week 11+ inspection and testing of electrical installations

7/30/2019 Week 11+ Inspection and Testing of Electrical Installations

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INSPECTION AND TESTING OF ELECTRICAL INSTALLATIONS

This book is intended to act as a working manual for those engaged inthe initial inspection or re-inspection of an electrical installation. It willalso assist trainees studying for City and Guilds courses 2360-1,

2360-2, 2360-C, 2400, 2391, NVQ and BTEC. The advice given, andthe methods suggested, are based on many years of practical experienceand therefore will hopefully not be considered too theoretical for thoseengaged in the Electrical Contracting Industry.

If this most important aspect of an electricians work is to be successfullycompleted, testing and inspection activities must be carefully prepared,executed and documented. The principles enclosed in this book will enablethat goal to be achieved with the minimum of errors.

References are made throughout to BS 7671, better known as the 16th.

Edition of the IEE Regulations, and their associated guidance notes.Possession of these documents is essential for any test engineer hoping toperform a high quality service.

It goes without saying that inspection and testing requires a suitable range ofinstruments that will be regularly checked for accuracy and, when necessary,regularly re-calibrated. (See appendix 3 for details of instrumentrequirements)

If the reader is unfamiliar with current terminology, he should turn to section25 where definitions of commonly used terms are given.

T


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CONTENTS

SECTION 1 Requirements for the initial testing of an electrical installation

SECTION 2 Inspection and testing of existing installations.

SECTION 3 Outline of instrument tests and the compilation of inspection check list.

SECTION 4 Determination of cable length and voltage drop.

SECTION 5 Tests of protective conductor continuity.

SECTION 6 Tests of main and supplementary equipotential bonds.

SECTION 7 Tests of Ring Circuit continuity.

SECTION 8 Tests of Insulation Resistance.

SECTION 9 Tests of Polarity.

SECTION 10 Tests of Earth electrode resistance and soil resisitivity.

SECTION 11 Tests of Line - Earth loop impedance.

SECTION 12 Tests of RCD effectiveness.

SECTION 13 Overcurrent provision survey.

SECTION 14 Organisation of a testing programme.

SECTION 15 Testing portable and transportable equipment.

SECTION 16 Measurement of earth leakage current.

SECTION 17 Operation of devices for isolation and switching.

SECTION 18 Testing of information technology equipment.

SECTION 19 Measurement of prospective short circuit current.

SECTION 20 Measurement of illumination.

SECTION 21 Testing of escape lighting.

SECTION 22 Discrimination between overcurrent protective devices.

SECTION 23 Urban distribution systems.

Appendix 1 Related principles.

Appendix 2 Definition of terms.

Appendix 3 Instrument requirements.

Appendix 4 Self-assessment exercise.

Appendix 5 Completion and inspection certificates


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SECTION 1

It is not only a basic safety need, but also a requirement of B.S. 7671 andthe Electricity at Work Regulations (1989), that any electrical installationshall be verified as safe to operate before being energised.

The term "safe to operate" means the user of the electrical installation will befree from the risk of fire, shock, burns and the injury from mechanicalmovement produced by electrical machines such as electric motors. Safe to

operate also means that the user of the installation should not requiretechnical knowledge in order to stay clear of the casualty department of thenearest hospital.

This verification procedure requires:

I A validation of the installation designii A visual inspection of the constructioniii Instrument tests

A well-constructed installation will not necessarily function correctly or safely

if cables, switchgear etc. have been incorrectly selected by the designer.Design assessment is not easily undertaken when the installation has beencompleted, the numerous facets of the installation requiring verificationcannot be assessed by a simple visual inspection. The inspector will have tohave an intimate knowledge of BS 7671, a good technical education andconsiderable experience.

If, in the circumstances, an assessment of design viability is not a practicalpossibility, the designer of the installation - if known - will have to berequested to affirm that the installation design meets all regulationrequirements. This affirmation will take the form of a signature in the

appropriate section of the Electrical Installation certificate which will be of atype published in Guidance note 3 - BS 7671 - certifying that all regulationshave been conformed with.

The foregoing assumes the installation is new. If not the inspection andtesting process will be a periodic one not requiring knowledge of the originaldesigner.

Incorrect selection of fuses, circuit breakers, and cables are a prime exampleof a design defect that is not obvious to the casual observer. To assess

overcurrent device efficiency for example, it will be necessary to haveknowledge of the short circuit fault levels, maximum current demand and any

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TESTING AND INSPECTION - BASIC REQUIREMENTS


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applicable correction factors that will modify the tabulated current rating ofinstalled cables. Or, more simply, the temperature of the cable can bemeasured on full load. If the insulation is PVC, the maximum permittedtemperature is 70 degrees Celsius. Any temperature below this indicates thecables are running at less than capacity. However the voltage drop could be

excessive if the circuit lengths are long, perhaps necessitating theinstallation of a larger cable.

Section 13 of this book will examine methods of assessing the provision ofovercurrent protection.

An inspection of an installation is far from a simple matter, particularly if theinstallation has been in service for some years. Well-designed installationstend to deteriorate with the passage of time, due to both the natural effectsof ageing but more usually because of the unwelcome attention of"electricians" of uncertain skills.

Physical defects cannot be detected by the application of instruments.Construction faults such as insecure fixings and inappropriate means ofisolation can only be detected by visual inspection, which will have to beconducted in a systematic manner.Before commencing the inspection and test, you are strongly advised toprepare a schedule of work, which will not only organise the sequence ofoperations, but their detail. If the inspecting team arrive on site without anypre-planning, lots of time will be wasted and nervous energy expended. Thismeasure is particularly important if the installation is already in service and

installation schedules and diagrams of practical value are non-existent.

It is also essential that an inspection ascertain that the installation is ofsufficient capacity to supply the demand. Loads could have grown beyondthe intention of the designer. Maximum current should be measured, and forboth distribution and final circuits. Comparisons are then made with thecurrent rating of the controlling overcurrent device and the connected cable.

Maximum demand may be measured by the use of a recording clamp meterleft in position for 24 hours.

We live in a litigious society - dont take chances and overlook anoverloaded installation - it could be expensive!

If the installation is new, the inspecting engineer will be required to inspectthe construction of the installation, ideally both during erection and oncompletion, to ensure that no hidden defects are left undetected.

Electrical installation inspection and testing is a potentially hazardousexercise. Any person responsible for this work must be "competent" in thesense that this term is used in the Electricity at Work Regulations 1989.


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Inspection and testing is not a job for apprentices! It carries a heavyresponsibility. Understand the job - act responsibly - act safely - getorganised!

Approved voltage testers should be available to ensure that any circuit

assumed to be dead is in fact so. If the installation has to be attended to live,suitable precautions must be taken. These measures above all else requirecompetence on the part of the test engineer. An electric shock at 400V is inquite a different league to one at 230V. Make sure that you don't have tolearn the difference by experience.

A competent person could be described as someone of relevant experience,technically qualified, mature, and trained to use all of the necessaryinstruments in a safe manner and, additionally, be able to accuratelyinterpret the obtained test data.

No inspection can be effectively carried out without the possession of currentdrawings of the installation and knowledge of the installation"characteristics". These characteristics will include:

(i) The maximum current demand(ii) The prospective short circuit current at each distribution board(iii) The external P-E loop impedance at each distribution board(iv) The nature of the overcurrent device at each distribution board(v) The means of earthing and equipotential bonding(iv) The presence of sensitive electronic devices.

All of the above will be examined in detail in later sections of this book. Ifdrawings of an installation are not available, then possibly a substantialamount of exploratory work will be required. Tracing a myriad of outlets ofdiffering kinds back to their parent circuit is a tedious and time-consumingbusiness. However, an electronic cable tracer makes this work"comparatively" easy by the simple expedient of sending signals down theinvestigated cables. A receiver attached to the outlet under test will displaythe trace number on a LCD. Additionally, this instrument is extremely usefulfor conducting tests of polarity as explained later.

No assumptions should be made that the installation has been logically

designed and constructed.

An essential for this exercise is a pad of adhesive labels for attaching to thevarious outlets, on which will be inscribed the circuit identification.

In summary, the inspection and testing requirements can be quite simplystated thus:

"The completed installation shall be inspected and tested to ensure that in allrespects the requirements of BS 7671 have been conformed with".

A tall order, but that's the objective.


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Regulation 712-3 lists salient points requiring particular attention, whichinclude the following,

* Connection of conductors;* Identification of conductors;

* Current carrying capacity of conductors;* Determination of voltage drop;* Presence of fire barriers;* Presence of devices for isolation and switching;* Space factor of conduits and trunking.

See also appendix A of the BS 7671 Guidance Note 3 for further adviceregarding visual inspections and subsequent assessment.

There is a requirement for a periodic inspection of installations, the details ofwhich are given in Section 744 o f BS 7671 and table 2.1.5 of GN 3.

Some installations, which have a mandatory requirement for periodicinspections and tests, are listed below

ANNUALLY

CinemasTheatresPetrol filling stationsCaravans and caravan sitesLeisure complexesPlaces of public entertainmentRestaurants and hotelsPublic housesLaunderettes

All other installations have only recommended periods between inspectionsand tests. For all buildings where people work, theElectricity at WorkRegulations 1989 regulation 4 (2) has a requirement formaintenance of the electrical installation, which will necessitate testing andinspection on a regular basis.


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SECTION 2

The re-test of an installation introduces hazards not present in a deadinstallation and it is an important mandatory requirement that the inspectorshall be deemed to be "competent" (EAWR reg. 16). The term indicates afamiliarity with the type of work, technical literacy and maturity.Diagrams, charts etc. and any other relevant information will be required. Intheir absence - as previously mentioned - a degree of cautious exploratorywork will be necessary.

A visual inspection shall be carried out with the installation de-energised, as

far as practicalities will allow - embracing as much of the installed equipmentas possible and attention being paid to the following factors:

Safety Does the installation present a shock risk? To ensure safety, measures employed to ensureprotection against overcurrent and earth leakage must be suitable for any given situation.

Wear Does the installation show any sign of wear or abrasion? Check portable tools and otherequipment.

Environment The environment to which an electrical installation is subject may cause rapid deterioration instandards of safety. Environmental factors will include high and low temperatures, exposure tothe elements etc.

Damage Damaged equipment is dangerous equipment. All facets of the installation must be regularlychecked for signs of damage.

Corrosion Has the environment a corrosive atmosphere? Does the electrical installation come into contactwith the elements? If so, is the choice of equipment suitable?

Age As with most things, an electrical installation will deteriorate with age - particularly conductor

insulation based on organic compounds> This deterioration cannot necessarily be checked byan insulation resistance test - a visual inspection is required.

Suitability All electrical equipment shall be suitable for the use to which it is put. Unsuitable selection cancause a dangerous situation to arise.

It should be emphasised that the Electricity at Work Regulation 14 does notpermit any live workingunless it is absolutely essentialfor the installation toremain energised. The avoidance of inconvenience is not considered areason for investigating an installation live. This regulation - for reasons ofconvenience - is commonly and lightly breached, but any accidents resultingthat attract the attention of the Health and Safety Inspectorate could result ina prosecution.

Some installations that are getting on in years defy logic and no assumptionsshould be made regarding any installed equipment. It is not unusual todiscover that two fuses may control a particular point or unexpectedly have400V at its terminals.

Never assume that a circuit dead - particularly if the circuit is three phase.It's your life at stake, so use your voltage tester intelligently. Remember thevoltage between two points of the same phase of a live circuit is zero! Butthe voltage with respect to earth will be 230V.The resistance of the human body is voltage dependant the higher the

voltage the lower the resistance. A 400V shock is of a much highermagnitude than the increase in voltage would suggest.

INSPECTION AND TESTING OF EXISTING INSTALLATIONS


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The facets of the installation that require a visual inspection are the same asfor an initial inspection with particular emphasis being given to switchingdevices and identification and notices and correct polarity.

It is recommended by GN3 that a random sample of 10% of all switches and

isolators shall be selected for an inspection, which will assess their electricaland mechanical condition. Where any defects are revealed, all switchingdevices shall be inspected and tested unless the cause of a defect can beclearly identified and be confirmed as locally confined. The test will be one ofcontinuity, verifying complete isolation has taken place when the isolator isopened.

It goes without saying that before any circuit is assumed to be dead it mustbe effectively tested with an approved voltage tester. Examples of voltagetesters are illustrated below.

The tester shown in Fig.1 is also a continuity indicator, giving a beep whenconnected in a circuit of resistance up to 500 000 .

This tester will indicate the approximate impressed voltage by theillumination of the appropriate LED. A self-test and battery test facility is alsoincorporated.

Fig.1 Approved voltage testers

The tester above will respond to the electric field that surrounds allenergised conductors and requires no direct contact with live parts. Howeverit will only indicate the presence of a voltage and not its value, nor will itprecisely point to the energised conductor.

Despite its limitations this tester is a very good aid to safe working and doesnot require a potential difference to indicate a potential danger.


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It is important that the inspecting engineer realises that the condition ofcable insulation cannot be determined solely by the application of aninsulation resistance test. It is perfectly possible for the insulation to betotally absent; if the live parts are physically separated, an IR test willindicate a reading of infinite resistance.

Insulation problems can result from ageing or overloading and it is absolutelyessential that a thorough, systematic visualinspection be conducted.

All parts of the installation shall be verified as being adequately provided bythe necessary devices for isolation and, if required, emergency switching. Itis not acceptable, for example, that a distribution board be isolated only by afuse or circuit breaker. Isolating devices breaking either two or three poles -depending upon the number of phases required, are needed. If the isolator issited out of line of sight, it should be provided with a means of locking off.

To maximise safety and convenience, notices or labels are required at thefollowing points:

* Where differing voltages exist,* Earthing and bonding conductors;* Where an RCD is fitted,* At socket outlets supplying equipment for use outdoors;* Caravan installations;* Where an installation is supplied from two differing sources,* Earth free locations.

Additionally at each distribution board a schedule should be fixed to theinside of the lid indicating circuit destinations, number and type of pointsserved and their location.


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Visual inspection. Shown below are photographs of defects discovered in theinspection process. Examine the photos carefully and determine the natureof the defects.

Fig.2


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SECTION 3

OUTLINE OF TESTING REQUIREMENTS AND THE COMPILATIONINSPECTION CHECK LISTS.

Instrument tests are required to reveal the "hidden" characteristics of aninstallation and must be conducted in a sequence indicated in section 713of BS. 7671, reproduced below.

Continuity of protective conductorsContinuity of final ring circuit conductorsInsulation resistanceSite applied insulationProtection by separation of circuitsInsulation of non conducting walls and floors

Protection against direct contact by enclosures duringerectionPolarityEarth - fault loop impedanceEarth electrode resistanceOperation of residual current devices

Only those tests in italics will be examined in the book. The other tests are ofonly marginal interest to the working electrician.

Additionally it may be necessary to measure:

Prospective short circuit current levelsLevels of illuminationPortable appliance safetyEmergency lighting effectivenessMaximum current demand

For a re-test of an installation the previous sequence of tests has now beendeleted. The testing sequence is largely determined by opportunity andappropriateness.When conducting a visual inspection, it is absolutely vital that observationsare recorded at the time, not at a later date. The fallibility of memory makesrecollections unreliable. Prepare and use check lists, not a loose-leaf pad.

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No inspection and test of an installation can be conducted without plans.This need is not only one of common sense and safety but also a

requirement of BS 7671 reg. 711-01-02 and 514-09-01. If none exist they willhave to be determined by an explanatory survey.

You are expected to determine the location and current rating of all majorfuses or circuit breakers controlling sub distribution boards and theassociated isolators.

All of this information can be summarised in a single line distributiondiagram, shown below, to be provided on the reverse side.

Fig.3

KWH

TNC-S E

CT

300 A x 3

160 A 100 A 100 A 63 A

switch fuse

main distribution board

lower ground floor ground floor - north ground f loor - south first floor

A B C D

all distribution circuit cables

are pvc/swa/pvc/4c

All sub distribution boards are TP&N


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SECTION 4

If the client or a professional body requires a declaration of cable lengthand voltage drop, the following procedure can be adopted in the absenceof any useful data.

The maximum permitted voltage drop allowable by BS 7671 is 4%, ofnominal voltage, from the origin of the installation to the furthest point ofutilisation. (Section 525)

At 400V, permitted voltage drop is 16V, at 230V, 9.2V

If the installation is a simple one consisting of a single distribution board, theprocedure to measure voltage drop will also be simple. For each circuit -when isolated - the P and N conductors are joined and the resistance of theloop measured at the distribution board. (See Fig. 2)

Circuit length = 29.4 x R x S metres

Where R = loop resistance S = cable cross sectional area in mm.2

Example: the loop resistance of a lighting circuit, shorted out at the furthest

point is found to be 0.7 . If the c.s.a. of the cable is 1.0 mm.2, what is thecircuit length?

Solution L = 29.4 x 0.7 x 1 = 20.6 metres.

The voltage drop may then be determined by reference to appendix 4 of BS7671.

Example: if the above circuit is carrying a current when fully loaded of 5A,the voltage drop will be:

Vd = Ib x L x mVd = 5 x 20.6 x 44 =4.53 Volts1 000 1 000

Assumed conductor temperature of 70o.

Voltage drop is within limits. The above calculation assumed a single-phasecircuit, wired in single core cable enclosed in conduit. If the installation werea three-phase one, the procedure would be identical, using two of thephases instead of phase and neutral to determine the circuit length. It shouldbe remembered that calculated three-phase voltage drops are line voltdrops, meaning the voltage difference between phases or lines, at the mainsand load ends of the circuit. A voltage drop along the length of a cable iscalled aphase volt drop.

The difference between permitted single and three-phase voltage drops is bya factor of 3 (1.732). The allowance for voltage drop on a three-phase

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Determination of cable length and voltage drop


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circuit is not more generous; its simply the same voltage measured from adifferent standpoint.

An alternative means of determining voltage drop on a single-phase circuit isby the use of the formula:

Vd = Ib x R x 1.29 Vd = 5 x 0.7 x 1.29 = 4.52 Volts

Where Ib is the load current and R the loop resistance.

The correction factor of 1.29 is used to convert the resistance measured withthe circuit dead at 20 deg. to that of the assumed cable operating

temperature of 70o.

For a three-phase circuit the formula becomes,

Vd = Ib x R x 1.12 (Where Vd = Line volts)

For example, if the circuit current and loop resistance were the same as inthe previous example, the voltage drop on a three-phase circuit would be,

Vd = 5 x 0.7 x 1.12 = 3.92 Volts (line)

or 3.92 /1.732 = 2.26 Volts (phase).

It should be noted that for a given current and length of run the phasevoltage drop on a three-phase circuit is only half that for a single-phasecircuit.

The easiest way of measuring voltage drop is to take a voltage measurementat the origin of the installation and others taken at points located at extreme

ends of circuits. The voltage drop is then determined by simply subtractinglocally measured voltage from mains values. The installation should, ofcourse, be fully loaded when measurements are taken.

Mains voltage must be measured. Dont assume that its 230V or 400V:mains voltage continually fluctuates throughout the day.

Mains and load voltages mustbe measured at the same time.

If the installation is a large one, cable lengths will have to be determined insections and the individual voltage drops added together to obtain theresultant voltage drop.


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Fig.4 Measurement of circuit length

Low

Link


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SECTION FIVE (Reg. 713-02)

The purpose of these tests is to verify the continuity of all protectiveconductors and to obtain a measurement of the combined resistance ofcircuit phase and protective conductors at each point on every circuit. Ashorting link at the distribution board between phase and earth will put theseconductors in series, creating the circuit whose resistance is to bemeasured.

This combination resistance is symbolised as R1 + R2. where R1 is the

resistance of the phase conductor and R2 the resistance of the protective

conductor. (See fig.6 P-E loop path.)

The above data can usually be used to determine the phase - earth loopimpedance for individual circuits, when the external P - E loop impedance(Ze) of the installation is known.

Method

As far as it is reasonably practicable to do so, remove all main andsupplementary bonds before any continuity measurements are made. Thereason for this measure is to minimise the lowering effect the equipotentialbonds will have on the recorded resistance. They act as parallel paths toearth-fault current and hence reduce any measured resistance.

It is required that earth-fault currents will be of sufficient magnitude due tothe low impedance of the earthing construction, without reliance on the mainand any installed supplementary bonding.

Prior to the tests commencing, an insulation resistance test must beconducted to ensure that there are no short circuits between neutral andearth. A N-E short circuit will produce lower resistance readings than wouldotherwise apply, and of course a fault of this nature would not produceexcess current when the circuit is energised, and hence go unnoticed.

Having firstly ensured that the installation is dead, install temporary linksbetween the phase bus bar(s) and the earth bar. (See Fig. 8)

Isolate all circuits, except the circuit under test.

Attending to all points of termination, measure the resistance between thephase conductor and earth, proving continuity. The resistance measured isthat of R1 + R2. Record this value on an appropriate form - an example of

which is included in this section. (See also section 24-25)

A later test requires the live measurement of external P-E loop impedance(Ze). When this value is known for the distribution board under test, it will be

possible to determine the values of phase - earth loop impedance for theindividual final circuits.

TESTS OF PROTECTIVE CONDUCTOR CONTINUITY


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Fig. 5 Measurement of R2

Low

MET

Main equipotential bonds

Extraneous conductive parts


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It should be realised that if the protective conductor is steel conduit ortrunking, the creation of unintentional parallel earth paths will be bothinevitable and unavoidable, every contact with earthed metalwork loweringconductor resistance. Therefore even if the conduit system were falling apart

with rust, if it made multiple contacts to earth throughout its length it wouldproduce a low continuity resistance.

However, if the inspecting engineer is satisfied of the presence of acontinuous and permanent protective conductor, these parallel paths willassist without danger the production of the desired earth fault current.

BS 7671 allows the use of a Phase-Earth loop impedance tester wheredoubt exists regarding the substance of a protective conductor. Thisinstrument generates a test current at 230V with all the attendant dangersthis voltage may introduce and the author would advise the sparing use ofsuch a method on safety grounds.

All P-E loop impedance testers will have fitted warning neons labelled P-Nand P-E. When lit they signal confirmation of correct polarity and earthcontinuity. Under no circumstances must a test be conducted if either ofthese lights is out. If the test circuit lacks continuity, the cable will be drivenup to full mains voltage for the duration of the test - clearly a potentiallydangerous situation.

Where possible, a 50V 25A continuity tester should be used in preference toan ohmmeter, which produces only 200 milliamps of test current. Thisinstrument will conduct a far more thorough test due to the high-test current.Additionally, the full current tester, having an AC output, will measureimpedance and not resistance, essential when testing continuity of cables

having a c.s.a in excess of 35 mm.2. Up to this cross sectional area,impedance and resistance coincide; beyond 35 mm.2 the impedance of thecable exceeds its resistance.

The magnitude of a short circuit current is determined by impedance, notresistance.

Unfortunately, a mains driven tester requires mains power, not alwaysavailable at the myriad of places where tests are to be conducted.

If the installation is being re-tested, care should be taken not to disconnectthe protective conductors of any part of the installation that is energised. If a

fault to earth were to occur whilst disconnected, the protective conductor -which may be exposed to touch - would become live up to the point of break.

For a new installation it may be convenient to use the recorded values ofR1+R2 to determine the P-E loop impedance, the ambient air temperature

should also be recorded at the same time.

Resistance will vary with temperature change and an assessment of likelyresistance under short circuit conditions will have to be made and used inany calculations. Further details will be given in section 11 which isconcerned with P - E loop impedance measurement.


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Fig. 6 The phase - earth loop path (TN-C-S)

11kV 230V

Fuse or circuit breaker

fault

Fault current

General mass of earth

Ze

R1

R2

Total impedance of the phase-earth loop path isgiven by the formula,

Zs = Ze + R1 + R2

Power transformer

R1+R2


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When a P-E fault occurs a circuit is created which is called the phase-earthloop path. The impedance of this circuit will have to be low enough to ensurethat the resulting current will activate the fuse or circuit breaker within 0.4seconds for a socket outlet circuit and 5 seconds for all others. This

disconnection time may have to be reduced in circumstances of special risk.

The components of this circuit are the resistance offered internal to theinstallation and the impedance encountered externally. Or expressedmathematically:

Zs = Ze + R1 + R2

See section 11 for documentation and assessment methods


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Measurement of protective conductor continuity

R1 + R2

Fig.8

Neutral

Earth

Low

All fuses in place or circuitbreakers switched on

Temporary links to be removedbefore switching on

Connection betweenphase and earth


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If the installation is TP&N dont forget to physically disconnect the neutralafter switching off. If a P-N reversal were to occur on any of the sockets itwould not be immediately evident and time wasting would result

Equipotential bonding


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SECTION 6

TESTS OF MAIN AND SUPPLEMENTARY BONDS (Regulation 713-2)

The purpose of equipotential bonding is to ensure that in the event ofa P-E fault the resulting current flow to earth will not produce apotential difference between items of simultaneously accessiblemetalwork. Shocks received due to earth faults will not only produce a

direct injury, but also cause an indirect injury such as that resulting from afall.

The magnitude of a p.d. will depend upon the fault current and the resistanceof the path between the point of fault and the main earthing terminal. It

should be noted that all other simultaneously accessible metalwork wouldalso be connected to the main earthing terminal. It follows, therefore, that ifthe resistance between adjacent metalwork can be reduced to a very lowlevel, fault voltages will be similarly reduced. This principle is followed wheninstalling supplementary bonds - when they are required.

If the fault voltage is Vf, the fault current, If and the resistance along the

earth fault path, R , then,

Vf= Ifx R.

Equipotential bonds may be either supplementary or main. Mainequipotential bonds are mandatory and have the additional function ofmaintaining the main earthing terminal at earth potential where the supply isby means of a TN-C-S (PME) service.

If the incoming main neutral conductor of a TN-C-S service were to becomedisconnected without the main equipotential bonds in place, all exposedconductive parts would become live without the necessity of an earth fault,rather a disconcerting characteristic. More on TN-C-S systems later.

Supplementary bonds are installed locally and are only necessary where the

effects of an electric shock are likely to be much more serious.Supplementary bonding is usually for installations of a special nature thatintroduce an enhanced shock risk. These installations are mostly describedin part 6 of BS 7671.

Supplementary bonding will also be required if the maximum permitted P-Eloop impedance is exceeded, the presumed logic being if an earth fault isextended beyond the normally limits, no voltage of a level likely to produce ashock will be produced for the fault duration.See regulations 413-02-15 413-02-28 471-08-01

T


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Method

It will not be obvious by an instrument test that there is continuity betweenitems of adjacent metalwork. The possibility will exist that the apparentcontinuity is maintained by a common connection to the general mass of

earth. Therefore, a visual inspection for the presence of bonds and a note oftheir c.s.a. is necessary. (See Fig.9)

If the only connection between items of exposed metalwork is that of thegeneral mass of earth, earth fault current will produce a dangerousdifference of potential for the duration of the fault.

Maximum resistance for main or supplementary equipotential bonds is notindicated, the formula shown in regulation 413-24 is intended forsupplementary bonds and may be generally used; this states:

R max


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The use of the above regulation can considerably reduce the number ofinstalled supplementary bonds - even in a bathroom and other areas ofincreased risk.

Fault voltage developed under earth fault conditions.

Fault voltage x - z = Ia x RxyFor example, if Ia = 500A Rxy = 0.3,Then, Vf = 500 x 0.3 =150V

Ohms

extraneous conductive part exposed conductive part

Test for supplementary bonding

MET

Break in the protective conductorwill go unnoticed because of theparallel earth path.

MET

x

y

z

Vf

Fig.9


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Load current flowing in the neutral conductor of a TNC-S system will, at theN-E service terminal, separate into all available paths connected to the

system earth.

This characteristic is illustrated in fig.10. Load current will flow through notonly the main equipotential bonds, but also through protective conductorsconnecting switchgear to earth, assuming intentional or unintentionalsupplementary bonds are installed.

When measuring these currents, care must be taken on interpretation. If thecurrents flowing in the protective conductor are large - in the order of 20A -they are most likely to be neutral load current. These currents will fall to zerowhen all loads are switched off. If, on the other hand, the current flowcontinues after load disconnection, they will be caused by insulationbreakdown.

Neutral current will flow away from the service earth, fault current towards it.Unfortunately, a clamp-meter is unable to detect direction of current.

Main earthingterminal

Extraneous metalwork

Neutral current

Fig.10


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Neutral-earth current produces no heat or arcs if a joint is disconnected andis therefore harmless.

Fig.11


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SECTION 7

TESTS OF RING CIRCUIT CONTINUITY (Regulation 713-3)

Two tests are required to verify the continuity of a ring circuit and are

illustrated in figures 15 and 16.

Method

If more than one ring circuit is connected to a distribution board, all circuitsshall be disconnected before any testing commences - don't forget to labelthem first! Accessing 13A sockets directly with instrument leads is ratherdifficult; therefore, prior to conducting any tests, construct a 13A adapterplug. This accessory will consist of a standard 13A plug to which areconnected three flexible leads terminating into shrouded 4 mm. plugs. See

fig. 12A.

Using the 13A adapter plug, connect the P and N plugs together andconduct continuity tests between the P and N conductors at the distributionboard. The pair between which the lowest and highest resistance lies shouldbe identified and marked (see figure 13). These ends represent the "start"and "finish" of the ring circuit.

Secondly, the possibility will always exist that ring circuit conductors areinadvertently crossed connected to two differing fuses or circuit breakers.Without disconnection, a wiring fault of this nature will not necessarily be

detected by an instrument test, If the installation consists of more that onering circuit, it is absolutely essential that an insulation resistance test beconducted between these circuits in order to ensure no cross connections

Test one

The phase conductor of one leg of the ring is joined to the neutral conductorof the other; a continuity test is conducted between the un-joined ends andthe resistance noted.

If the cable used in the construction has an integral earth, it shouldsubstitute for the neutral and the test be repeated. It should be noted that thecross sectional area of a protective conductor in pvc/pvc cables is less thanthat of live conductors and consequently the resistance would be somewhathigher.

Copper protective conductors enclosed in steel conduit or trunking must beformed into a ring. Measurements of ring circuit continuity where cpc's areenclose in metal conduit or trunking is rather tedious, requiring thedisconnection of each socket from its box and the disconnection of the earthtails.

The test procedure for test 1 is illustrated in fig.12B

T


31/134

Page 31 of 135

Alternatively each of the circuit conductors may be tested separately.

Test two

The un-joined phase and neutral conductors are connected as shown infigure 16. Using the 13A adapter, a resistance measurement is madebetween P, N and E at each socket outlet. The resistance measurements fora given pair of conductors should be the same in each case. If test oneincorporated phase and neutral connected together, the resistancemeasured will be a quarter of that measured in test 1. If the loops weretested separately the resistance expected in test 2 will be half of thatmeasured in test 1.

Resistance values will vary with the physical size of the circuit. The followingtable gives some guidance regarding expected measurements. Any recorded

values widely at variance with those below will possibly indicate looseterminations. Note, values shown indicate that test 1 tested the circuitconductors separately.

It should also be noted that test 2 will not give consistent results whentesting between P and E if the cable used is pvc/pvc.

* 2.5 mm.2 at 25 deg. For 1.5mm.2 multiply by 0.6. For 4.0mm.2 multiply by 1.6

Total cable length Test one* Test two*

30 m. 0.22 ohms 0.11 ohms50 m. 0.37 ohms 0.18 ohms60 m. 0.44 ohms 0.22 ohms

70 m. 0.52 ohms 0.26 ohms80 m. 0.58 ohms 0.29 ohms

Fig.12 B

NN

0.4

NN 0.2

A 13A plug adapted forthe attachment of aninsulation/continuitytester

Fig.12B

Table 1


32/134

Page 32 of 135


33/134

Page 33 of 135

Zeroing out lead resistance priorto taking measurements

Test one

Conductors disconnected in preparationfor the test

Test conducted 0.04

Fig.13

Removal of ring circuitconductors at the distributionboard


34/134

Page 34 of 135

Note should be taken of the current rating of the overcurrent device and the cross sectionalarea of the connected cables that form the ring circuit.

Test one may be conducted at the distribution board if more convenient than testing at asocket.

Fig.14


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Inspection and Testing. A.W.Croucher 2 004 page35

SECTION 8

The purpose of this test is to ensure that all live and protectiveconductors are effectively separated from each other by means of highresistance insulation, hence ensuring safety by confining current to theintended path. Current flow directly to earth, to neutral, or to another

phase, will produce shock and fire risks. A leakage current of 200 mA flowingto earth through a concentrated fault resistance of 1 000 will result in apower dissipation of 40 Watts. Enough power to start a fire, but not enough toblow a fuse or trip a mcb. Think of the temperature attained by a filament

lamp of this power.Insulation resistance can be regarded as consisting of a myriad of individualparallel connected leakage paths. The larger the installation - in accordancewith the rules of parallel circuit theory - the lower will be the insulationresistance. This principle applies to individual circuits and completeinstallations.

Assume that the resistance values indicate theinsulation resistance of individual circuits withrespect to earth.

The overall insulation resistance of thedistribution board will be equal to

2.37 M, less than that of any individual circuit

BS 7671 requirements are to conductan insulation resistance test of thecomplete installation. Sub division willonly be permitted between distributionboards, not between circuits.

Prior to any test commencing, any electronic controls such as lamp dimmersshould be disconnected. The search for their location should be a thoroughone. The application of 500 V D.C. usually means permanent damage andreplacement, which can remove much of your profit margin in less than asecond. Make sure that those lamp dimmers and computers etc. aredisconnected.

If removal of electronic equipment is not a practical possibility it should beshorted out for the duration of the test.

Beware of thinking that a P-E test will not cause damage. If a N-E short hasoccurred, a P-E test will have the same effect as a P-N test if equipment is

left connected, i.e. the test voltage is applied across equipment terminals.

TESTS OF INSULATION RESISTANCE (Regulations 713-4 713-5)

T

m m m5 9 10 m100

Fig.15


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Method

Small single phase installations

As previously stated, the installation must be tested as a whole: sub-divisioninto individual circuits is not permitted unless fault finding.

N - E test

This test should be conducted first in order to ensure that test voltages arenot inadvertently applied to sensitive equipment.

Minimum IR. acceptable - 0.5 megohms with the qualifications stated overleaf

P - N test

All fuses must be inserted, all current-using equipment disconnected, and alllighting switches on. For a 230V installation a test instrument capable of anoutput of 500 V - 1 mA D.C. shall be applied and a minimum insulationresistance of0.5 megohms shall be obtained. It should, however, be notedthat insulation resistance values of less than 2 megohmswould usually signalunacceptably low insulation resistance for one or more of the tested circuits,unless the installation is well above average size. If this is the case eachcircuit should be investigated to determine whether the low insulationresistance is concentrated in a particular circuit. It is strongly recommended

that the minimum insulation resistance measured on any individual circuitshall be not less than 2.0 megohms. This requirement is automatically met ifthe overall resistance is 2 M or more.

Low insulation resistancecan be caused by surfacetracking betweenterminals. The conductive

track will be caused byeither condensation orcarbon contamination.

Fig.16


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If equipment disconnection is not a practical proposition, the controllingswitch may isolate equipment. But be sure to make an on-site assessment ofthe chances of a switch line -neutral fault existing.

The insulation resistance tester shall be positioned - wherever practicable -at the mains end of the distribution circuit or, alternatively, at the distribution

board bus bars.

P - E tests

All fuses are inserted. Current-using apparatus may remain connected,assuming no N-E shorts exist. Controlling switches should be left on. Thepreviously stated results and qualifications are applicable.

Three-phase and neutral installations

Firstly, in the absence of a 4-pole isolator, the neutral shall be disconnectedfrom the incoming supply in order that a test may be conducted betweenneutral and earth - neutral being at zero potential.

The test shall include the distribution circuit (sub main), with the IR testerpositioned at the main distribution board fuse/s.

Test of insulation resistancebetween phase and cpc.

Fig.17


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The following insulation resistance tests shall be conducted as an absoluteminimum,

Connected together Insulation resistance teststo

RYBN ERYB NRY BR Y

The values and qualifications previously stated apply in each case.

If the installation consists of a number of distribution circuits with connecteddistribution boards, each shall be tested separately.

Warning! The direst consequences imaginable can result if the main neutral isnot reconnected into a three-phase distribution board before the supply isrestored. Phase voltages will be destabilised resulting in an excessive voltagebeing applied to one phase causing damage and raising the possibility of fire.

Although not a requirement of the BS 7671, it is advisable to conduct aninsulation resistance test between circuits in order to ensure that no

Phases linked for insulationresistance testing

Measuring insulationresistance

Fig.18

Fig.19

Fig.20

Table 2


39/134


interconnections of phase conductors or sharing of neutrals has taken place -the consequences of which can be extremely serious, preventing individualcircuit isolation.

Precautions

As previously pointed out, electronic equipment will usually be permanentlydamaged if subjected to the test voltage and should therefore bedisconnected before any tests are conducted. Such devices will includedimmer switches, delay timers, power controllers and electronic starters forfluorescent fittings.

Certain classes of semi conductors are likely to be affected by the electricfield created by the test voltage, even if these components are not part of thetest circuit. If such devices are installed, alternative methods of investigationshould be considered (see Section 16).

Electronic components cannot be guaranteed safe from voltages applied

during an insulation resistance test, even when separated from the testvoltage by an isolating transformer. The initial pulse of voltage will bereflected into the electronic equipment, with uncertain results.

If any of the disconnected equipment has a conducting enclosure, which isrequired to be connected to protective conductors, an insulation resistancetest shall be conducted between live conductors and earth. In the absence ofany applicable British Standards, a minimum insulation resistance of 0.5megohms shall be obtained.

Periodic inspection

The test methods and procedures required are identical to that for a newinstallation, with the additional precaution of ensuring that the installation istruly dead.

Ensure that all means of isolation are identified and in good working orderand no circuits are fed from two sources. Dont forget to use your voltagetester.


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Tests of Insulation resistance 1

Test of Insulation Resistance between live conductors and earth

neutral

earth

M



Connection between

phase and earth.

Note the test can beconducted at anypoint in theinstallation that isconvenient

Insulation resistance tester

Fig.21


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Tests of insulation resistance 2.

Test of Insulation Resistance between live conductors and neutral

neutral

earth

M




Fig.22


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Test of insulation resistance 3

Test of Insulation Resistance between phases

neutral

earth

M


Temporary link to be removedbefore switching on

Connection betweenphase and earth.

Note the test can beconducted at any

point in theinstallation that isconvenient


Fig.23


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0.02M

NN

If bridging loads are connected is not possible to locate a fault by switching off circuit breakers.This principle is illustrated above. A N-E fault exists on circuit #1. All circuit breakers except that of

circuit #5 is switched off; the fault is ostensibly located on this circuit, indicated as a P-E fault.To find the location of an insulation resistance fault all of the neutral conductors must be removedfrom the terminal block

Fig.24


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SECTION 9

The purpose of polarity testing is to ensure that all single pole devices suchas fuses, single pole switches, thermostats etc., are connected in the phaseconductor only and the three phases are correctly identified throughout the

Periodic testing

Tests shall be made using the described method to verify that:

(i) All single pole devices are connected in the phase conductor only.(ii) All multi-pole devices are connected to the identified live conductors, e.g.

no phases are crossed.

If it can be established that no alterations or additions have been made since

the last test and inspection of the installation, the above tests need onlyinclude 10% of the any installed points excluding socket outlet circuits.

All socket outlets must be tested for polarity.

If any reverses of polarity are detected, all points on that particular circuitmust be tested for correctness of polarity and the sample testing on theremaining circuits connected to the distribution board increased in frequencyto 25%. Any further cases of reversed polarity found in the 25% sample willrequire a 100% test of the completed installation.

A simple, cheap, but effective polarity-testing tool is illustrated in figure 30.

Consisting of few components, it can easily be made in less than an hour.To use the device - having firstly ensured that the installation is dead - a 9Vbattery is attached to the phase (+ ve) and neutral (- ve) bars of thedistribution board. All switches are closed.

The p.d. will be distributed around the installation.

Attending to the points to be tested, the red terminal is attached to the phasetermination and the black terminal to the neutral. If polarity is correct, thegreen LED will be illuminated; if not, the red LED will be lit.

It is important, when testing for correctness of polarity, that the neutral andearth connections are physically disconnected if a discrimination is to be

made between these conductors. If the installation is single-phase, it ismerely necessary to open the main isolator. However, if the installation isthree-phase, controlled by a 3-pole isolator, the neutral is unswitched and willhave to be unbolted from its terminal block.

It is important to note that polarity cannot be verified live using a voltagedetector of the type illustrated in Fig.26. If an N-E reversal has taken place itwill go undetected on a live test, resulting in load current flowing to earth. Theconsequences of this will be the opening of an RCD or the continual flow ofearth current, with all the attendant hazards this event will possibly produce.

TESTS OF POLARITY (Regulation 713-13)


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Polarity testers

Polarity testers of a type that rely on the measurement of voltage can falsely indicate correct polarity

Polarity tester

9V battery

neutral

R

G

light emitting diodes

A C AC

R B

G R

Polarity tester

Tests of polarity

E P

N

polarity tester

all lamps lit

A proprietary polarity testerfor use on live circuits.Testers of this type do notmake a distinction between

neutral and earth.

Fig.25

Fig.26


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SECTION 10

f the supply system is TT, that is, a system without a mains distribution ofprotective conductors - an earth electrode will have to be installed locally.I

No electrode can be in contact with the general mass of earth without anintervening resistive barrier. This resistance is called earth electroderesistance, which will be confined, to the immediate vicinity of the electrode.

The structure of electrode resistance is a complicated one, modelling series-connected concentric resistive hemispheres, each shell decreasing in

resistance with distance from the electrode.

Possibly the simplest concept of earth electrode resistance is that of twoconductors - one of which is earth - connected by a chain of resistors ofsteadily decreasing value. Starting at the low resistance end, investigationsoutwards would reveal increasing resistance which is incrementally reduceduntil the point is reached when no further significant increase is detectable(see Fig.31).Measurable increase in resistance only occurs close to the electrode,occupying a finite area, known as the earth electrode resistance area.

In order to ensure that earth fault current is of sufficient magnitude to operateprotective devices, earth electrode resistance must be measured andassessed.

A simple earth electrode installation is unlikely to result in a resistance lowenough to meet the earth loop impedance requirements of regulation413-02-08. Therefore, the installation of a residual current device is essential.

Method

Using an instrument dedicated to the testing of earth electrodes, connect thecircuit as shown in figure 28.

It is important that the electrode under test (E) and the current electrode (C)are sufficiently spaced as to be outside each other's resistance areas.

In general, the distance between E and C should be at least 10 times thedepth of the electrode under test. For example if the electrode were 3 m long,the spacing would have to be at least 30 m.

TESTS OF EARTH ELECTRODE RESISTANCE (Regulation 713-15)


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Connecting the instrument as shown, three readings would be taken:

(i) with the potential spike midway between E and P;

(ii) with the potential spike moved 10% of d closer to E;(iii) with the potential spike moved 10% of d closer to C;

The average of the three readings is taken and the maximum deviation fromthe average is expressed as a percentage of the average.

The estimated accuracy of this method is 1.2 x percentage deviation.Accuracy in this context means the possibility of + or - 5% error.

It all sounds very complicated but the following example will show that theprocedure is relatively simple.

Example: test 1 60 test 2 56 test 3 62 Average = (60+56+62) = 59.33

3

Maximum deviation = 59.33 - 56 = 3.33 % deviation = (3.33/59.33) x 100

= 5.6%

Accuracy = 1.2 x 5.6 = 6.74 % Accuracies exceeding 5% are not

acceptable and possibly indicate that the C and E electrodes are too close.

The regulation requirements concerning permitted resistance must be linkedwith those applicable to the installed RCD, i.e.

Zs


48/134


Periodic inspection

If any doubt exists regarding the suitability of an installed earth electrode, itshall be tested using the method and assessment techniques described.

The guidance notes and the BS 7671 suggest that earth electrode resistancemay be measured with an earth loop impedance tester. Due to the likelyexternal nature of the electrode and the 230 V test current, extreme cautionwill be necessary. If the electrode is open circuit or very high resistance withrespect to earth, the electrode and its connecting cable will be driven up to

mains voltage for the duration of the test. This method is likely to contravenethe requirement of the Electricity at Work Regulations regarding live working.

Tests of earth electrode resistance

It may not be possible to measure Electrode resistance in the way previously

suggested due to inaccessibility of the soil. If this is the case a simplifiedmeasurement may be made.

An earth electrode andconnected earthingconductor, installed in anaccess pit.

Earth electrode resistancetester

Electrode undertest

Potential electrode

Current electrode

23

Resistance

area

Fig.27

Fig.28


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The potential terminal is not used; the current terminal is connected to asuitable reference electrode, such as a water pipe etc. The measurementmade will be that of the combined electrode resistance. Because itsimpossible to divide up the measurement, the reading will have to be

considered that of the electrode under test.

The simplified measurement is therefore in error on the safe side.

33

Earth electrode resistancetester

Electrode undertest

Simplified measurement ofearth electrode resistance

Fig.29


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Soil resistivity

33Mains socket

P-E loop tester

Main equipotential

bonds disconnected

Electrode under test

Measurement of earthelectrode resistanceusing a mains poweredphase earth loopimpedance tester

Fig.30


51/134


If an earth electrode is to be installed, it will be advantageous for theelectrode to be sited in a location where the soil resistance is at a minimum.

If the test engineer possesses a Null Balance tester - of the type illustrated -soil resistivity may be measured.

C1 P1 P2 C2

+-

scale multiplier

decade resistors

n x 100 n x 10 n x 1

press

The null balance earth tester

The null-balance earth tester

This instrument will enable measurement of earth electrode resistance - soilresistivity and protective conductor continuity.

To use, the scale multiplier is selected e.g. x 1, x 0.1 etc. The decaderesistors are then adjusted until the centre zero galvanometer is balanced.The indicated resistance is that displayed by the decade resistors multipliedby the scale multiplier.

a

Measurement of soil resistivity

Fig.31


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The resistivity of the soil is calculated by the formula

= 2 Ra

Where R is the resistance measured:

a = the distance in centimetres between electrodes.

= the soil resistivity in ohms-cm.

The depth at which the electrodes are to be buried should be 1/20 of that ofdistance a.

The resistivity is measured at depth a.

For example, if the resistance measured was 100and the distance betweenelectrodes 4m. then using the above formula, the soil resistivity will be,

251 327 /cm.

Fig.32


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SECTION 11

TESTS OF EARTH FAULT LOOP IMPEDANCE(Regulation 713-10)

If the dangers of earth fault currents are to be minimised, fault duration must

be carefully controlled by means of the circuit protective device, fuse, circuitbreaker, or RCD. This control, however, will only be effective if theimpedance of the earth fault circuit is sufficiently low to generate the requiredoperating current.

Earth fault currents are usually driven at 230V, which is, of course, a constantvalue. It follows therefore that variations in earth fault current are determinedonly by variations in P-E loop impedance.

Ip = Uo/Zs Where Ip = prospective short circuit currentUo = nominal voltage to earth

Zs = phase-earth-loop impedance

The maximum fault duration for a 230 V socket outlet circuit likely to supplyhand held equipment is 0.4 seconds (Regulation 413 - 02 - 08). For maximumdisconnection times for voltages other than 230 V, see table 41A.

If the installation is a temporary supply for construction site use, thedisconnection times are given in table 604A, and for agricultural andhorticultural installations, table 605A is applicable.

Tables 41B1, 41B2 and 41D indicate maximum operational values of P - E

loop impedance permitted for a particular overcurrent device, connected load,and current rating. If these values are not exceeded, the fault current will besufficient to produce the required disconnection time. It should be noted thatvalues given in these tables represent operational conditions not the limitsmeasured on a loop impedance tester. For tabulated limitations of measuredvalues see GN 3 tables 2B to 2D.

Complicated - but the flow chart included in this section will makeassessment relatively simple.The maximum disconnection time for circuits containing only fixed equipment

is 5 seconds (Regulation 413-9 [i]).

As previously stated, if a distribution board has connected a mixture of fixedequipment and socket outlets, either:

(i) The impedance of the protective conductor associated with thedistribution circuit shall not exceed that indicated in Table 41C, or,(ii) Supplementary bonding shall be installed at the distribution board thatmust connect to the same extraneous metalwork that is connected to themain equipotential bonds. (See regulation 413-02-13)

For example, if the largest fuse in such a distribution board were to be ratedat 40A BS 88, it could be seen by reference to table 41C that the maximum


54/134


permitted protective conductor resistance from the DB earth terminal to themain earthing terminal is not to exceed 0.29?

To achieve a disconnection time exceeding 0.4 seconds, but not exceeding 5seconds, loop impedance values for a particular fuse or circuit breaker of a

stated current rating shall not exceed those indicated in tables 41B2 or 41Das applicable.

Method

The measurement of P-E loop impedance is essentially conducted on a livecircuit but Regulation 13 of the Electricity at Work Regulations 1989 requiresthat circuits shall be made dead before any work is carried out on or near anenergised conductor unless it is unreasonable for it to be isolated.

This regulation is absolute, which means it must be conformed with. Cost orinconvenience is not to be a paramount consideration. The implication forloop impedance testing is that it shall be organised is such a way as tominimise the time spent near live conductors.

Therefore, it is to be strongly recommended for an initial verification that livetesting is confined to the measurement of that part of P - E loop impedanceup stream to the distribution board (Ze) only. For a periodic test it is usually

not possible to follow the above procedure due to difficulties in disconnectionof supply.

As previously stated, cables having a cross sectional area not exceeding 35mm.2 have negligible reactance and the previously measured R1+ R2 values

will also indicate the internal impedance of the earth fault circuit; therefore:

Zs = Ze + R1 + R2

where Zs is the total P-E loop impedance of the circuit.

The above formula is an approximation of total impedance because itassumes that all of the components are in phase with each other. In reality,

that is far from true. Phasor arithmetic decrees that 1 + 1 will only equal 2 if inphase; otherwise 1 + 1 will be equal to less than 2. Therefore an error existsin the formula, acting on the safe side.

Cables larger than 35 mm.2 have an impedance in excess of their resistanceand if the above formula is to be used, then ideally impedance Z1 + Z2 would

be measured, requiring the application of an instrument that has an a.c 50Hzoutput.

Prior to the measurement of Ze, the main equipotential bonds shall be

removed and all downstream circuits isolated. This measure will obviate

reliance on equipotential bonding for lowering Ze. If the installation is in use

the main equipotential bonds mustnot be disconnected.


55/134


Before attaching the P - E loop impedance tester to the installation it shouldbe isolated. For safety reasons the instrument should not be directly appliedto live parts if at all avoidable. After attachment, the supply should beenergised and the test conducted.

Measurement of Ze is a potentially dangerous business. The test is quite

often conducted in a confined space, inadequately lit, with the additionalhazard of 400V exposed to touch.

Measurement of Ze must only be undertaken by confident and competent

electricians.

The loop impedance tester should be of a modern design - older instrumentsare usually of low safety standards. Instrument leads should have substantial

insulation and should be protected by in line H.B.C. fuses.

To take into account an increase in temperature and hence resistance underfull load conditions, as a rough rule of thumb, the recorded values should notexceed 0.75 of the relevant value indicated in BS 7671 tables 41B and 41D,which are based on a conductor temperature of 70 deg.

A more precise assessment may be made of conductor resistance (R1+R2)by consulting the table below, which sets out multiplying factors to correct forresistance increase due to the temperature rise occurring on full load current.

Test ambient temperature(deg.C) Correction factor

5 1.0610 1.0415 1.0220 1.00Measured resistance is multiplied by the above factor.The resulting value will then be multiplied by the factor given below..Insulation type Correction factor PVC 1.20

85 deg. rubber 1.531.42

90 deg. thermosetting plastic 1.601.48

* where the protective conductor is not part of a composite cable.The method advised to determine Zs, suggests that a substantial part of the

externalP-E loop impedance could be contributed by the impedance of thedistribution circuit but the comparatively large cross sectional and surface

area of this cable will ensure that the temperature and hence resistance rise

Table 3

Table 4


56/134


will be of little consequence. Hence the correction factors should be appliedto the measured value of R1 + R2 alone.

Example: A test of p-e loop impedance is measured and found to be 0.35at10 deg. If R1 + R2 + 0.25what will be the amended resistance?

Solution: Zs = Ze + (R1 + R2)1.2 x 1.04 Zs = 0.1 + 0.25 x 1.2 x 1.06 =0.418

Except where the supply system is TT, the impedance of the public mainsdistribution system is negligible and contributes little to Zs in percentage

terms. This reasoning applies particularly to built up areas of high demanddensity. In rural areas with overhead supplies the external loop impedancewill be of a considerably higher proportion of Zs.

It should also be noted that the P-E loop impedance of an installation wouldnot necessarily be of a fixed value for the life of the installation. Changes indemand will be reflected by changes in the mains structure, which couldresult in the installation or removal of power transformers with aconsequential change in loop impedance. Hence the importance of periodictesting.

See section 23 for more information about the structure of the mainsdistribution system.

For an installation fitted with an RCD, different considerations apply whenassessing Zs values. Because the device is sensitive to very small earth

leakage currents it will usuallyoperate faster than any connected fuses orcircuit breakers. If the earth leakage current is sufficiently high (usually in therange 30 - 500 mA), it will open the offending circuit in less than 20milliseconds. This means that other overcurrent devices have no earthleakage role and therefore no reference need be made to tables 41B and41D for circuits protected by an RCD.

An evaluation of the phase earth loop impedance when an RCD is installed isdetermined by an application of the formula below:

Zs


57/134


installation having its own characteristics and must be tested separatelyusing the procedure described above and illustrated in figure 41.

(See regulation 541-01-02)

The measurement of P-E loop impedance is an inexact science. Theoperational values of P-E loop impedance will be considerably lower thanthose calculated or measured, due to the parallel paths provided by theequipotential bonding - all of which will carry earth fault current.Temperature rises and hence multiplying factors under earth fault conditionsassume a fully loaded cable prior to the earth fault occurring, operating at a

temperature of 70o. While this condition is not impossible, it is unlikely. Aconsequence of this will almost certainly be loop impedance lower, and earthfault current larger than that predicted. Protective conductor current will belower than that in the phase conductor due to parallel earth paths such aswater pipes etc., producing a lower temperature rise.

BS 7671 guidance notes also make an implied assumption that the faultycircuit has a protective conductor formed by a cable core. If a cable trunkingor a conduit supplements the protective conductor, the large surface areaand low resistance will ensure that no appreciable temperature rise onprotective conductors takes place

. Tests of external P-E loop impedance (Ze)

Warning! Dont try this athome.

Tests of Ze should bemade as near to theservice as possible.

Main earthing terminal400A TN-C-Sservice

Fig.33

Fig.34

Live !!!


58/134


Dont forget a professional organisation will produce professional reports.Take note of the example given later in this chapter - use it as a model forreports of your own design. And remember, a loop impedance test will usuallytrip an RCD.

Phase - Earth loop impedance requirements of BS 7671

Set out below are some of the maximum permitted operational phase-earthloop impedance values and the corresponding currents at 230V required todisconnect a circuit with a phase-earth fault. These values of loop impedanceare abstracted from tables 41B1, 41B2 and 41D and refer to circuits fullyloaded (70 deg.) at an ambient air temperature of 20 deg.

For example if a ring circuit is protected against overcurrent by a 32A fusemanufactured to BS 88, the maximum P-E loop impedance and minimumcurrent would be 1.09and 211A respectively.

BS 88 fuses

Rating 6 10 16 20 25 32 40 50

Zs (ohms) socket outletcircuits

8.89 5.33 2.82 1.85 1.50 1.09 0.86 0.63

Zs (ohms) fixed equipmentcircuits

14.1 7.74 4.36 3.04 2.40 1.92 1.41 1.09

Rating 6 10 16 20 25 32 40 50

I2 sockets - Amps 25.8 A 43.1 A 81.5 A 124 A 153 A 211 A 267 A 365 AI2 fixed equipment - Amps 16.3A 29.7A 52.7A 75.6A 95.8A 119A 163A 211A

Circuit Breakers

Type 1 current rating 6A 10A 16A 25A 32A 40A 50A 63Zs Ohms 10 6 3.75 3 2.4 1.88 1.5 0.95

Current for instantaneousdisconnection

23A 38.3A 61.3A 76.6A 95.8A 122A 153A

Type 2

Zs Ohms 5.71 3.43 2.14 1.37 1.07 0.86 0.69 0.54


40.3A 67A 107A 168A 215A 267A 333A

Type BZs Ohms 8.0 4.80 3.0 1.92 1.50 1.20 0.96 0.76


28.8A 48A 76.7A 119A 153A 191A 239A

Table 5


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Type 3Zs Ohms 4.00 2.40 1.50 0.96 0.75 0.60 0.48 0.38


57.5A 96A 153A 240A 306A 383A 479A

Type D

Zs Ohms 2.00 1.20 0.75 0.48 0.38 0.30 0.24 0.19


115A 191A 306A 479A 605A 766A 958A

Alternately, p-e loop impedance values may be obtained from GN3 and usedwithout modification.


60/134


The measurement of P-E loop impedance on a large installation.

TEST

earth/neutral phase

l oo p te st P SC te st

20

200

2000 2-60A

20-600A

0.2-20kA

press and

release

DIGITAL PSC/LOOP TESTER

P-E

P-N

N-P TEST

earth/neutral phase

l oop te st P SC t es t

20

200

2000 2-60A

20-600A

0.2-20kA

press and

release

DIGITAL PSC/LOOP TESTER

P-E

P-N

N-P

E

Zsm Ze

0.970.56

Protective conductor continuitycan best be determined bysubtracting Ze from Zsm

Fig.35

Fig.36


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If the measured value of phase-earth loop impedance is too high whenreferred to tables GN3 2a to 2D, you may be in the unfortunate position of notonly being responsible for reporting the situation to your client, but also beingrequired to suggest a solution to the problem.

If the circuit in question supplies socket outlets and fuses provide earthleakage protection, the disconnection time may be raised to five seconds ifcertain conditions are met. A five second disconnection will enable highervalues of Zs to be tolerated.

For example, the measured value of P-E loop impedance for a 30A ringcircuit is found to be 1.6, the protecting device is a 30A rewirable fuse,maximum Zs is 1.2and the resistance of the circuit protective conductor fromthe furthest point to the main earthing terminal is 0.5.

Reference to table 2D reveals that the maximum permitted value of Zs is0.91 excessive. However regulation 413-02-12 allows an extension of thedisconnection time to five seconds if the protective conductor resistance doesnot exceed that given in table 41C, which in this case will be 0.58 , thecriteria, has been met and therefore reference may be made to table 2Aii forthe limiting value of Zs - which is 2.21.

It follows that a test result that is apparently outside acceptable limits

becomes acceptable, looking at the problem from a different angle.

The circuit would also meet regulation requirements if an RCD were to befitted and/or additional supplementary bonding were to be installed. (Seeregulation 413-02-15).

Periodic inspection and testing

Measuring p-e loopimpedance at a floor socket

Fig.37


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It may not be possible for the installation to be made dead for testingpurposes; therefore live testing will have to take place. Optional testsmethods are illustrated.

A composite measurement ofp-e loop impedance on aluminaire. For this test anextension lead may be

necessary.

Fig.38


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SECTION 12

TESTS OF RESIDUAL CURRENT DEVICE EFFECTIVENESS(Regulation 713-12)

n RCD will detect an imbalance in either the three line and neutral currents in a TP&Ncircuit or P - N current in a single phase circuit. An imbalance in this context means thatthe sum of the circuit current does no equal zero. This situation will be interpreted by theRCD as an earth fault, between either a neutral or a phase conductor and earth. These

devices can achieve great sensitivity and can give a measure of protection against directcontact, although it should be emphasised that the protection referred to is against electrocutionnot electric shock.

An RCD is not an overload-detecting device nor will it operate under short circuit conditionsbetween live conductors.

The fault current causing the RCD to trip is symbolised as:

In (I delta n)

Residual current devices can be categorised as either:

(i) (Non-delayed) RCDs to BS 4293 (In 30 mA )(iii) (Non-delayed) RCDs to BS 4293 (In 30 mA

Selecting a suitable socket outlet, the circuit is energised and an RCD tester is connected: No

equipment shall be connected to the circuit under test.The test procedure is as follows,

(i) Selecting In/2, and applying the test current for 2 seconds, the deviceshould not trip.

(ii) Selecting In and applying the test current, the device should trip in a timenot exceeding 200 mS. if an RCBO - 300 mS. If In


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BS 4293

When a test current equal to 100% of In is impressed on the device, it should trip within therange 50% t + 200 ms to 100% t + 200 ms. where t is the stated time delay in seconds. For

example, if the RCD was declared to operate at 2 seconds with a residual operating current of100 mA - with a test current of 100 mA it should trip within the range 2 x 0.5 + 0.2 = 1.2 secondsand 2 + 0.2 = 2.2 seconds.

BS EN 61008

When a test current equal to 100% of In is impressed on the device, it should trip within therange 130 to 500 mS.

Most RCD testers will provide means of injecting the test current on either the commencement ofthe positive or negative half cycle. Both options should be chosen and that producing the

longest delay time logged.

Additionally, the test button should be operated to verify the mechanical efficacy of the device.It should be noted that the earthing installation may be totally defective and the device will trip.The operation of the trip button merely displays the integrity of the mechanical and magneticelements.

It is perfectly possible for all earthing to be totally absent and operation of the test button willcause the circuit breaker to trip. Therefore, although a test of the circuit breaker is important, it isnot a substitute for an instrument test.

With the increasing intrusion of electronic controls into power equipment such as thyristors, d.c.-or a.c. with a d.c. component may leak to earth in the event of a fault. The dangers of thesecurrents are no less real than that produced by a.c.

If the possibility of a d.c. fault exists on an installation the RCD should be tested for a d.c.response. Standard RCDs will either not trip, or trip only with a larger current than that rated.When purchasing an RCD tester, ensure that it has a d.c. test facility.


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RCD ControlTest of an RCD

Fig.39

Fig.40


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SECTION 13

OVERCURRENT SURVEY

It is absolutely vital that the current flow in all parts of the installation - cables and equipment -does not exceed design levels, if this is not the case, obvious hazards can result.

Any survey of an electrical installation that has been in use some time should includea survey of current demand and efficacy of overcurrent protection.

With the passage of time, demand may bear little relationship to that intended by the designer.Loads grow, and there is great temptation to accommodate those loads without a reinforcement

of the system. Cables, fuses, circuit breakers, services etc. may all be dangerously overloadedto an extent that insulation damage is a distinct possibility.

The nature of this survey is illustrated in the report form shown overleaf.

Firstly, the tabulated current ratings of all overcurrent devices and the cross sectional areas ofall connected cables are noted. Tabulated current ratings are those presented in table form inappendix 4 BS 7671

Choosing a time when all circuits are at maximum demand, current flow is measured - using aclamp meter - at each final and distribution circuit.

Load P N

To solenoid

Search coil

Fig.41


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Then the current ratings of the cables are compared with that of measured demand and thecurrent ratings of the connected fuses or circuit breakers. The current rating of a cable must benot less than that of the protective device and must not be exceed by the maximum load current.

When making an assessment of cable current ratings, determine if any correction factors apply.

Correction factors modify tabulated rating and will exist for:

AMBIENT TEMPERATURE

Cables are rated at an ambient temperature of

week 11+ inspection and testing of electrical installations

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