paper - electrical installations 2

Wolfgang Hofheinz serves on the National Committees of the

German Commission for Electrical, Electronic & Information Technologiesof the Deutsches Institut für Normung(DIN) and Association for Electrical,Electronic & Information Technologies(VDE). He has been with the BenderCompany since 1975, becoming thecompany’s Executive Vice-Presidentin 1995. He has been an invited

speaker at many national andinternational conventions and is

the author of various publicationsand books on the subject ofelectrical safety. These include

Elektrische Sicherheit in MedizinischGenutzten Räumen (2001), Protective

Measures with Insulation Monitoring(2000), Schutztechnik mit

Isolat ions -Überwachung (2000)and Fehlerstromüberwachung in

Elektrischen Anlage (1998). Englishtranslations are pending. He is

the holder of two patents in thefield of insulation monitoring of

unearthed direct current systems:Swiss Patent no. 613313 (1979)

and German Patent no. DE2546997 C2 (1984). Mr Hofheinz

studied electrical engineering at theUniversity in Giessen, specialising

in communications engineering andholds a Dipl.-Ing.

a report by

Wo l f g a n g H o f h e i n z

Member, National Committees of the German Commission for Electrical,

Electronic & Information Technologies of DIN and VDE

I n t r o d u c t i o n

Nowhere in daily life is there a more indispensablerelationship between safety and life than in themedical profession. The multitude of accidents andsubsequent personal and material damage demonstratethat safety cannot always be guaranteed by humanintervention. This is especially the case in the hospitalenvironment, where the application of electricaldevices in diagnostic and therapy, for example, meansexposure to increased electrical hazards, e.g. throughfaults in the current supply or defective devices.

Ha z a r d s i n M ed i c a l S i t e s

The safety degree is influenced by many factors thatcan be categorised essentially into the following threehazard groups:

• hazards through faulty devices;

• hazards through faulty application; and

• hazards through procedural causes.

A risk always remains through unexpected orunavoidable faults but, with consequent safetyresponsibility, the risk can be reduced significantly.

Legislation has documented its responsibility for careand safety when installing and applying electricaldevices and electrical installations through many regu-lations and standards. This means that the constitutionalright of the patient in terms of physical and mentalprotection in medical locations is safeguarded.

The responsibility of the electronic industry in thisarea lies in the intricate knowledge of known and as-yet-unknown single fault conditions of the totalelectrical installation. The foremost aims of all safetymeasures taken should therefore be:

• patient safety – to protect the patient from beingunsettled, damaged or subjected to unreasonableand repeated therapy due to a single fault condition;

• simple but effective operation – to enable the

medical staff, who often are electro-technicallyinexperienced, to simply and effectively deal witha single fault condition and, henceforth, preventany additional workload; and

• clear and comprehensive documentation – tosupply clearly structured information on theproject and application of electrical installation inmedical locations to designers, architects andoperators, so that the occurrence of a single faultcondition is prevented to the best of theirtechnical know-how.

The reliability of power and reduction of currentleakage is also an essential safety aspect, which can beresponsible for the following:

• abnormal or impaired responses of the patient andorgans;

• body functions can be temporarily or permanentlytaken over by machines;

• fire and explosion hazards; and

• electric and magnetic interference.

I n t e r n a t i o n a l E l e c t r o t e c h n i c a lC omm i s s i o n R e g u l a t i o n 6 2 A

In view of the application of a multitude of electro-technical equipment in use in the medicalenvironment, measures must be taken to preventelectrical accidents. This was the driving idea for theInternational Electrotechnical Commission (IEC)Regulation 62A (IEC 62A), Common Aspects ofElectrical Equipment used in Medical Practice, publishedas a draft in 1976 by IEC Technical Committee(TC) 62A. Besides numerous indications aboutgeneral installation, this draft contained two essentialarguments for the installation of ungrounded powersupply systems (information technology (IT)systems) in hospitals:

• reliability of power supply; and

• reduction of leakage currents.

E lec t r i ca l Sa fe ty – Ungrounded Power Supp ly Sys tems in Med ica l S i te s

B U S I N E S S B R I E F I N G : H O S P I T A L E N G I N E E R I N G & F A C I L I T I E S M A N A G E M E N T 2 0 0 3

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Both aspects are still valid today; however, theemphasis of the measures taken in this respect varyfrom country to country. The application andmonitoring of ungrounded power supply systems inhospitals are reflected in the different nationalstandards. The following are still valid basic statementsaccording to IEC TC 62A.

• A patient may not be able to respond normally ina hazardous event. This may be due to illness,unconsciousness or anaesthesia or the patient maybe connected to electrical apparatus fortherapeutic reasons.

• The natural electrical resistance of the skinnormally provides important protection againstelectrical current. With some treatments, thisprotection may be short-circuited, e.g. throughthe insertion of a catheter into the patient’s bodyor by treating the skin when an electrode has tobe placed on a patient’s body. The human heartis more sensitive to electrical current than otherparts of the body. Electrical current couldinhibit natural heart activity and could lead toheart failure.

• Electro-medical equipment could be used topartly or permanently support or substitute vitalbodily functions. A fault in the device or a powerfailure could be life-threatening.

• A mixture of flammable anaesthetics, disinfectantor cleaning substances with air or with oxygenand nitrous oxide may present fire/explosionhazards.

• Electrical and magnetic interference, e.g. from thepower supply, could disturb the reproduction ofaction potentials, such as electrocardiogram orelectroencephalogram equipment.

The use of an ungrounded power supply system maybe desirable for the following reasons.

• It improves the reliability of power supply in areaswhere power failure may cause safety hazards forpatient and user.

• It reduces the leakage currents of devices to a lowvalue, thus reducing the touch voltage of theprotective conductor through which the leakagecurrent may flow.

• It reduces the leakage currents of devices to a lowvalue, if approximately balanced to earth.

It is necessary to maintain the impedance of thesystem to earth as high as possible. This is achievedby restricting the following:

• the physical dimensions of the medical isolatingtransformer;

• the system supplied by this transformer;

• the number of medical electrical devicesconnected to the system; and

• through high internal impedance to earth of theinsulation monitoring device (IMD) connected tosuch a circuit.

S t r u c t u r e

The operation of the ungrounded power supplysystem with isolation monitoring and alarm forms thecentre of the power supply of Group 2 locations (seeFigure 1). The base for the IT system is an insulatingpower source against ground. An isolatingtransformer is added, with a nominal power range setto 3.15 kilovolt-amperes (kVA) to 8kVA, e.g.according to the Deutsches Institut für Normung(DIN) standard VDE 0107: 1994-10. More set valuesare: maximal secondary voltage alternating current(AC) 230V, maximal inrush current under no loadconditions 8xIn, short-circuit voltage uo and no-loadcurrent io ≤ 3%.

The ungrounded power supply system also bears thefollowing advantages:

• it has no direct connection between liveconductors and earth;

• in the case of an insulation fault, only a smallcapacitive current will flow;

• a fuse is not triggered;

• the power supply is maintained; and

<U

PE

PA

>θ

MK

• The ungrounded power supply has no direct connection between live conductors and earth

• In case of an insulation fault, only a very small capacitive current will flow

• A fuse is not triggered

• Power supply is maintained

• Insulation Monitoring Device (IMD) signals alarms

Group 2

Figure 1: Structure of an Ungrounded Power Supply System


• IMD signals alarms.

The essential advantage of an IT system is alreadyevident in the event of a single fault condition.Only a small current IF flows, the value of which is determined through the system leakagecapacitance CE. This does not trigger a fuse, thesupply voltage is maintained and the operation ofthe installation preserved.

Downtime safety is not the only argument for the use of a medical IT system. It also reduces the leakage currents of devices to a low value, if the medical IT system is approximately balanced to ground.

Mon i t o r i n g

Increasing the availability of supply – a substantialaspect – is guaranteed with the IT system. Whenoperating an IT system at single fault condition,attention has to be given to the fact that the originalungrounded (IT) system has turned into a groundedsystem. An additional fault could then lead totriggering the short-circuit protection therebyswitching off the system. To prevent thisoccurrence, the insulation resistance is permanentlymonitored through an IMD. Internationally, thefollowing three measuring principles have provedthemselves here:

• resistance monitoring with an IMD;

• monitoring of impedance through a line isolationmonitor (LIM); and

• monitoring of load and temperature.

R e s i s t a n c e M o n i t o r i n g w i t h a n I M D

The task of the IMD is to monitor the insulationresistance between the active phase conductors andearth and to report a certain drop below a set value(<50kOhm (kΩ). The IMD is connected betweenthe active phase conductors and earth andsuperimposes a measuring voltage to the system. Atthe occurrence of an insulation fault, the circuitbetween the system and earth closes over theinsulation fault RF, so that a measuring current Im

occurs that is proportional to the insulation fault. Thismeasuring current causes a voltage drop at themeasuring resistor Rm, which is electronicallyevaluated. If the voltage drop exceeds a set value,which is equal to falling below a set insulationresistance, an alarm is triggered via light-emittingdiode (LED) alarms and alarm contacts. The inherentsystem leakage CE capacitance is only charged tomeasuring voltage and, after a short transient reaction,does not affect the measurement (see Figure 2).

The remote signalling and testing combinationrequired for the visual and audible alarms in medicallocations must be mounted in such a way that thesignal can be noticed by the staff on duty.

M o n i t o r i n g o f I m p e d a n c e

T h r o u g h a n L I M

A device known as an LIM (an IMD to measureimpedance) should always be included in anungrounded power supply system. The LIMmonitors the impedance of the conductors to earth.It is installed into an isolated power distributionsystem, as illustrated in Figure 3. It is designed in sucha way that a green LED alarm, visible conspicuouslyto the staff in locations for medical use, lights upwhen the system has reached a sufficient impedanceto earth. The red LED alarm lights up and sounds anaudible warning signal as soon as the prospective faultcurrent (consisting of resis-tive and capacitive leakagecurrents) of an ungrounded power supply systemreaches the threshold of 5mA (2mA in Canada).Means are provided for resetting the audible warningsignal while leaving the read alarm LED activated. Adistinctive visual signal indicates that the audiblealarm has been reset. When the fault is eliminatedand the green LED alarm lights up again, the audiblealarm is automatically reset.

• IMD connected between the phase conductor

and earth

• Measuring voltage Um generated by G is

superimposed on the system via the coupling Ri,

the measuring resistance Rm and the low pass

• An insulation fault RF between system and earth

closes the measuring circuit, measuring current

Im is flowing

• Im causes a voltage drop Um proportional

to the insulation fault RF at the measuring

resistance Rm• Display indicates the ohmic insulation resistance

With an IMD

G

RA A-ISOMETER

RIRm

Ri

RF

®

Low pass

With a LineIsolation Monitor (LIM)

• LIM monitors impedance of the ungrounded power supply

THC

C

• Display indicates the total hazard current in mA

e1RF1 RF2

ImACe2

UN

Figure 2: Monitoring of an Ungrounded Power Supply System with an IMD

Figure 3: Monitoring of an Ungrounded Power Supply System with an LIM

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M o n i t o r i n g o f L o a d a n d T e m p e r a t u r e

To avoid overloading the isolating transformer, arespective installation must be designated in order toprotect the transformer and supply conductorsbetween primary and secondary terminals and thedistribution bus from overloading or overheating.When the rated current or the temperature is overrange, an acoustic or optical alarm is released.

Wor l dw i d e D e v e l o pmen t

The application of ungrounded power supplysystems in hospitals began between 1920 and 1930 inthe US. Alarming numbers of fires and explosions inoperating theatres, in which flammable anaestheticswere used, shocked the experts. In 1939 the NationalFire Protection Association (NFPA) began bydeveloping regulations for the safe application of

Bo x 1 : E l e c t r i c a l S a f e t y i n M ed i c a l S i t e s A c c o r d i n g t o I E C 6 0 3 6 4 - 7 - 7 1 0

After completion of IEC 62A, work was again taken up in Working Group 26 of IEC TC 64: ElectricalInstallations of Buildings, Electrical Installations in Hospitals and Locations for Medical Use OutsideHospitals. This working group, with members from Germany, the UK, France, the Netherlands, Italyand Finland published Draft 364: Electrical Installations of Buildings, Part 7: Particular Requirements forSpecial Installations or Locations, Section 710: Medical Locations.

Clause 710.413.1.5 describes the medical ungrounded power supply system as follows:

“In Group 2 medical locations, the medical IT system shall be used for circuits supplying medical electricalequipment and systems intended for life-support or surgical applications and other electrical equipment locatedin the ‘patient environment’ excluding equipment listed in 710.413.1.3

For each group of rooms serving the same function, at least one separate medical IT system is necessary. Themedical IT system is to be equipped with an insulation monitoring device in accordance with IEC 61557-8with the following additional requirements:

• the AC internal impedance shall be at least 100kΩ;

• the measuring voltage shall not be greater than 25V DC;

• the measuring current shall, even under fault conditions, not be greater than 1mA DC;

• the indication shall take place at the latest when the insulation resistance has decreased to 50kΩ. Ameasuring device shall be provided to test this facility;

• the indication shall take place, if the earth or wiring connection is lost.

Monitoring of overload and high temperature for the medical IT transformer is required.

For each medical IT system, an acoustic and visual alarm system incorporating the following components shallbe arranged at a suitable place such that it can be permanently monitored (visual and audible signals) by themedical staff:

• a green signal lamp to indicate normal operation;

• a yellow signal lamp, which lights when the minimum value set for the insulation resistance is reached.It shall not be possible for this light to be cancelled or disconnected;.

• an audible alarm, which sounds when the minimum value set for the insulations resistance is reached. Thesignal may be silenced;

• the yellow signal shall go out on removal of the fault and normal condition is restored.

Where an equipment is supplied from one single dedicated IT transformer, the latter can be installed withoutan insulation monitoring device.”

4

Country Standard Isolated Insulation Load Transformer Maximum Transformer Transformer

system monitoring monitoring temperature transformer regulation leakage

monitoring load current

yes no ohmic impedance yes no yes no

Australia AS 3003 X X X X X * AS 3100 25mA

Austria ÖVE-EN7 X X X X 8kVA ÖVE-M/EN60472 *

Belgium TN 013 X X X X X 8kVA NEN 3615 500 PF/35µA

Brazil NBR13.543 X X **) X ***) IEC 742 *)

Canada CSA Z31.1 X X X X 5kVA CSA C22:1 15µA

Chile N.SEG 4EP79 X X X X 5kVA * *

Finland SFS 4372 X X X X * IEC 601-1 0.5mA

France NF C15-211 X X X X 7.5kVA NF C74-010 0.5mA

Germany DIN VDE 0107 X X X X 8kVA DIN VDE 0551 *

Ireland TC10 X alt.x alt.x X X 8kVA IEC 742 *

Italy CEI 64.4 X X X X 7.5kVA CEI 11-11 0.5mA

Japan JIST 1022 X X X X 7.5kVA JIS C0702 0.1/0.5mA

The Netherlands NEN 3134 X X X X 1.6kVA NEN 3615 500 PF/35µA

Norway NVE-1991-FEB X X X X IEC 742 *

Spain UNE20-615-80 X alt.x alt.x X X 6.3kVA UNE 20-399-78 0.5mA

Switzerland MED 4818 X X X X 8kVA EN60742 *

Hungary MSZ 2040 X X X X 4kVA 6.3kVA MSZ 9229 *)

US NFPA 99 X x X X 10kVA NFPA 99 50µA

Table 1: Standards by Country


*) no regulations

**) the standard does not clearly define the method of measurement

***) the standard stipulates that the transformer must be built according to IEC 60742, but does not indicate the maximum capacity

alt.x) measuring of the ohmic resistance or impedance=20

electro-technical equipment in hospitals. In 1944 thefirst regulation, Safe Practice in Hospital OperatingRooms, was published in the US.

Through the increasing application of electrical med-ical devices, the subject of micro-shock and non-stopoperation became increasingly popular and becamean integral part of the standards listed in Table 1.

S a f e t y C o n c e p t s

A safe device is not the decisive factor in guaranteeingsufficient electrical safety for patients, doctors andmedical staff. The requirement is foremost for theoptimal co-operation of the following:

• safety of devices;

• safety of the location; and

• safety of the application.

In other words, the highest degree of safety for thepatient, doctors and their assistants is only achievedwhen a sufficiently safe constructed and maintaineddevice is applied by a responsible operator in a medicallocation with installations according to the regulations.

Furthermore, when problems in hospitals in terms oftechnical safety are considered, the questions of

protection from radiation and fire, safety of theenvironment, hygiene and electromagnetic tolerancecan no longer be disregarded.

S a f e t y M e a s u r e s f o r M ed i c a l E l e c t r i c a lD e v i c e s A c c o r d i n g t o I E C 6 0 6 0 1 - 1

Regular testing of medical electrical devices is anessential aspect of the safety concept in hospitals. It isembodied in European law and the request fortesting is stipulated in the European MedicalProducts Act (MPG), the implementation ofRegulation 93/94/EWG in June 1993.

Today, the international standard IEC 60601:1988(European Standard EN 60601-1:1990) is used forperiodic tests. According to this standard thefollowing tests must be conducted:

• resistance of protective conductor;

• earth leakage current;

• enclosure leakage current;

• patient leakage current; and

• patient auxiliary current.

It is requested that all leakage currents are measured 5

under the following ‘normal’ and ‘single fault’conditions:

• Normal conditions:

– normal and reverse polarity of the supplysystem;

– operational earth is or is not connected toprotective earth; and

– applied part of type F is or is not connected toprotective earth.

• Single fault conditions:

– open circuit of the protective conductor (notvalid for earth leakage current);

– break of one of the line conductors; and

– voltage ≤110% of the highest nominal value ofthe nominal voltage is applied to each appliedpart of type F against earth.

IEC 60601 is a standard for type tests and productiontests that is used also for periodic tests since adedicated standard for periodic tests is not available inmany countries. For this purpose, a draft Europeanand IEC standard is currently being prepared.

Con c l u s i o n

The patient is the focus of attention in the hospital.Even the slightest power failure can impair a successfuldiagnosis and therapy and therefore be life threateningto the patient. Therefore, the un-grounded powersupply system with insulation monitoring is used,because it guarantees the comprehensive protection ofpatients, doctors and medical staff.

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