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Measurements on IT power installation

Version 1.1, Code No. 20 750 205

Measurements on IT power installation Table of content

2

1. Preface........................................................................................................................3

2. Fundamentals ............................................................................................................4 2.1 Main advantages....................................................................................................4

3. Related standards......................................................................................................6

4. Safety conditions in IT power supplies ...................................................................9 4.1 Normal operating condition ....................................................................................9 4.2 Normal operating condition (decreased insulation resistance).......................................10 4.3 First fault condition ...............................................................................................11 4.4 Double fault condition...........................................................................................13

5. Measurements..........................................................................................................14 5.1 Monitoring insulation resistance and earth leakages............................................14 5.2 Testing IT installations .........................................................................................14 5.3 Insulation test .......................................................................................................15 5.4 Continuity test ......................................................................................................16 5.5 LINE impedance test............................................................................................18 5.6 LOOP impedance test..........................................................................................19 5.7 RCD tests.............................................................................................................20

5.7.1 Checking the RCD operation .........................................................................21 5.7.2 Checking the protection effectiveness of installed RCD ................................22

5.8 Single fault leakage current test / Contact voltage measurement ........................24 5.9 Checking the actual alarm limit of the IMDs .........................................................27 5.10 Selective leakage / fault current measurements.................................................28 5.11 Earthing resistance ............................................................................................30

5.11.1 Three wire earthing resistance test with internal generator .........................30 5.11.2 Two wire earthing resistance test with internal generator............................31

5.12 Rotary field check, polarity checks .....................................................................32

6. The Eurotest family of installation testers.............................................................34

Measurements on IT power installation Preface

3

1. Preface The main objective of this handbook is to make the electrical installer more familiar with measurements on IT power supply systems. There are many differences between IT and the most commonly used TN and TT power systems. It is similar with the measurements. Most of measurements are the same in all three systems, but some of them differ significantly when performed on IT installations. Few measurements are characteristic of the IT systems only. Therefore electrical installers are often confused when they have to measure safety parameters on an IT installation. This handbook tries to explain which measurements are to be made in IT system, when and in how they are performed. It will be shown that with modern METREL installation testers all tasks related to safety of IT installations can be completed in a fast and easy way. Figures in this document are simplified. Installation components that are not relevant for the described problematic are left out. Where the topic can be well described on base of one-phase system the situation in three phase system is not presented. More information about measurements in IT power supply systems can be found in other METREL documents:

Handbook “Measurements on electric installations in theory and practice” User manuals of METRELs Eurotest family of installation testers. Handbook “Insulation measuring techniques”.

Authors of this handbook hope that the reader will be better acquainted with measurements in IT power systems after reading it. Feedbacks are highly appreciated. Note : This handbook cannot serve as a substitute for national and international regulations.

Measurements on IT power installation Fundamentals

4

2. Fundamentals IT supply system is a power supply system where live conductors are insulated from ground (PE) – it is an ungrounded supplying system. The system is without direct connection to the ground (only different fault and leakage currents flow) or the connection is provided through relatively high artificial impedance. All exposed conductive parts of the electrical installation are earthed and connected to the earthing point of the system (definition for IT supply system). Beside the standard protection measures known from the TN/TT systems the IT system can also include indication (fig. 1).

TN, TT IT

INDICATION:- IMD- RCM- Insulation fault location

Fig. 1: Protection measures in power supply systems

2.1 Main advantages Two main advantages of IT systems compared to ordinary TN/TT systems are:

Since all live (current carrying) conductors are isolated from earth a first fault to earth does not lead to the tripping of disconnection devices. The power system is still operational, vital processes are not interrupted.

Indication of power supply safety condition can be included. IMDs (insulation monitor devices), ELMs (earth leakage monitors), RCMs (residual current monitors) or similar devices are often installed in IT systems. They constantly monitor the leakage and fault currents in the system and are trigger an alarm if the insulation resistance drops below a predefined level. The insulation usually deteriorates gradually (trough ageing, pollution, moisture). If monitored permanently an insulation problem can be predicted in most cases before troubles arise.

IT systems guarantee a higher level of fire protection than TN/TT systems because alarms are triggered before high-energy fault currents and sparks occur (that can cause a fault).

Measurements on IT power installation Fundamentals

5

These facts are very important. Furthermore, IT systems are deployed: Anywhere where working with flammable and explosive materials (mining,

chemical industry, pharmacy), In areas where the supply is not allowed to be interrupted (computer systems,

industrial processes). Typical areas of use are:

Medical surgery rooms, Emergency lighting in communal facilities, Mining industry, Ships, Computer power supply systems, Explosive atmospheres, chemical industry.

It has been proven that in these areas the number of incidents (power failure with consequences, accidents, fire) is reduced because of using IT installations.

Measurements on IT power installation Related standards

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3. Related standards A) International standardization

Requirements for electrical installations Some standards listed below contain special provisions for IT system while others are valid for all supply systems.

IEC 61140, Protection against electric shock – Common aspects for installations and equipment

IEC 60364 consists of the following parts, under the general title Low-voltage electrical installations: Part 1: Fundamental principles, assessment of general characteristics, definitions Part 2: Void Part 3: Void Part 4: Protection for safety Part 5: Selection and erection of electrical equipment Part 6: Verification Part 7: Requirements for special installations or locations

IEC 60364-1: Electrical installations of buildings - Part 1: Fundamental principles, assessment of general characteristics, definitions

IEC 60364-4-41 to 44: Electrical installations of buildings – Protection for safety – Protection against electric shock IEC 60364-4-42, Electrical installations of buildings – Protection for safety – Protection against thermal effects IEC 60364-4-43: Electrical installations of buildings – Part 4-43: Protection for safety – Protection against overcurrent IEC 60364-4-44, Electrical installations of buildings – Protection for safety – Protection against electromagnetic and voltage disturbances IEC 60364-5-51: Electrical installations of buildings – Part 5-51: Selection and erection of electrical equipment – Common rules IEC 60364-5-52, Electrical installations of buildings – Part 5-52: Selection and erection of electrical equipment – Wiring systems IEC 60364-5-53, Electrical installations of buildings – Selection and erection of electrical equipment Isolation, switching and control IEC 60364-5-54, Electrical installations of buildings – Part 5-54: Selection and erection of electrical equipment – Earthing arrangements, protective conductors and protective bonding conductors IEC 60364-5-55, Electrical installations of buildings – Selection and erection of electrical equipment – other equipment IEC 60364-6-61, Electrical installations of buildings – Part 6-61: Verification, testing and reporting IEC 60364-7 (all parts 7), Electrical installations of buildings – Part 7: Requirements for special installations or locations


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Special locations Medical installations IEC 60364-7-710, Electrical installations of buildings – Part 7-710: Requirements for medical installations Electrical installations of ships and of mobile and fixed offshore units IEC 60092-201, Electrical installations in ships - Part 201: System design - General IEC 60092-202, Electrical installations in ships - Part 202: System design - Protection IEC 60092-401, Electrical installations in ships - Part 401: Installation and test of completed installation IEC 60092-501, Electrical installations in ships - Part 501: Special features - Electric propulsion plant IEC 61892-1, Mobile and fixed offshore units - Electrical installations - Part 1: General requirements and conditions IEC 61892-6, Mobile and fixed offshore units - Electrical installations - Part 6: Installation Installation testing and monitoring IEC 61557-1 to -7, 10, Electrical safety in low voltage distribution systems up to 1000 V a.c. and 1500 V d.c. – Equipment for testing, measuring and monitoring of protective measures parts 1 to 7 and 10. Special for IT: IEC 61557-8, Electrical safety in low voltage distribution systems up to 1000 V a.c. and 1500 V d.c. – Equipment for testing, measuring and monitoring of protective measures – Part 8: Insulation monitoring devices for IT systems IEC 61557-9, Electrical safety in low voltage distribution systems up to 1000 V a.c. and 1500 V d.c. – Equipment for testing, measuring and monitoring of protective measures – Part 9: Equipment for insulation fault location for IT systems IEC 62020, Electrical Accessories – Residual current monitors for household and similar uses (RCMs)


8

B) National standardization (some examples) General requirements BS7671 (UK), NEC National Electrical Code (USA), DIN VDE 0100 series (Germany) Installation monitoring ASTM F 1134-88, ASTM F 1207-89 (USA), UTE C 63-080/10.90 (France) Special locations Medical locations DIN VDE 0100-710:1994-04, Amendment to DIN VDE 0100-710:1994-04, DIN VDE 0100-710:2002 (=IEC 60364-7-710), DIN VDE 0107, DIN VDE 0108 (Germany), NFC 15-100 (France), AS3003 (Australia), ÖVE-EN7 (Austria), TN 013 (Belgium), NBR13.543 (Brazil), CSA Z31.1 (Canada), N.SEG 4EP79 (Chile), SFS 4372 (Finland), TC10 (Ireland), CEI 64-4 (Italy), JIS T 1022 (Japan), NEN 3134 (The Netherlands), NVE-1991-FEB (Norway), UNE20-615-80 (Spain), MED 4818 (Switzerland), MSZ 2040 (Hungary), NFPA 99 (USA) Underground mining VDE 0118 Protective measures in underground mining (Germany) Ships DIN VDE 0129-201

Measurements on IT power installation Safety conditions in IT power supplies

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4. Safety conditions in IT power supplies

4.1 Normal operating condition Figures 2 and 3 show single- and three-phase normal operating IT systems. Contact voltages and leakages are below the threshold levels. IMD is installed and indicates normal condition. In normal operating conditions the impedance to the ground consists of:

Capacitances and insulation resistances of supply wires to the ground (CLEAK, RLEAK) 1 2,

Capacitances between primary and secondary windings (CPRIM-SEC) of the IT supply transformer 3,

Impedances formed by Y capacitors (EMC protection) in mains section of connected devices (CX, CY) 4 .

Depending on application additional artificial impedance (ZART) to the ground can be applied. Typical values of the artificial impedance are from 100 to several 10 kΩ 5.

By selecting appropriate transformer, installation cabling and optional artificial impedance to ground the maximum leakage currents can be kept under the desired threshold level.

L1

L2

RE

DEVICE

RPEC

IMDOK

ZART

cx

cy cy

CPRIM-SEC

LEAKAGE CURRENTS

1

3

4

5

3 2,1 2,

Fig. 2: Single-phase IT power supply system in normal operating condition. Paths of

leakage currents are shown.


10

230V

230V

230V

IMDALARM OK

ZART

L3

L2

L1

N

Fig. 3: 3-phase IT power supply system in normal operating condition

4.2 Normal operating condition (decreased insulation resistance) Figure 4 shows a normal operating IT system with an insulation problem between L1 and PE conductor (RPOLUTION). The insulation / resistance is still above the limit value. The IMD noticed that the insulation resistance decreases slowly and indicates a warning. In this case the advantage of IT system becomes obvious. The decreasing insulation trend is recognized and the problem can be eliminated before it will cause any troubles (danger, fire etc.)!

Typical reasons for decreasing of insulation resistance are ageing of insulation, pollution, moisture etc. This can happen in electrical installations and devices. Sudden shorts and other failures are not so often. Refer to METREL handbook Insulation measuring techniques for more information about insulation.

L1

L2

RE

RPEC

IMD

230V

t

R

Fig. 4: Normal operating condition on the limit. The fault current increases as the RPOLLUTION decreases.


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4.3 First fault condition Figures 5 and 6 show a low resistance path between one line conductor and the PE conductor through equipment failure. If the resistance drops close to zero ohms this failure is called a single fault (or first fault). Both systems are still operational and safe. The installed IMDs indicate the single fault in system. The IT power system in figure 5 has all exposed conductive parts connected by a protective conductor to a system earth electrode. It has been converted to a TN-like system due to the single fault condition. The IT power system in figure 6 has exposed parts earthed individually and in groups. It has been converted to a TT-like system.

Conclusions:

Usually no dangerous contact voltages occur on exposed conductive parts since the leakages are just slightly higher than in normal condition.

The other live conductor is slightly more stressed because the voltage on it has risen to nominal supply voltage. It is unlikely that this will cause any serious trouble.

If installed the IMD will trigger the alarm. The fault reason must be found and removed as soon as possible!

When the IT power system is converted to a TN-like system, overcurrent disconnection device (fuse) will not react in single fault condition.

Where the IT power system is converted to a TT-like system in case of a first fault, RCDs should be installed. They will usually not trip in single fault condition. If the contact voltage UCSF rises above 25 V or 50 V the RCD must trip out. As it can be seen the ISFL currents are not much higher during the single fault condition than in normal condition (only a half of the high impedance path is shorted). Therefore the contact voltages almost always stay below the limit.

Note: Sometimes (in larger systems) an installed RCD can trip out during a single fault condition because of relatively high existing fault and leakage currents. There is nothing wrong with the safety of the installation if this happens. It depends on the individual application whether this is acceptable.


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L1

L2

RE

DEVICE

RPEC

230V

ISFL1

IMD

UCSF

0 V230 V

IT TN

Fig.5: Single fault between L and PE conductors. IT system is converted to TN system. Because of simplicity only a portion of the path for the fault currents is shown.

L1

L2

RE

IMD

230V

IT TT

ISFL1

Fig. 6: Single fault between L conductor and conductive part on earth potential. IT system is converted to TT system. Only a portion of the path for the fault currents is shown.


4.4 Double fault condition If a further insulation fault happens before the first fault is removed, higher and dangerous fault currents occur in the IT power system. The disconnection devices must react in the same way as in TN or TT systems. This means that the overcurrent disconnection devices or RCDs must react in parts of a second (protection against fire and dangerous contact voltages)! The system is not operational anymore! If protection is provided with an overcurrent device the fault loop current (IPFC) must exceed the breaking current of the installed disconnection device (fig. 6).

DEVICE

L1

L2

RE

RPEC

IMDALARM

230V

OK

IPFC

Fig. 7: Double fault on IT to TN converted system, protection with overcurrent

disconnection devices. If protection is provided with an RCD the fault loop current (IΔ) must be higher than the RCD’s nominal current to assure the disconnection in due time (Fig. 7). It will be shown later in this handbook that the position of the RCD is very important.

L1

L2

RE

IMD

230V

ZART

IFAULT

Fig. 8: Double fault on IT to TT converted system, protection with residual current device.

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Measurements on IT power installation Measurements

14

5. Measurements

5.1 Monitoring insulation resistance and earth leakages Insulation Monitoring Devices (IMDs), Earth Leakage Monitors (ELMs), Residual Current Monitors (RCMs), and similar devices are permanently monitoring (measuring) the condition of the IT system and trigger an alarm if the insulation resistance or leakage impedance drops below or approaches a predefined limit. Some monitoring systems are suited to localize the fault too. Standards IEC 61557-8 and IEC 61557-9 define the specifications of monitoring devices.

5.2 Testing IT installations Beyond the on-line insultation/leakage monitoring different parameters must be regularly measured on an IT installation. Portable installation testers are used for these tests. Some precautions must be taken if measuring with standard installation testers that are mainly suited for TN/TT systems. New METREL testers include an IT system measuring mode that simplifies the measurements significantly. The standard IEC 60364-6-61 describes the measurements and tests that should be performed on an electrical installation as initial verification. The standard includes IT systems too. For the requirements of measurements of safety measures for IT system as defined in IEC 60364 standards, the standards of IEC 61557 series are defined:

Insulation tests according to IEC 61557-2. Continuity of earth connection and equipotential bonding according to IEC

61557-4. Line impedance according to IEC 61557-3. Loop impedance* according to IEC 61557-3. Earthing resistance measurement according to IEC 61557-5. RCD tests* according to IEC 61557-6. Contact voltage measurements* according to IEC 61557-6. Phase sequence according to IEC 61557-7.

* These tests differ significantly from that in TN/TT installations. The tests and differences are described in the following chapters of this document. Other important tests are:

Measuring of leakage/fault currents in the installation. Single fault leakage current measurements. Checking the actual alarm limit of the IMDs. Wall socket connection checks.


15

In order to meet the requirements of IEC 60364 some tests on IT systems can be carried out only if an artificial single fault is made. The standards give no exact information about this – detailed instructions do not exist yet. This handbook tries to explain this topic. Warning The operator must be aware of that when an artificial single fault is created:

The electrical installation is stressed through voltage increase on the other line conductor,

A second fault (caused by the measuring instrument itself or a failure in the installation) during the single test condition can lead to the disconnection of the power supply, safety problems because of impaired protection measures etc.

5.3 Insulation test Typical measurements in IT system:

Insulation resistance RISO between phase (live) conductors (L1, L2, L3) 1,2,3. RISO between each phase (live) conductor (L1, L2, L3) and PE (conductor,

terminals) 4,5,6. Test conditions / limits:

Installation switches should be opened (de-energized supply). Devices, appliances, motors should be disconnected or switched off (not

necessary if RISO is above limit with items connected or switched on). Test voltage 500 V. Limit from 0.5 MΩ up to several MΩ.

Suited for:

Best suited for initial verifications before the installation is energized. Particularly suited for periodic tests on installations (result can be wrong because

of installed lamps and loads between line to line tests).

Note: The measuring procedure is the same as for TN/TT systems.


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Connection diagram

230V

230V

IMD

Zartificial

RISO

L1

L2

L3

PE

2 3 4 5 6

Fig. 9: Insulation tests

5.4 Continuity test

Typical measurements in IT system: Continuity RLOW of PE conductor between main potential equalizer (MPE) and PE

terminals (outlets, etc). 3 Continuity RLOW of local equipotential bonding between PE terminals and bonded

conductive parts. 4 Continuity RLOW of equipotential bonding between MPE terminal and bonded

conductive system (water, gas, lighting system, CTV). 2 Continuity RLOW of main earthing connection between MPE terminal and earthing

probe. 1

Test conditions /limits: Test current must be higher than 200 mA. Limits are from 0.1 Ω up to 2 Ω.

Suited for:

Initial verifications and periodic retests. Fast safety checks on energized system are possible. Checking of additional local potential bonding.


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Notes: The measuring procedure is the same as for TN/TT systems. If the IT system is protected with overcurrent disconnection devices only this test

is especially important! Connection diagram

L1

L2

RPEC

IMD

230V

Zartificial

MPEPE

2 3 4

R 200mA

Fig. 10: Continuity tests


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5.5 LINE impedance test Typical measurements:

Line impedance ZLINE between phase (live) conductors (L1-L2, L1-L3, L2-L3) at different locations, such as origin of installation 1, wall sockets 2 3 4 5 , device terminals 6.

Prospective short current IPSC between phase (live) conductors at different locations: origin of installation 1, wall sockets 2 3 4 5, device terminals 6.

Test conditions / limits:

Limits are up to several Ω, depending on installed fuse (view METREL installation testers User manuals for more information). The latest METREL installation testers have the fuse / limit base already included.

Suited for:

Initial verifications and periodic retests, Fast safety checks of wall sockets, device terminals, Safety check in case of adaptations, prolongation of wiring, changing fuses etc.

Notes:

The voltages between conductors differ from that in TN/TT system, besides that the measuring procedure is the same as for TN/TT systems.

The short circuit currents are flowing mainly through the live conductors as in TN/TT system.

Connection diagram

230V

230V

230V

IMD

L1

L2

L3

PE

DEVICE

4 5 6

ZLINE

RCD

2 3

Fig. 11: Line impedance tests


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5.6 LOOP impedance test The LOOP impedance test is applicable if the IT system converts to TN system during the single fault and the protection is provided with collective earthing and overcurrent devices (fuses). Standard LOOP tests as in TN/TT system are not suited for IT systems in normal operation. Only small leakage / fault currents would flow during the test. A single fault condition (R0 Ω) converts the IT system into a TN like system. The leakage /fault currents will just slightly increase despite of the fault. In double fault condition the fault currents become high as they in TN system. Typical measurements:

Loop impedance ZLOOP between phase (live) and PE conductors (L1-PE, L2-LPE, L3-LPE) at different locations, such as origin of installation 1, wall sockets 2 3 , device terminals .

Prospective short currents IPSC1, IPSC2, IPSC3 between phase (live) and PE conductors at different locations, such as origin of installation, wall sockets 1 2 3 , device terminals .

Test conditions /limits:

Limits are up to several Ω, depending on installed fuse (view METREL installation testers User manuals for more information). Latest METREL installation testers have the fuse / limit base already included.

A single fault must already be made before starting the test. The second fault is made by the instrument during the measurement.

LIM

NOMLOOP I

UZ

2

LIMPFC II 2

Eq. 1 Eq. 2

UNOM……nominal supply voltage IPFC……..prospective fault current ZLOOP…..fault loop impedance

ILIM…….current of the fuse that causes a disconnection within a safe time frame 2………conservative safety factor

Suited for:

Initial verifications and periodic retests Safety check in case of adaptations, prolongation of wiring, changing fuses etc.

Notes:

The fault loop path depends on the place of the single fault. A good practice is to place the single fault at the origin of the installation.


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Factor IPFC in METREL instruments should be set to 0.5 for loop testing in IT systems.

If testing on working installation, consider possible consequences in case of disconnection!

Warning The operator must be aware of:

The electrical installation is stressed through voltage increase on the other line conductor during the single fault condition.

A second fault (caused by the measuring instrument itself or a failure in the installation) during the single test condition can lead to the disconnection of the power supply, safety problems because of impaired protection measures etc.

Connection diagram

230V

230V

230V

IMD

L1

L2

L3

PE

2 3

ZLOOP

IPFC

SINGLEFAULT

Fig. 12: Fault loop measurements in IT to TN converted system- single fault inserted intentionally by operator.

5.7 RCD tests RCD tests can be divided into two subgroups:

Checking the functionality of the RCD itself. With these tests correct operation of the RCD itself is checked.

Checking the contact voltages. With this test protection effectiveness of the RCD is checked.


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5.7.1 Checking the RCD operation It is obvious that the RCD test current IN cannot be generated in an IT system during normal operation and the RCD test cannot be carried out. Therefore the simplified connection to check the operation of installed RCDs is preferred 1. Typical measurements:

Disconnection time tN at nominal currents I x1/2, I x1, I x5. Trip out current I, trip out time t at nominal trip out current.

Test conditions /limits: RCD type (A, AC, B, general, selective) and nominal value must be considered

too. Refer to METREL installation testers User manuals for RCD test limits.

Suited for:

Initial verifications and periodic tests. Fast safety checks of RCDs in the fuse cabinet.

Connection diagram

230V

230V

230V

IMD

RE

1

RCDNt,I

RCD

Fig. 13: Simplified test of RCD operation – connection at the RCD


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5.7.2 Checking the protection effectiveness of installed RCD As described in chapter 4 RCDs are often used if the IT system converts to a TT system during a single fault. During normal operation of the IT system the RCD test current IN cannot be generated and the RCD test cannot be carried out. When a single fault condition occurs the fault currents slightly increase. In case of a high earthing resistance the contact voltage UCSF could rise above the safety limit of 50 V. Usually the RCD test currents cannot be generated in a single fault loop. The contact voltage at single fault UCSF test (on base of earth test RE and single fault leakage current ISFL test) is better suited for checking protection effectiveness in a single fault condition (described in chapter 5.8). In double fault condition the fault currents become higher and RCD tests can be performed normally. The protection effectiveness of the RCD dramatically depends on the place where the faults occur. Different fault combinations are analyzed in the table below. In actual situations the fault paths can be much more complex so care must be taken when placing RCDs to ensure their effective operation! Position of 1st fault

RCD trips Condition for disconnection

Safety condition for effectiveness of protection

before RCD

- -

behind RCD

possible SFLN II CSFSFLLOCALE UIRV )()25(50 _

Table 1: Effectiveness of RCDs in single fault condition.

Position of 1st fault

Position of 2nd fault

RCD trips Conditions for disconnection

Safety condition for effectiveness of protection

before RCD

before RCD

- -

behind RCD

before RCD

OTHERLOCALE

NOMN RR

UI

_

CNLOCALE UIRV _)25(50

behind RCD

behind RCD

- -

Table 2: Effectiveness of RCDs in double fault condition.

UNOM……….nominal supply voltage UCSF………..contact voltage in single fault condition (calculated) UC………..contact voltage in double fault condition (measured with RCD test)


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RE_LOCAL…local individual or group earthing (named RS, RE in testers) ROTHER…….parallel resistance of other earthings that form the paths for returning

currents ISFL………single fault leakage current (measured with ISFL test) IΔN… ……RCD nominal current Typical measurements:

Disconnection time tN at nominal currents I x1/2, I x1, I x5 1 2 3. Trip out current I, t at trip out current 1 2 3. Contact voltage tests UC1, UC2, UC3 1 2 3.


RCD type (A, AC, B, general, selective) and nominal value must be considered, too.

Refer to METREL installation testers User manuals for RCD test limits. Contact voltage UC and UCSF limits are 50 V for normal environmental conditions

and 25 V for aggravated environmental conditions. A single fault must already be made before starting the test. The second fault is made by the instrument during the measurement.

Suited for:

Initial verifications and periodic retests. Verification of newly installed local earthings. Verification of newly installed RCDs.

Notes:

The fault loop path depends on the place of the faults and influences the effectiveness of the RCD. A good practice would be to place the single fault at the origin of the installation during measurement.

If testing on working installation, consider possible consequences in case of disconnection!

Warning The operator must aware of:


A second fault (caused by the measuring instrument itself or a failure in the installation) during the single test condition can lead to the disconnection of the power supply, safety problems because of impaired protection measures, etc.


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Connection diagram

230V

230V

230V

IMD

ZART

RE

RCDCU

3

RCD

SINGLEFAULT

RE_OTHER

2

Fig. 14: Complete RCD test

5.8 Single fault leakage current test / Contact voltage measurement

ISFL is an appropriate parameter for evaluating the effectiveness of the earthing in IT systems that converts to a TT system during a single fault. The purpose of this test is to check if contact voltages stay inside the safety limits during single fault condition. As shown in chapter 4.3 the leakage/fault currents are increased by a certain amount during the single fault and can rise above the limit values. The calculated result UCSF1 returns the expected contact voltage in a single fault. Typical measurements & calculations:

ISFL1, ISFL2, ISFL3 for individual or group earthings UCSF1, UCSF2, UCSF3 (calculated) for each individual or group earthing


25

The earthing resistances of individual and group earthings must be known for the UCSF calculation:

SFLxLOCALPECLOCALECSFx IRRU )( __ VUCSFx )25(50 Eq. 3

UCSF…………contact voltages at single fault on earthing (calculated) ISFL………… single fault leakage current (measured) RE_LOCAL… ...earthing resistance of each individual or group earthing (measured) RPEC_LOCAL…resistance of PE conductor of each individual or group earthing (measured) Test conditions /limits:

Single faults must be performed during the test. In a three phase system between L conductors, L conductors and PE conductor (PE terminal). In a single phase system between L conductors and PE conductor (PE terminal). An A-meter must be placed in series with the single fault 1, 2. Alternatively the single fault current in the local earthing can be measured with leakage current clamp 3.

Expected ISFL currents are in range of mA. The UCSF limit is 50 V for normal environmental conditions and 25 V for aggravated environmental conditions.

Suited for:

Initial verifications and periodic retests. Safety check when that new wiring or devices (that can contributes to

leakage/fault currents) are added. Safety check in case that the protective earthing system was changed.

Notes:

The single fault can be made between the live and PE conductor /PE terminal. RPEC can be measured with the Continuity test. RE can be measured with the Earth test. Performing the single fault is easy. Some METREL instruments perform the

single fault automatically during the test. Warning: The operator must be aware of:


A second fault (caused by the measuring instrument itself or a failure in the installation) during the single test condition can lead to the disconnection of the power supply, safety problems because of impaired protection measures, etc.


26

Connection diagram

IMD

ZART

RE

RCD

mA mA

2

artifitialsingle fault

mA

3

Fig. 15: Single fault leakage current with A-meters. Because of simplicity the measurement is shown in a 1-phase system.

IMD

RE

RCD

artifitialsingle faultis performedby instrument

ISFL

Fig. 16: Single fault leakage current with some METREL measuring instruments. Because of simplicity the measurement is shown in a 1-phase system.


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5.9 Checking the actual alarm limit of the IMDs IMDs and other monitoring devices usually include a self-test option for the alarm circuit. The alarm circuit can be double checked by applying an adjustable resistor between line and PE conductors. Test conditions / limits:

An adjustable resistor is applied between each L conductor and PE (earth potential) 1 2.

The response values of IMDs depend on application and are from several kΩ up to several 100 kΩ. For IT systems in hospitals a frequently used limit is 55 kΩ.

Suited for:

Periodic and occasional checks of IMDs and ELMs Adjusting IMDs

Notes:

If other leakage (connected appliances) or fault current sources already exist they must be considered. Otherwise the result could be impaired (the alarm will be triggered faster then expected)!

This test is included in some METREL instruments. Warning: The operator must be aware of:

The electrical installation is stressed through voltage increase on the other line conductor during the connection of the resistor.

A second fault (caused by the measuring instrument itself or a failure in the installation) during the connection of the resistor condition can lead to the disconnection of the power supply, safety problems because of impaired protection measures, etc.

Connection diagram

L1

L2

RE

IMD

230V

ZART

2

adjustableresistor

Fig. 17: Checking the alarm limit of the IMD device.


28

L1

L2

RE

IMD

230V

ZART

2

adjustable resistorinside instrument

IMD

,

Fig. 18: Checking the IMD with some METREL measuring instruments

5.10 Selective leakage / fault current measurements In chapter 4.1 leakage and fault currents flowing in a normal operating power supply are shown. The currents increase with the size of the installation and number of devices connected to it. If the sum of these currents exceeds the expected level they can cause different troubles:

Tripping of RCDs in normal operation or single fault condition. Alarm indication of monitoring devices. If the installation protective measures are improper, excessive leakage and fault

currents can even result in dangerous contact voltages in the system. Installed IMDs and ELMs detect excessive currents but cannot localize the problem. Different types of fault location systems (fixed, portable) are used for solving this problem in critical applications. A simple method for localizing excessive leakage currents causing problems is the selective leakage / fault current measurement with current clamps.

Typical measurements: Selective leakage /fault current measurement ILF of the system 1, fuse circuit 2,

individual devices 3, earthing connections, etc.


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Test conditions / limits: Installation must be energized. Currents in individual PE wires can be measured. If a device is considered as problematic it should be switched on/off or put into

different operation modes while measuring the current through its protective conductor(s).

Applied limits depend on the application. Some general hints can be followed: - The current should be at least three times smaller than the rated IΔN of the

installed RCD. - The current should be significantly lower than the set limit of the

monitoring device.

ILF < 0.25 UNOM / RLIM Eq. 4

ILF…………leakage/fault current UNOM …nominal mains voltage RLIM….set limit on the monitor device

Suited for:

Localizing problems caused by excessive leakage/fault currents in the system Fast checking of leakage/fault current in the power system

Notes: The clamp current measurement could be influenced by near EM fields (high

load currents in line conductors, larger metal surfaces on high potential like in fuse cabinets, etc. ).

Some types of electronic devices (frequency converters etc) can produce DC leakage currents. DC currents are not detected with AC current clamps!

L1

L2

RE

DEVICEIMD

230V

OK

CURRENT

2 3

RE

RCD

Fig. 19: Selective leakage /fault current measurements with current clamps


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5.11 Earthing resistance The best way to measure earthing resistance in IT systems is the so called 3-wire test (see figure 20) with internal signal generator. This method is appropriate for measuring system, individual and group earthing resistances. The system earthing resistance is usually a substantial part of fault loop paths. It is used for contact voltages calculations in combination with the single fault leakage current ISFL (see chapter 5.8) The easier so-called 2-wire test can be used if one can assure that there are no connections between individual earthings or groups of earthing. 5.11.1 Three wire earthing resistance test with internal generator Typical measurements:

System 1, individual 2, group 3 and earthing resistance RE measurement with three probes .


Limit values :

RE_LIMIT < UCLIM / IΔN

Eq. 5

UCLIM….system limit contact voltage (25 V or 50 V) IΔN……..nominal current of RCD

Suited for:

Initial verification and periodic retests of earthing system Safety check if earthing components are changed

Notes:

The size of the earthing system must be known to place the probes at an appropriate distance.

If RE is too high: - The earthing connection must be improved. - Or an appropriate disconnection device must be installed that will trip out

during the single fault (complicated design). - Or additional equipotential bonding should be provided.

The measuring procedure is the same as for TN/TT systems.


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Connection diagram

L1

L2

IMD

230V

OK

ZART

EARTH

S

H

RE

RPEC

2 3

E

S

H E

S

E H

Fig. 19: Earthing resistance test

5.11.2 Two wire earthing resistance test with internal generator Typical measurements:

Individual earthing resistance RE measurement without probes 1. Group earthing resistance RE measurement without probes 2.

Test conditions /limits:

For limits view Eq. 5 Suited for:

Initial verification and periodic retests of earthing system. Safety check if earthing components are changed.

Notes:

It must be assured that there are no additional connection (not intended for equipotential bonding) between the measured earthings. An unknown connection can lead to completely wrong results!


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If RE is too high - The earthing connection must be improved. - Or an appropriate disconnection device must be installed that will trip out

during the single fault (complicated design). - Or additional equipotential bonding should be provided.

The measuring procedure is the same as for TN/TT systems. Connection diagram

L1

L2

IMDALARM

230 V

OK

ZART

EARTH

RE

RPEC

2

RE_INDIVIDUAL RE_GROUP

E H,SE H,S

Fig. 20: Earthing resistance test

5.12 Rotary field check, polarity checks Typical measurements :

Checking direction of rotary field on three phase outlets and device terminal 1,2. Fast checking of voltage conditions, polarity on wall sockets, terminals, etc 3.

Notes: The measuring procedure is the same as for TN/TT systems Voltages between phase and PE conductor differ from that in TN/TT systems.


Connection diagram

230V

230V

230V

IMD

ZART

L1

L2

L3

PE

3

VOLTAGE

2

Fig. 21: Rotary field and polarity check

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Measurements on IT power installation The Eurotest family of installation testers

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6. The Eurotest family of installation testers

Eurotest IM Eurotest XE Eurotest AT Eurotest XA Remarks

IT system mode

IT voltage symbols; limits,

decisitions modified for IT

system

Insulation tests

Insulation between ALL terminals can be measured at the same time

Continuity

Line impedance

(R) (R) (Z)

Loop impedance

(R) (Z) (Z) Measuring method with current probe

RCD operation

tests

Earthing resistance

with internal generator

2 and 3 wire tests

Rotary field

Leakage/fault currents

From 0.1mA

Monitoring device alarm

check

Adjustable resistor into L1-PE and L2-

PE

Single fault current / contact

voltage at single fault

ISFL1, ISFL2 in one

step

METREL d.d.Measuring and Regulation Equipment Manufacturer Ljubljanska c. 77, SI-1354 HorjulTel: + 386 (0)1 75 58 200 Fax: + 386 (0)1 75 49 226E-mail: metrel∞metrel.si http://www.metrel.si

© 2012 METRELNo part of this publication may be reproduced or utilized in any form or by any means without permission in writing from METREL.

Measurements on IT power installation on... · IEC 60364-5-54, Electrical installations of...

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