overview of available options to achieve EMC control.

4.2 Circuits and componentsThe sensitivity to EMI of a signal circuit is dependent uponthe input impedance. The higher the input impedance, themore sensitive the circuit is to EMI. The effect of an un-symmetry is also greater for high impedance inputs. By usingminimum bandwidth for the signal inputs, the equipment'sresponse to EMI is reduced. Since standard integratedcircuits having very large bandwidth are commonly used,filters or other components should be applied to reduce thebandwidth. When possibility of common-mode voltagesoccur in extremely sensitive equipment, unsymmetry in cablerouting and the equipment's impedances to ground must beavoided. Good symmetry can be achieved by using abalanced isolation transformer or specially symmetricequipment.

Relay coils and/or relay contacts which have directconnection to semiconductors contacts and/or secondarycontacts in the electronic cabinet, must be EMI suppressed. Arelay without such protection must not be connected tosupply voltage circuits for the electronics. All other relaycoils and operating coils should be suppressed if possible.

4.3 FilteringConstruction and application of EMI filters in each caseshould be considered in the actual situation. The equipmentmanufacturer's specifications should be followed where theseare available.

Power supply for electronic equipment should normally befiltered. This is valid both for DC and AC voltages.Electronic equipment representing heavy load should be fedvia separate lines from the main switch board. Power supplyfilters should have separate shields (boxes). Filters should beused at the signal input of sensitive equipment and the noisesource if needed.

The distance between input and output terminals on the filtershould be maximised. If a filter is used in a symmetric circuit,the filter should be balanced. Filters should be used at allanalogue inputs. Filters should be used at all digital inputs ifthe filter itself does not introduce functional degradation.

The use of galvanic isolators should be considered for thefollowing cases:

� Isolation of sensitive measuring and control equipmentfrom noisy AC power supply.

� Isolation of noise generating equipment from noisesensitive equipment when both types are using the sameAC power supply.

� For minimising differential-mode noise (noise acrosswinding) resulting from common-mode noise (noisebetween winding and ground).

� For maximum common-mode noise voltages.� Separate measuring equipment and cables galavanically

from central, electronic signal processing equipment. Thisis to prevent that unintentional supply of high voltages onsignal and control cables should damage the signalprocessing equipment.


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DET NORSKE VERITAS

Isolation can be done in many different ways, depending onthe application. For signal and control circuits both optocoupler and isolation transformers can be used. For powersupply system isolation transformers can be used. Isolationtransformers should have a ground screen between thewindings to reduce the capacitive coupling. The capacitancebetween the windings should be much less than 1pF. Anyisolation device has to withstand the highest noise voltagelikely to occur in the system.

Figure 4-2 Preferred grounding of cable screens.

4.4 Screens and shieldsElectric/electronic equipment which is susceptible to EMI ormight be EMI emitting should generally have a metallicshield. From an EMI viewpoint, it would be favourable to usescreened cables everywhere, with the screen groundedeverywhere. For all EMI susceptible and EMI emitting cableson board, conductive screens are recommended. Manycommonly used ship's cables having copper braided screensare satisfactory for reduction of capacitively coupled EMI-voltage, shield factor 60 dB provided satisfactory groundingof the screens.

Commonly used ship's cables will usually have a modestshield factor for magnetic fields at frequencies up to 1 kHz,the best achievable is down to 2 dB. If better shielding effectat low frequencies is desired, a screen made of magneticmaterial should be used. At higher frequencies the shieldfactor increases at two -and multi point grounding , e.g. 60dB at 1 MHz. Cables should be as short as possible, androuted as close as possible to the main ground system (deckand/or floor) when routed outside cable trays. This will alsohave a certain shielding effect.

In order that the screening of the conductor and theequipment to which it is connected shall be completelyeffective against high frequencies, all outer cable screensmust be in contact, all around, with the screening enclosureof the equipment.

Preferred low frequency shield grounding for both shieldedtwisted pair and coaxial cable are shown in Figure 4-2. Theshield grounding in A through C are grounded at theamplifier or structure, but not at both ends. When the signalcircuit is grounded at both ends, the amount of noisereduction possible is limited by difference in ground potentialand the susceptibility of the ground loop to magnetic fields.The preferred shield ground configurations for cases wherethe signal circuit is grounded at both ends are shown in D andE. A transformer may be used to break the ground loop. Incase E the shield is grounded at both ends to force someground-loop current to flow through the lower impedanceshield, rather than the centre conductor. In case of circuit Dthe shielded twisted pair is also grounded at both ends toshunt some of ground loop current from the signalconductors. If additional noise immunity is required, theground loop must be broken. e.g. using transformers, opticalcouplers or a differential amplifier. Note that an unshieldedtwist pair, unless it is balanced, provides very little protectionagainst capacitive pickup, but is very good for protectionagainst low frequency signals. The effectiveness of twistingincreases with the number of twists per length.

In exceptional cases in which signals of frequencies from 0Hz to about 10 MHz must be transmitted via a coaxial cablein an unsymmetrical system e.g. video lines, analoguesignals, it is necessary to prevent the flow of an interferencecurrent e.g. with net frequency in the cable outer conductor,the return conductor of the signal circuit. Since in this case,on account of the large frequency range, it is not possible todepart from coupling the cable screen to ground at each end,it is necessary to use double screen circuit. In this case theinner conduits and the inner screen form a normal coaxialsystem which must be driven as such. The circuits connectedtogether have, however, only a connection to ground at oneend (preferably at the transmitter end or according to themanufacturer's specifications). The outer screen musthowever, be connected all around at both ends to theparticular equipment housings.

The cable screen can only be effective against magneticfields if it is electrically connected at both ends, so that acurrent can flow in the screen.


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Single ended connected cable screens are only effectiveagainst electrical fields and only when the screened length isnot greater than �/10 of the highest frequency in the EMCzone be considered. In this case it must be taken into accountthat the equipment with working frequencies below e.g. 100kHz however can be interfered by higher frequencies. It isequally possible that these equipment form interferencesources for frequency ranges being far away from theirworking frequencies.

In order to prevent the electrostatic charging ESD of objects,grounding of metallic enclosures is highly recommended.Insulating material such as floor coverings, seating etc.should have a limited conductivity to avoid charging-up. As atarget value, specific resistance of the insulating materialshould be taken as 107 ohm cm. Rotating and movable parts(machine parts, propeller, shafts, swing-out equipment etc.)are to be conductivity connected to ground e.g. by means ofgrounding brushes, slide contacts, conducting grease etc.

Efforts to reduce EMI inside and outside the radio room havetwo purposes:

a) Prevent other equipment on board from disturbingtransmitting from and receiving to the radio room.

b) Prevent the radio station from disturbing other equipmenton board.

The radio room must be shielded. Normally the steelbulkhead and the steel deck in the radio room will form anatural part of the shield. All joints in the shield should becontinuously welded where practically possible. If the wholeshield or parts of it must be joined in another way, e.g. bybolted connections, special efforts (e.g. EMI gaskets) shouldbe executed to make the shield as electrically 'tight' aspossible. All necessary apertures (door and windowopenings, ventilation openings etc.) in the shield should bemade as electrically non-penetrating as possible. This involvee.g.:

� EMI gaskets around doors (door, or door panel of samematerial as the rest of the shield).

� Multi-layer screened covers or honeycomb aperturecovers at ventilation openings.

� Conductive glass in windows not facing free air.

4.5 WiringCables may be grouped in five different classes, according toEMI generation and EMI susceptibility. (There are cables ofdifferent functions in each class because the number of actualfunctions are greater than the suitable number of interferenceclasses).

The classification is schematic, and in some instances therewill be a matter of judgement whether a cable belongs to e.g.group A, B or in two groups.

Note:As a general rule, all different cable classes A through E shouldhave separate routing, and the distance should be as large aspossible. However, the benefit from separation is not linearlydependent upon the separation distance and the first tens ofmillimetres are the most significant

Table 4-2 shows the minimum distances between cables ofthe different classes when routed on cable trays or directlyonto the steel hull etc.


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DET NORSKE VERITAS

Class Classification Function (examples)

A EMI generating, not EMI susceptible,24-600V, DC, 50-60Hz, 400Hz.High power and voltage up to 10kV

Power cables.Control cables in circuits using mechanical contacts and relay coil

B Slightly EMI generatingSlightly EMI susceptible 0,5-50V, lowfrequency

Telephone cablesSignal cablesSynchro circuits (60-400Hz)

C EMI generating,EMI susceptible 0,1-5V, 50�, pulse0,1-24 V, DC

Video signals, Data transmissionAnalogue measuring values after converter.

D Highly EMI susceptible10�V-100mV, 50�- 2000�,DC, AF, HF

Receiver antennaHydrophone and microphone cables.Analogue measuring values

E Highly EMI generating Radio transmitterSonarRadar modulatorThyristor controls

Table 4-1 Cable classes.

Cable class

EMI generating,not EMI

susceptible

Slightly EMIgenerating and

susceptible

EMI generating,EMI susceptible

Highly EMIsusceptible

Highly EMIgenerating

A B C D ECase 1: Unscreened cables paralleled over more than 2 metres.

ABCDE

00,250,250,500,25

0,250

0,250,250,25

0,250,25

00,250,50

0,500,250,25

00,50

0,250,250,500,50

0Case 2: Unscreened cables at crossing angle 90�1)

ABCDE

00

0,150,300,15

00

0,150,150,15

0,150,15

00

0,30

0,300,15

00

0,30

0,150,150,300,30

0Case 3: Screened and grounded cables

ABCDE

00

0,100,10

0,252)

00

0,100,10

0,252)

0,100,10

00

0,50

0,100,10

00

0,50

0,252)

0,252)

0,502)

0,502)

01) If the crossing angle is less than 90�, the distances should be increased towards the values in case 1.2) Steel tube or conduit having wall thickness of at least 1,5 mm around class E cables, delete the separation requirements.

Table 4-2 Minimum distances in metres between cables of different classes when routed on cable trays ordirectly onto the steel hull.


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If there are only screened cables of reasonably good qualitye.g. copper or iron braided screen with outer non-metallicsheath, the distances may be reduced, except for cables inclass E. To allow distance reduction between class E andother classes, cables of class E must be routed in tubes,conduits or boxes having a minimum wall thickness of 1,5mm steel with good electrical connection to the hull at leastin both ends.

Special cables that may be both highly EMI generating andhighly EMI susceptible (e.g. combined cables from antennato transmitter/receiver e.g. VHF), i.e. cables that mayalternate between class E and D, should be routed separatelywith distances as shown in Table 4-2, case 1. Alternativelysuch cables may be routed in separate steel tubes having awall thickness of at least 1,5 mm . Then there is norequirement to separation.

Using only screened cables in the installation, the distances inmeters should be as per table Table 4-2, case 3.

4.5.1 Installation of cable traysCables should be mounted either directly to the conductinghull or on trays made of at least 1,5 mm perforated steelplate. The plate is for mechanical reasons usually made witha bent edge. To reduce EMI this edge should be higher thanthe height of the cable bundle. The cables should as far aspractically possible, be installed in a single layer.

The plate sections should be welded together and to thefasteners which in turn are welded to the hull. Alternativelyone might use at least 1 screw and lock washer per 0,2 mjoint. The distance between cable trays for different cableclasses is specified in Table 4-2. At bulkhead feed-throughthe distance between the trays might be reduced over a shortdistance.

Single core cables for AC having current rating in excess of250 A, and single core cables for DC with high ripplecontent, should not be mounted directly to the hull or othermagnetic material, but at a distance of at least 50 mm. If not,large losses and additional voltage drop will occur due tomagnetic hysteresis.

4.5.2 Conductors in cablesIn a multi-conductor cable the different circuits shouldnormally have the same function and the same power/voltagelevel. Deviation from this rule might be accepted if a cable isrunning between parts of a single self-contained system.

Twisted pairs should have a pitching of at least 10turns/meter. Analogue and digital signals should haveseparate cables. Signal conductor and return should beadjacent conductors in the same cable, and the difference inpower/voltage level between the circuits in one cable shouldnormally not exceed one order of magnitude.

Using circuits with twisted pair cables and symmetricterminations appreciably greater difference in power/voltagelevel can be tolerated. Such circuits could also be used forlow level signals as an alternative to coaxial cable, especiallyfor low frequencies. In special cases it might be necessary touse coaxial cables, double screen cables, cable routing intubes, conduits, etc. This must be considered in eachindividual case and in agreement with the equipmentsupplier.

4.6 GroundingGrounding is one of the primary ways to minimise unwantednoise and pick-up. Proper use of grounding and shielding inconjunction can solve a large percentage of all noiseproblems.

In the most general sense a ground can be defined as anequipotential point or plane which serves as a referencevoltage for a circuit or system. It may or may not be at earthpotential. If ground is connected to the earth through a lowimpedance path, it can then be called an 'earth ground'.Safety grounds are always at earth potential, whereas signalgrounds are usually but not necessarily at earth potential. Inmany cases, a safety ground is required at a point which isunsuitable for a signal ground, and this may complicate theproblem.

In the context of EMC it is imperative to think of 'ground' asa path for the current to flow instead of an equipotentialsurface. Currents with frequency components from DC towell above 100 MHz typically pass through 'ground'. Atfrequencies in the MHz range resistance of the conductor,even including skin effect, is negligible compared with theimpedance due to the ground conductor inductance.

There are two basic objectives involved in designing goodgrounding systems:

� �o minimise the noise voltage generated by currents fromtwo or more circuits flowing through a common groundimpedance.

� To avoid creating ground loops which are susceptible tomagnetic fields and differences in ground potential.Grounding if done improperly however, can become aprimary means of noise coupling.


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DET NORSKE VERITAS

S0V

S0V

S0V

S0V

S0V

S0V

Alternative 2

Galvanic isolator

Central unit Periferal equipment

Alternative 1Isolate the reference voltageat both receiver and sender

Ground loop

Central unit

Central unit Periferal equipment

Periferal equipment

Figure 4-3 Ground loops

When grounding, the reference conductors, the frequencyrange of the signals to be transmitted, the mode oftransmission as well as the electromagnetic environment areto be taken into account. For frequencies f < 100 kHz onlythe point of symmetry in a symmetrical transmission can begrounded. In an unsymmetrical system the referenceconductor is only to be grounded at one point. One referenceconductor can be used for several signal conductors. Ifseveral reference conductors are used, these are to begrounded at only one point, the common ground point.

For frequencies f > 100 kHz and for pulse techniques areference conductor system grounded at the common point isno longer applicable.

As a general rule, equipment housings must be grounded. Forequipment whose dimensions are smaller than �/10 for thehighest considered frequency, it is normally sufficient toground the housing at one point. If the housing dimensionsexceed �/10 then the housing is to be grounded along thelongest edge at several points at separations not greater than�/10 in order to reduce the antenna effect of the housing. Forseparations of less than 0,3 m between ground points, ingeneral no improvement is to be expected. The highestfrequency considered is dependent on the electromagneticenvironment in which the equipment operates.

In the radiation field of an antenna, metal parts can act assecondary radiators. If these metal parts have connection withground or with each other which varies strongly with time(loose contacts) or is corroded (semiconductor effect), thenthese varying contact resistance can cause new frequencies(harmonics, interference spectra) to arise, which by means ofthe antenna effect of the metal parts can be radiated andconsiderably disturb radio receiving. Movable rods, links,ladders, turn-buckles, cables, doors, hatch covers and toolsetc. are therefore to be connected or isolated.

4.6.1 Main ground systemMetallic hull and superstructure, including details in thesewhich are welded together, are presumed to make asatisfactory main ground system. If metallic parts of the hullor superstructure are bolted together, they are presumed to bepart of the main ground system, provided measures have beentaken to secure good and permanent electrical conductingcontact at the bolted joints.

In non-metallic parts of the ship, e.g. a plastic superstructure,the main ground should be formed by interconnected copperbus-bars of at least 50 mm² along all cable routings.Aluminium superstructures which are provided with insulatedmaterial between aluminium and steel in order to preventgalvanic action, are to be grounded to the hull. For thispurpose, corrosion-resistant metal wires or bands are to beused. Provisions are to be made for preventing galvanicaction at the terminals of these connections e.g. by using'Cupal 'terminals when copper wires or bands are connectedto the aluminium constructions).

4.6.2 Signal reference systemThe signal reference system consists of the electricalconducting material ( copper bars) of a common referencebetween communicating electronic measuring -and/or controlequipment. This reference can be either connected to themain ground system or be floating. The signal referencesystem is constructed as a 'star' network emanating from thefunctionally central instrument in the communicatingelectrical /electronic systems.

4.6.3 Ground network configurationsWhen a galvanic conducting part of an installation, e.g. asignal reference system, or a cable screen is connected to themain ground system in one point only, this is called a singlepoint grounding. When the conducting part of an installationis connected to the main ground system in severalgeographically separated points, e.g. at each instrument on acontrol system, this is called a multi point grounding.


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DET NORSKE VERITAS

4.6.4 Grounding rulesThe signal reference system should be grounded at thefunctionally central instrument, the rest of the system beinginsulated from ground. As mentioned above, the referencesystem might also be floating. If the reference within anelectronic system must be grounded in several points, e.g. atmany instruments geographically separated, the referencemust be split by means of galvanic separation to avoidground loops via the signal reference system. The connectionbetween the reference system and the main ground systemshould be as short as possible and not in common with anyother grounding except at one point at the main groundsystem.

Signal cable screen between equipment operating at or beingsusceptible to frequencies having a wave length �>20 L,where L is the cable length, should have single pointgrounding. (Wave length �=300/f �m�; where f=frequency�MHz�). Below 100 kHz single point grounding is generallyrecommended.

1MHz 300m

1

100MHz 3m

10MHz 30m

0.1MHz 3000m10 100m

Cable length

Freq. �

Multi-point

Single-point

Figure 4-4 Single-point and multi-point groundingof signal cable screens

The grounding should be made at the cable end where thecircuits connected provide the lowest impedance to ground.This is usually in the functionally central instrument, wherethe reference system is also grounded. Exemption from this iscable screens from thermocouples which might have aground point at the sensor end.

Signal cable screen between equipment operating at high, -orlow frequencies, and being susceptible to frequencies havinga wave length �<20L, should be grounded at least at bothends. Above 10MHz, multi point grounding is generallyrecommended. In a transition region from 100 kHz to10MHz, both single point and multi-point grounding may becombined for complex systems.

Power cable screen (DC, 60 Hz, 400 Hz, etc.) should begrounded to main ground whenever possible, at least at bothends. This is safety grounding, which is also favourable froman EMI point of view. Above deck the cables are especiallyexposed to radio signals. If the highest radio frequency is 22MHz, the power cable screen above deck should be groundedat least at every 2,5 m (approx. 0,2 �).

Single core cables for AC and special DC-cables with highripple content (e.g. for thyristor equipment) are to begrounded at one end only.

All metallic racks, cabinets, cases, etc. surroundingelectric/electronic equipment must be grounded. Large unitsshould have several ground points distributed around theunit.

To prevent electrostatic charging of insulated mountedmetallic parts in the vicinity of antennas or cable routing,these should also be grounded.

The equipment supplier's specification regarding thegrounding network should be considered according to theguidelines given above. In case where the two specificationsdo not agree, one should carefully examine the totalgrounding system to avoid inferior overall solutions.


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DET NORSKE VERITAS

CPU

I/O

I/O

ACDCDC

Power filterclose to entry Filter

Instrument earth bar

RE-postPE-post

1m

Optionally IS-bar

IP

Galvanic separator

Cable screen isolated

Cable screenisolated

Central unit

Armour isolated from the screen to be connected to PEInstrument earth bar to be:�� isolated, or�� connected to the frame, or-grounded to frame by HF

capacitors e.g. 20cm between the capacitors

Figure 4-5 Grounding of cable screens.

4.6.5 Shielding, procedures and choice of materialsCabinets, racks, etc. should be all seam welded to givecontinuous, homogenous joint of the separate parts. For anyopening in the shield (cabinet, rack, etc.), the largest diametershould be less than �/40 of the EMI that might disturb theequipment.

For equipment working with signals having fast rise times(less than 10 nsec.): L< 25 t; where

L = largest acceptable aperture �mm�; t = rise time of signals(pulse) �nanoseconds�

When using EMI gaskets on doors, covers, etc. in cabinets,racks and boxes, one should ascertain that there is a sufficientpressure on the gasket to achieve good contact to bothsurfaces. The surfaces for the gasket must be electricallyconducting , i.e. surface treatment like painting, plasticcoating etc. must be removed. Possible natural deformationor compression of the gasket must be taken care of (at doors,covers, etc.)

Doors, hinged covers, etc. should preferably have acontinuous (long) hinge to ascertain better electrical contact(higher pressure, evenly distributed). Especially this isimportant where EMI gaskets are used.

4.6.6 Grounding, procedures and choice of materials.The technique for making the ground connection and type ofmaterials used in the straps, conductors, etc. and the jointsare of great importance to have a lasting, good connection.

The ground connection can be done either by direct bondingto the main ground (the hull) or by a dedicated ground lead.The latter is the case when the equipment is mounted onvibration isolators.

The mating surfaces should be clean and free from oil andoxides. Special attention should be paid to corrosion at thejoints.

The dedicated ground conductors should be solid, flat,metallic conductors, or a woven braid configuration wheremany conductors are effectively in parallel. The conductormust be of a sufficient cross section. Ground straps should beprotected against corrosion. Ground straps should be as shortas possible. The ground strap, connecting two points togethershould be insulated if possible, to prevent undesired metalliccontact between the ground strap and other items.

If components such as instruments, signal lamps, etc., aremounted on hinged doors in a rack, the door should beconnected to the rack via a separate flexible ground strap.The same applies to doors in radio rooms or other roomswhere the bulkhead and decks serve as shielding againstEMI. At such doors care should be taken to ascertain a goodelectrical connection all around the door by means of EMIgaskets.


December 1995

DET NORSKE VERITAS

Welded or brazed bonds are preferred over all other types.Bolted connection or shrinkage is preferred over solderingwhenever practical. Especially when mechanical boltconnection or shrinkage is used in joints between groundpoint and ground conductor, it is important that all matingsurfaces are cleaned to base metal (made electricalconducting). Be aware of that anodised aluminium has apoorly conducting surface. To ascertain good contact usewasher and serrated lock washer or equivalent.

When using cable trays, cable 'ladders', etc. that are notwelded together, good and lasting electrical connectionbetween the different sections of the cable tray, cable 'ladder',etc., should be taken care of by using bonding straps or byscrews and lock washers. Make all cable trays electricallycontinuous.

Any ground connection should provide a low resistance pathto ground, AC and/or DC. Depending on environment,structural details, etc., there will practically be one or moremetals suitable as ground connector. Due to corrosionpossibilities it is important that metals having largeelectrochemical potential difference do not make contact witheach other, e.g. copper and aluminium. Generally speaking,metals having low electromotive force (EMF) are corrodingmore rapidly than those having higher EMF. When differentmetals are combined, it is desirable to use metals from thesame group. In cases where contact between metals havinglarge EMF difference are unavoidable, metals having thelowest EMF are used in easily changeable parts such as bolts,lock washers, nuts, etc., such that more solid and'unchangeable' parts having higher EMF.

All radio room equipment must have its individual groundconnection directly to the main ground system. Use of one ormore common ground buses which in turn are connected tothe main ground system should be avoided because this canlead to noise voltages via common impedance (commonmode coupling effects).

The radio transmitter's cabinet should be grounded at severalplaces. If special efforts are made to assure good electricalcontact through the bolted connection fastening the cabinet tothe ship's hull (shield around radio room), such a groundingwill often be sufficient. Dedicated ground straps arenecessary if the transmitter is mounted on isolating vibrationdampers.

Power and instrumentation cables to/from equipment that arenot situated in the radio room must not be routed through theradio room (i.e. not inside the shield around the radio room).

Cables, including metallic cable screen that go to/from theradio room may carry considerable amounts of EMI.Especially when the radio transmitter is operating, cablescreens on cables in the radio room can carry considerablecurrents in the radio frequency range (about 2-30MHz).Cables screens should therefore not go through the wall(shield) in the radio room, but be terminated in the shieldaround the radio room. As a general practice all cablesto/from the radio room should have noise reducing filters atthe point where they leave the radio room. Special effortsshould be made to reduce coupling of EMI via the powersupply system to/from the radio room. E.g. isolationtransformers with grounded metal screen between primaryand secondary windings could be used for AC supplies. DCpower supply could e.g. go via a converter/rectifier system.(This system must be relatively free from EMI).

The commonly used frequencies for the radio station arebetween 2-30MHz. In this range (actually 100kHz-10MHz)the electrical and magnetic coupling between circuits aredetermined by several variables. (But normally the electricfield is predominant). The correct grounding method will bemulti-point ground or a combination of single and multi-pointground of cable screens.

Screened power cables in and near by the radio room andtransmitter antennas should normally be grounded at least atboth ends. All cables passing transmitter antennas within acertain distance should be shielded. The precautions takenmust comply with governmental regulations.

4.6.7 Above deck installationsEquipment installed above deck is strongly exposed to directradiation from antennas (radio, radar, etc.) on board or fromland stations and passing ships. Special efforts often has to beexecuted to prevent these signals from disturbing equipmentabove deck, or propagating down into the ship.

All cables passing in the vicinity of a transmitting antenna forcommunication and navigation equipment must be screened.For common omnidirectional ship's radio antennas theminimum distance in meters is given as :Rmin = 0,1 Po

where Po is the nominal transmitting power in watts. Thisminimum distance must be adjusted if the transmittingantenna is directional, or effectively radiated power area ischanged in an other way. Shielding of the cable can be donein different ways, such as using screened cable, route thecable in metal conduit, having a steel bulkhead between thecable and the transmitting antenna, etc. By using filters toprevent the EMI from following the cable down into the ship,the above mentioned minimum distance can be reduced.

Electronic equipment above deck should have a metal coveror other protection against electromagnetic fields. For criticalcases it should be observed that two thin shields providebetter shielding than one equally thick shield (provided thetwo are separated).


December 1995

DET NORSKE VERITAS

When the frequency increases the current will flow closer tothe surface (skin effect). With respect to grounding it isimportant to consider the surface area (not the cross-section).In order to keep the impedance at a low level, the distancesshould be as small as possible. Up to 10kHz mainly magneticfield, the coupling is by induction. The objective is to createa safe and low impedance return for noise currents. If thereturn has a higher impedance than the signal circuit, thenoise may intrude the signal.

Figure 4-6 Avoid pig-tails.The surface area is reduced and is not able to carry noisecurrents above some kHz.

The high low frequency current that may flow in the pigtailwill be routed in parallel with the signal line and may pick-upnoise. Terminate the screen in a suitable cable conduit at theentry of the enclosure.

The checklist that follows is intended to summarise in shortform the more commonly used noise reduction techniques.


December 1995

DET NORSKE VERITAS

EMC Check ListItem Y/N Comments

A. Suppressing noise at source:Enclose noise source in a shielded enclosureFilter all leads leaving a noisy environmentLimit pulse rise timesRelay coils should be provided with some form of surge dampingTwist noisy leads togetherShield and twist noisy leadsGround both ends of shields used to suppress radiated interference(shield does not need to be insulated)B. Eliminating noise coupling:Twist low-level signal leadsPlace low-level leads near chassis (especially if circuit impedance ishigh)Twist and shield signal leads (coaxial cable may be used at highfrequencies)Shielded cables used to protect low-level signal leads should begrounded at one end only

Coaxial cable may be used at high frequencies with shield groundedat both ends.

When low-level signal leads and noisy leads are in the same connector,separate them and place the ground leads between themCarry shield on signal leads through connector on separate pinAvoid common ground leads between high and low level equipmentKeep hardware grounds separate from circuit groundsKeep ground leads as short as possibleUse conductive coating in place of non conductive coatings forprotection of metallic surfacesSeparate noisy and quiet leadsGround circuits at one point only (except at high frequencies)Avoid questionable or accidental groundsFor very sensitive applications, operate source and load balanced togroundPlace sensitive equipment in shielded enclosuresFilter or decouple any leads entering enclosures containing sensitiveequipmentKeep the length of sensitive leads as short as possibleKeep the length of leads extending beyond cable shields as short aspossibleUse low-impedance power distribution linesAvoid ground loops Consider using the following devices for breaking ground loops:

Isolation or neutralising transformers; Optical couplers; Differentialamplifiers; Guarded amplifiers; Balanced circuits

Use steel cabinets of 1-2mm thickness Avoid using plastic enclosuresAluminium superstructures to be grounded by CUPAL-foils every 10mWhen coaxial cables are used the cable screen may be used as a returnand ground

It may be advantageous to use a filter to prevent ground loops

Avoid coaxial cables with BNC connectors Use preferably twinax or triax having an outer screen that does notcarry signal

Avoid using RS 232 for data communication RS232 is not balanced i.e. conductors do not have the sameimpedance with respect to ground and to all other conductors.RS485 is better. TTY current loop 20mA is balanced, sender andreceiver are separated (low capacity)

Proper connection and clean the surface Use stainless steel bolts, zinc spray, lock washers, etc.Copper straps for grounding should have a cross section of at least25mm²

Use at least one strap per metre of the cabinet and preferably at bothbottom and top

C. Reducing noise at receiver:Use only necessary bandwidthUse frequency selective filters when applicableProvide proper power supply decouplingBypass electrolytic capacitors with small high-frequency capacitorsSeparate signal, noisy, and hardware groundsUse shielded enclosures


December 1995

DET NORSKE VERITAS

5. Testing for EMC

5.1 GeneralThe type of test required should be determined on the basisof interference to which the equipment may be exposed whenit is installed, taking into consideration the arrangement ofthe circuit i.e. the manner of earthing the circuit and shields,the quality of shielding applied and the environment in whichthe system as a whole is required to work.

It is impossible to simulate all conditions that may beencountered in the field. But a very good indication as tosusceptibility of an equipment can be obtained from theapplication of a few standard tests. The tests should beconsidered to be basic tests which cover a sufficiently widerange of interferences to test industrial and maritime processmeasurement and control equipment.

Interference susceptibility tests are essentially equipmentwithstand-tests designed to demonstrate the capability ofequipment to function correctly when installed in its workingenvironment. Interference tests should be carried out with thesystem 'live' i.e. with the functional signals present, which inpractice may be simulated. When considering the severity ofinterference tests to be applied the intention is to simulate, asclosely as possible, the conditions which can actually exist innormal applications. It is therefore appropriate that high testvalues should be chosen but not extreme values.

For reasons of comparison of equipment it is necessary tosimulate a test signal that is relatively uniform andrepeatable.

5.1.1 Conducted interferenceProducts can be susceptible to a wide range of interferencesignals that enter it via the AC power cord. An obviousexample is lightning induced transients. Thunderstormsfrequently strike power transmissions lines and transformerstations. Circuit breakers are intended to momentarily clearany faults and reclose after a short while.

The intent of the conducted emission limits is to restrict thenoise current passing out through the product's AC powercord. The reason for this is that the noise currents will beplaced on the common power net of the installation. Thecommon power net of an installation is an array ofinterconnected wires in the installation walls, and as suchrepresents a large antenna. Noise currents that are placed onthis power net will therefore radiate quite effectively, whichcan produce interference.

5.1.2 Radiated interferenceThe purpose of these tests is to insure that the product willoperate properly when it is installed in the vicinity of highpower transmitters. The common types of such transmittersare AM and FM transmitters and surveillance radars.Manufacturers test their products to these types of emitters bysubjecting the product to typical wave form and signal levelrepresenting the worst-case exposure of the product anddetermining whether the product will perform satisfactorily.If the product cannot perform satisfactorily in suchinstallation, this deficiency should be determined prior to itsmarketing so that fixes can be applied to prevent a largenumber of customers complaints and service calls. Refer toDNV Certification Note No. 2.4 for details regarding testingfor EMC.

R F IS p ik e sE S D

C O N D U C T E D R A D IA T E D

P o w e r le a d s M a g n e t ic f ie ld

C o n tro l a n ds ig n a l le a d s

A n te n n a te r m in a l

E le c t r ic a n de le c t r o m a g n e t ic

C O N D U C T E D R A D IA T E D

P o w e r le a d s M a g n e tic f ie ld

E le c t r ic a n de le c t ro m a g n e t ic

C o n t ro l a n ds ig n a l le a d s

E M CT E S T S

E M IS S IO NT E S T S

S U S C E P T IB IL IT YT E S T S

Figure 5-1 An overview of EMC tests.Tests in shaded boxes are not within the scope of CN 2.4.


December 1995

DET NORSKE VERITAS

6. Rules and regulationsThe requirements imposed by the governmental agencies arelegal requirements and generally cannot be waived. Theserequirements are imposed in order to control the interferenceproduced by the product. However, compliance with theseEMC requirements does not guarantee that the product willnot cause interference. It only allows the country imposingthe requirement to control the amount of electromagnetic'pollution' that the product generates. On the other hand EMCrequirements that the manufacturers impose on their productsare intended to result in customer satisfaction. They areimposed for the purpose of insuring a reliable, qualityproduct.

From a classification point of view, the safety of the vesselscan be verified by well planned function testing of equipmentand systems, possibly if required backed up by selectedmeasurements of radiation levels and EMI currents flowingon cables and cable screens. This may however lead tosituations where the problems are revealed close to or duringthe sea trials.

Implementation of formal and complete EMC managementprocedures based on susceptibility and emission levels for allequipment backed up by equipment tests over a widefrequency range, provides a systematic method of predictingproblem areas and planning for the optimum solutions. Thismethod may be an expensive approach to solving problems,but the expenses must be balanced against the benefits. TheDNVC approach to assisting Owners and Yards in EMIquestions is to provide assistance and guidelines on a case-by-case basis taking the specialities of each vessel intoconsideration.

Further, DNV has in connection with Type Approvalschemes for instrumentation equipment introducedsusceptibility EMI testing. These tests requirements are verysimilar to those required by other classification societies. Thetest levels provide a compromise between costs anddocumentation of equipment properties, i.e. one is able toscreen out equipment which is definitely not suitable forinstallation on board.

6.1 EEC and the EMC DirectiveThe EMC Directive 89/336/EEC was enforced January 1st,1992 and is mandatory to the member countries. Hence thenational legislation of these countries must be adjusted in linewith the EMC-directive.

The EEC/EMC-directive comprises equipment and systems.A system shall be tested and approved as a whole system. Aninstallation can however, seldom be tested as such. Thereforeone must assume that the requirements of the directive havebeen met if the individual parts of the installation have beenEMC approved (-which in practice may turn difficult sincethe EMC performance depends on a number of measureswhich need to be taken care of during the installation e.g.cable routing, grounding, screening). The distinction betweensystem and installation can in some instances be hard todefine. A computer system comprising a PC and someperipheral units should be regarded as a system, if the unitsare of the same make and destined to be integrated. On theother hand a computer system that should be configured bythe user, should be regarded as an installation if the units aredifferent makes.

In order to release a product in the EEC countries, a proofmust be produced justifying that the EMC-directive iscomplied with. This shall be stated by the manufacturer.Documentation must be produced to verify such compliance.As a proof of compliance with the EEC regulations, theproduct in question shall be marked with a sticker; 'CE'(Communauté Européenne). To demonstrate that a piece ofequipment complies with a harmonised standard, themanufacturer (or agent) shall provide a declaration ofmanufacture. Verification of compliance according to thestandard requirements should be by means of a test protocolissued by an impartial testing laboratory, or in case themanufacturer has the necessary resources, by himself.

Compliance without complete fulfilment of the harmonisedstandard requirements shall be documented in a technicalconstruction file. Radio equipment for broadcasting shall beprovided by an EEC-type approval certificate issued by anotified body. Note that the product requirements therewithmay apply several directives according to the so-called 'newapproach', which needs to be complied with by all e.g. EMC-directive 89/336/EEC, 91/263/EEC and 89/392/EEC. Notethat the latter directive refers to the 73/23/EEC (LVD)regarding safety of personnel.

6.2 A comparison between EU Directive89/336/EEC and the requirements of DNVCThe data in the following tables gives an overview of thetesting requirements of EU and DNVC respectively. Notethat in some cases the tests are based on different test set-upsand a straight comparison may therefore be awkward.

The following documents have been used:

� EN 50081-1 January 1992 Electromagnetic compatibility- Generic emission standard Part 1: Residential,commercial and light industry

� EN 50081-2 August 1993 Electromagnetic compatibility -Generic emission standard Part 1: Industrial environment

� EN 50082-1 January 1992 Electromagnetic compatibility- Generic immunity standard Part 1: Residential,commercial and light industry (To be replaced in the nearfuture)


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� EN 50082-2 March 1995 Electromagnetic compatibility -Generic immunity standard Part 2: Industrial environment

� DNVC Classification Note 2.4 May 1995 -Environmental test specification for instrumentation andautomation equipment

EmissionEN 50081-1 DNVC CN 2.4 Remarks

Enclosure 30-230 MHz

30 dB�V/m at 10 m distance

230-1000 MHz 37dB�V/m at 10 m distanceEN 55022 Class B

AC mains 0-2 kHz

EN 60555-2EN 60555-3

0,15-0,50 MHz 66-56 dB�V/m quasi peak56-46 dB�V/m average

0,50-5 MHz 56dB�V/m quasi peak46dB�V/m average

5-30 MHz 60dB�V/m quasi peak50B�V/m averageEN 55022 Class B

0,15-30 MHz See basic standardEN 55011

EmissionEN 50081-2 DNVC CN 2.4 Remarks

Enclosure 30-230 MHz

30 dB�V/m quasi peak(40 dB�V/m *)

Measured at 30 m (10 m*) distance

230-1000 MHz 37 dB�V/m quasi peak(87 dB�V/m *)

Measured at 30 m (10 m*) distance

EN 55011 *) Changed in EN 55022AC mains 0,15-0,50 MHz

79dB�V/m quasi peak66dB�V/m average

0,50-5 MHz 73dB�V/m quasi peak60dB�V/m average

5-30 MHz 73dB�V/m quasi peak60B�V/m average

EN 55011

Immunity - Enclosure portEN 50082-1 DNVC CN 2.4 Remarks

Radio-frequency 27-500 MHz 80-1000 MHzelectromagnetic field. 3 V/m (rms) 10 V/m (rms)Unmodulated 80 % AM (1 kHz)

IEC 801-3:1984 ENV 50140Electrostatic discharge 6 kV contact

8 kV air dischargeIEC 801-2:1984 IEC 801-2:1992


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Immunity - Input and output DC portsEN 50082-1 DNVC CN 2.4 Remarks

Fast transients 0,5 kV 1 kVCommon mode 5/50 Tr/Th nsSignal and control lines 5 kHz (rep.freq.)

IEC 801-4:1988Fast transients 0,5 kV (peak) 2 kVCommon mode 5/50 Tr/Th nsInput and output ports 5 kHz (rep.freq.)

IEC 801-4:1988

Immunity - AC input and AC output power portsEN 50082-1 DNVC CN 2.4 Remarks

Fast transients 1 kV 2 kV5/50 Tr/Th ns

5 kHz (rep.freq.) 2,5 kHz (rep.freq.)IEC 801-4:1988


Slow transientsSurge test

1 kV (differtial mode)2 kV (common mode)

1,2/50 �s (voltage surge)8/20 �s (current surge)

� polaritiesPRENV 50142

Immunity - Enclosure portEN 50082-2 DNVC CN 2.4 Remarks

Radio-frequency 80-1000 MHzelectromagnetic field. 10 V/m (unmod., rms)Amplitude modulated 80 % AM (1 kHz)

ENV 50140Radio-frequency 900 � 5 MHzelectromagnetic field. 10 V/m (unmod., rms)Pulse modulated 50 % duty cycle

200 Hz (rep.freq.)ENV 50204/50140

Power-frequency 50 MHzmagnetic field *) 30 A/m (rms)

EN 61000-4-8 *) apparatus containing devicessusceptible to magnetic fields only

Electrostatic discharge 4 kV contact 6 kV contact8 kV air discharge

EN 61000-4-2 IEC 801-2, 2nd edition


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Immunity - Ports for signal lines and data buses not involved in process control etc.EN 50082-2 DNVC CN 2.4 Remarks

Radio-frequency 0,15-80 MHzcommon mode 10 V/m (unmod., rms) 3 V/m (unmod., rms)Amplitude modulated 80 % AM (1 kHz)

150 � source impedanceENV 50141 ENV 50140

Fast transients 1 kV5/50 Tr/Th ns

5 kHz (rep.freq.)EN 61000-4-4 IEC 801-4

Immunity - Ports for process, measurement,control lines, long bus and control linesEN 50082-2 DNVC CN 2.4 Remarks

Radio-frequency 0,15-80 MHzcommon mode. 10 V/m (rms) 3 V/m (rms)

80 % AM (1 kHz)ENV 50141 IEC 801-6 Draft

Fast transients 2 kV 1 kV5/50 Tr/Th ns

5 kHz (rep.freq.)EN 6000-4-4 IEC 801-4

Immunity - DC input and DC output power portsEN 50082-2 DNVC CN 2.4 Remarks

Radio-frequency 0,15-80 MHzcommon mode. 10 V/m (rms) 3 V/m (rms)



5 kHz (rep.freq.) 2,5 kHz (rep.freq.)EN 6000-4-4 IEC 801-4


Radio-frequency 0,15-80 MHzCommon mode 10 V/m (rms) 3 V/m (rms)



5 kHz (rep.freq.) 2,5 kHz (rep.freq.)EN 6000-4-4 IEC 801-4

Immunity - Earth portEN 50082-2 DNVC CN 2.4 Remarks

Radio-frequency 0,15-80 MHzCommon mode 10 V/m (rms) 3 V/m (rms)



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7. EMC management

7.1 GeneralImplementation of EMC management control is a systematicapproach to EMC to help in decision making, describeremedial measures and make possible an estimate of the timeand cost expenditure. It will normally be easier to remedythose problems that do surface in the design in contrast toproblems arising at the final testing after installation.

The management control procedures1 serve to bring in asystematic manner the possible interference in and betweenthe systems, to investigate them qualitatively andquantitatively and to form the basis for working out theremedial measures for EMC.

It consists of setting up a general list as well as a review planto take in all the equipment of the system that can have aneffect on the EMC of the system:

In the data list are to be included:

� EMC requirement as to emission of disturbance.� EMC specification values for equipment already

developed. Data to be obtained from the manufacturer. Ifthere are no measured values available, then estimatevalues and values from experience may be put in.

� Consequence classes.� Data concerning the transmitting and receiving equipment� Data on the electromagnetic environment, that the

equipment is at times or continuously put into, e.g. dataon useful emissions, on emission of disturbance, onimmunity to disturbance and on the field strengths ofstrong transmitters in the neighboring systems as well asthe requirements to take into consideration the thresholdsof disturbance of sensitive sensors (antenna, receiverinput) in these neighboring systems.

� Equipment installation location, antenna sites.� Scaled drawings of the entire system or part of the

system, from which data can be taken on the spatialarrangement and the placing of the equipment, antennaand cabling.

� Information concerning the cable installation e.g. type,routing. paths, lengths, and other characteristics of thecable installation which are relevant to EMC can be seen.

� Information concerning the screening, grounding,earthing and further measures for potential equalization inthe system.

1 The procedures are based on the method described in DINVDE [Ref 3.]

Production of an influence schematic is helpful in indicatingthe sources, coupling paths and receivers of EMI. See Figure7-3. All the equipment of the system are to be classified intothe following consequence classes according to theconsequence of disturbance. This classification serves to setthe immunity margins to characterise the valence ofdisturbances in the processing of the EMC analysis.

Class Margin Consequence of disturbance

0 0 dB No harmful effect1 6 dB Equipment for which disturbance of the

function can lead to an additional load onthe operators or to limitation on thesystem efficiency

2 10 dB Equipment for which disturbance of thefunction can lead to wounding, damage tosystem or to restriction of efficiency ofthe system

3 20 dB Equipment for which the disturbance ofthe function can lead to loss of life, toloss of the system, or unjustifiablerestriction on system efficiency

Table 7-1 Consequence of disturbance andimmunity margins.The allowable level for emission of disturbance dependsessentially on the decoupling of equipment units, lines andcables from the system's receiving antennas and from eachother. The inherent coupling attenuation of various systemsdepends primarily on design and dimensions of theequipment location.

The coupling attenuation of an equipment location isessentially characterised by:

the distance of equipment units, lines and cables from theantennas and between each other,

metal surfaces for decoupling, and the existing screening against antennas and other

equipment units, at the place of installation.

As shown in Table 7-2 systems are classified by means ofthese characteristics into decoupling levels. The decouplingis given by a typical value. This value may be correctedaccording to the actual situation.


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Equipment locationcharacteristics

Decoupling

(�10 dB)

Typical EMC features Examples

No screening and nodecoupling metalsurfaces

Level 00 dB

Lines and cables have large antenna heights and loopwidths; therefore little decoupling from each other andfrom the antennas

Ships mainly of wood orplastic, or graphite composite.

No substantial screening,but decoupling metalsurfaces

Level 115 dB

Lines and cables in contact with or closely above themetal surfaces have minimum antenna heights and loopwidths; therefore decoupling from each other and fromthe antennas by at an average level

Ships with metal deck,superstructures with non-metalfloor surface

Screening as well asdecoupling metalsurfaces

Level 230 dB

Antenna heights and loop widths of lines and cablessame as for level 1; average antenna decoupling ofequipment to screened areas

Ships with metal deck andelectrically conductingbulkheads, tanks

Closed screening ormultiple screening e.g.by nested screens

Level 350 dB

Very high tightness against electromagnetic fields;average antenna decoupling of equipment within closedscreens or double screening

Submarines, metal ships withseveral metal decks on top ofeach other

Table 7-2 Decoupling levels and typical characteristics (not including distance related decoupling)The levels of equipment emissions of disturbances are basedon a measuring distance r � 1m assuming reflection-freepropagation of electric and magnetic fields. For otherdistances the respective coupling values between source andsink of disturbance must be calculated or corrected accordingto the nomograph Figure 7-1.

-80

-60

-40

-20

0

20

40

60

80

0.01 0.03 0.1 0.3 1 3 10 30 100 300 1000Distance r in metres

0.3 MHz1

3

10

30

>48

<48

100

300

1000

3000 MHz

Figure 7-1 Coupling attenuation increase for the electricfield (approximately also magnetic) of magnetic dipole asa function of distance r in metres and frequency in MHz.

The field strength present at the place of equipmentinstallation and the disturbing currents flowing through linesand equipment housing are caused by the internal transmittersof the system or, in some cases, by external transmitters,which are operated either continuously or for short periods inthe immediate vicinity of the equipment.

Figure 7-2 gives examples of field strength values fortransmitting antennas. The most important feature is thescreening attenuation resulting from metal structure of thesystem provided the place of equipment installation lieswithin this structure. In addition, the system's dimensions areof importance as large distances from the transmittingantennas may result, in some cases, in a significant reductionof the field strength produced.

Table 7-3 contains field strength data for various frequencysub ranges and for typical systems. In addition to fieldstrength data for 'external equipment', i.e. equipmentincluding associated cabling which is located within anantenna radiation field, these tables also include field strengthvalues for equipment installed within the system structure('internal equipment'), and the respective screen attenuationlevels.

Column 'Reason for requirement' contains a short descriptionof the specified basic conditions from which the requirementshave been derived. If these basic conditions do not exist, thefield strength requirements can be adapted by converting thecurves in Figure 7-2 (inclusion of other screened attenuationlevels and other antenna distances).

Immunity margins shall be determined during system analysisand ensured by respective measures during the integration ofthe system.

The voltages coupled into lines as a result of incident fieldsare dependent upon the ambient field, and the routing of theline.


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Figure 7-2 Examples of distance-dependent fieldstrength values for transmitting antennas. Distancer in metres.

The immunity to the disturbances at terminals of powersupply lines, unscreened lines, and screens (microsecond andnanosecond pulses) are primarily dependent upon the powersupply characteristics typical for the system (power supplies)and load connected. The peak values of disturbing pulses tobe expected in a specific system, can only be determined withaccuracy required for the exact specification of limitingvalues by means of a system power supply analysis.

Within the scope of this analysis, all characteristics of thesystem power supply (power generators, supply network andall inductively and capacitively coupled components of thecontrol and signal network, load) which are essential to thegeneration, propagation and effects of the disturbancingpulse, must be included and evaluated. Conductive couplingsare difficult to predict and estimate. When data are notavailable, fill in with a yes or no answer or a description ofthe signals on the information sheet.

Ship External equipment Internal equipment

Frequency dB�V/m Reason for requirement dB�V/m Reason for requirement

Metal150 kHz to 1,5 MHz

166 Internal MF transmitter (approx. 500 W)equipment approx. 5 to 10 m away fromantenna

130 Internal MF transmitter (approx. 500W) 36 dB screen attenuation

Wood, plastic150 kHz to 1,5 MHz

166 Internal MF transmitter (approx. 500 W)equipment approx. 5 to10 m away fromantenna

160 6 dB decoupling due to metalsurfaces or installation outside high-intensity radiation field

Metal1,5 MHz to 30 MHz

160 Internal HF transmitter (approx. 1000 W)equipment approx. 4 m away from antenna

130 Internal HF transmitter (approx. 1000W) 30 dB screen attenuation

Wood, plastic1,5 MHz to 30 MHz

160 Internal HF transmitter (approx. 1000 W)equipment approx. 4 m away from antenna

160 Internal HF transmitter (approx. 1000W) no screen attenuation assumed

Metal30 MHz to 1 GHz

146 Internal UHF transmitters (100 W)equipment 4 m away from antenna

130 Internal UHF transmitters (100 W),20 dB screen attenuation assumed

Wood, plastic30 MHz to 1 GHz

146 Internal UHF transmitters (100 W)equipment 4 m away from antenna

146 Internal UHF transmitters (100 W),no screen attenuation assumed

Metal1 GHz to 40 GHz

170 Internal radar transmitter, equipment outsideof antenna main lobe and approx. 10 m awayfrom antenna

140 Internal radar transmitter, 25 to 30 dBscreen attenuation assumed

Wood, plastic1 GHz to 40 GHz

170 Internal radar transmitter, equipment outsideof antenna main lobe and approx. 10 m awayfrom antenna

170 Internal radar transmitter, equipmentoutside of antenna main lobe, noscreen attenuation

Table 7-3 Typical field strength values


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Example:Emitter: MF radio transceiver: 500 W, 200 V/mCoupling: Metal ship, distance between emitter and receiver 15 m.Receiver: Computer based system, EM susceptibility 10 V/m

Parameter Emitter Coupling Receiver

Decoupling 30 dBDistance-dependent reduction 30 dBEM platform 166 dB�V/m 60 dB 106 dB�V/mAdditional attenuation 0 dB 0 dBConsequence class 10 dBRequired susceptibility 116 dB�V/mSpecified susceptibility 140 dB�V/mMargin between noise and susceptibility 24 dB�V/m

Table 7-4 Example of estimated margin between noise and susceptibility

Rn

E1

En

ER4

E6 R7

R2

R3

R5

Legend of symbols:

EM receiver, i.e. sensitive equipment; 'n' is the reference number

EM emitter; 'n' is the reference number

Cn

Cn

Cn

Coupling path; EM radiated signal; 'n' is the reference number

Couplingpath; EM conducted signal; 'n' is the reference number

Couplingpath; EM radiated and conducted signal; 'n' is the reference

C8

C9

C10

C13

C11

C12

Figure 7-3 Influence schematic


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Record sheet for EMC managementProject: Page:System: Sign:Reference, drawing, data sheet: Date:

CharacteristicEmitter Coupling Receiver

Specifications:Equipment, systemOperational:What is the system intended to do?When is the system operative?Stand alone, or part of a larger system?With what equipment does the system interface directly and indirectly ?Is the system required to operate continuously or intermittently?Are there critical sequences of operations involving this system?How will the system be maintained, operated, and supported?Consequence of failureConsequence classInstallation details:Location (type of facility in which the equipment to be installed, e.g. machineryspace, accommodation, bridge, open deck)What other equipment will be in the same installation?Cable specification:- Type,- Class,- RoutingSingle point or multi point groundingProtection against electrostatic dischargeType of filter applied- Input- Output- Power supplyRequirements:Basic power requirements?Signal inputs, and their range of frequency and power?Signal outputs, and their range of frequency and power?Sensitivity requirement for receiving equipment?Requirement to emission of disturbanceRequirement to total harmonic distortion [%]Requirement to electrostatic discharge, ESD [kV]Conclusions:Decoupling level � dB �Distance-dependent reductionEM environmental platform- Radiation- Conduction- Frequency � MHz �Anticipated attenuation by additional means � dB �Safety margin i.e. consequence class � 0,6,10,20dB�

Required susceptibility-Radiation � dB �-Conduction � dB �-ESD � kV�

-Harmonic distortion � % �-Power supply variations � % �Specified susceptibility-Radiation � dB �-Conduction � dB �-ESD � kV�

-Harmonic distortion � % �-Power supply variations � % �Estimated margin between noise and susceptibility-Radiation � dB �-Conduction � dB �-ESD � kV�

-Harmonic distortion � % �-Power supply variations � % �Instructions for filling in the form� Use one record per item.� Use parameter values whenever available and compatible units e.g. dBV/m; dBA/m ; MHz� Shaded boxes are not relevant information.


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Figure 7-4 Conversion of units

References

[1] Paul R. Clayton; Introduction to Electromagnetic Compatibility; John Wiley & Son, Inc. 1992

[2] Automation for Safety in Shipping and Offshore Petroleum Operations; A.B. Aune andJ.Vlietstra, North-Holland Publishing Company 1980.

[3] DIN-VDE-Taschenbuch, Elektromagnetische Verträglichkeit 3, VDE-Verlag Beutch 1989.

[4] IEEE Std. 519-1992 Recommended Practise and Requirements for Harmonic Control onElectrical Power Systems.

[5] MIL-HDBK-237A, Electromagnetic Compatibility Management Guide for Platform Systemsand Equipment, 1981.

[6] Electromagnetic Interference Guidelines for Installations and Proposal for Test of Equipment;P.Gulbrandsen, K.Fotland, T.Heimly, H.Liland; DNV Paper Series No. 80 P008 1980.

class notes 45.1: electromagnetic compatibility · classification notes no. 45.1 det norske veritas...

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