Insulation monitor

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An insulation monitor (also insulation monitoring device ) monitors the insulation status in IT networks (local low-voltage networks without operationally earthed star point ). It reports that the insulation resistance has fallen below a minimum .

In the past, only a current or voltage measurement to earth was carried out for this purpose (symmetry deviation), but today it is required that symmetrical insulation faults are also detected, which also requires a test current.

In the event of a fault, if the insulation fault current is exceeded, either only a warning can be given or the relevant network outlet can also be switched off.

history

The first insulation monitor was developed by Walther Hans Bender in Frankfurt / Oder and registered for a patent at the Reich Patent Office in 1939 under the name A-Isometer (patent certificate 722348). This was the first actively measuring insulation monitoring and earth fault display device for three-phase systems in lignite mining. The technical term isometer was registered on July 31, 1952 at the German Patent and Trademark Office under the registration no. 622564 registered.

application

Insulation monitor with earth fault location

Insulation monitors are used where power supplies or their outlets have to be single- fault- proof, i.e. where a simple fault (single-pole earth fault ) must not lead to a failure of the power supply or the outgoing line concerned, such as in operating rooms and intensive care units, in railway signal boxes , in critical areas in industry, in conventional power plants and photovoltaic systems, on ships or in electric vehicles.

The first earth fault is in principle harmless in IT networks as long as the leakage capacitance of the system is small enough, which is usually the case when the network is small. It must be reported by the insulation monitor so that it can be rectified promptly. If a first earth fault is not rectified, there is a risk that the person touching it will close a circuit across both fault locations in the event of a second fault. For this reason, the localization and elimination of insulation faults is mandatory through DIN VDE 0100-410 (VDE 0100 Part 410): 2007-06 . Since troubleshooting in the system is often a challenge, fixed and mobile devices are used to localize earth faults ( English Earthfault Detection System , EDS ).

A related method with a similar objective, which is used in the high-voltage area as in medium-voltage networks , is earth fault compensation , also known as resonance star point earthing .

Insulation monitoring devices in accordance with DIN EN 61557-8

This part of IEC 61557 specifies requirements for insulation monitoring devices (IMD), which continuously improve the insulation resistance RF to earth of unearthed IT AC voltage systems, of IT AC voltage systems with galvanically connected DC circuits with nominal voltages of up to AC 1000 V and of unearthed IT DC systems up to DC 1500 Monitor V, regardless of the measurement method.

According to DIN EN 61557-8 (VDE 0413-8), the following applies: Insulation monitoring devices must be able to monitor both symmetrical and asymmetrical insulation deterioration according to their specified measuring principle. A symmetrical deterioration in insulation occurs when the insulation resistance of all conductors in the network to be monitored is (approximately) evenly reduced. An unsymmetrical deterioration in insulation occurs when the insulation resistance, e.g. B. of a conductor is reduced much more than that of the other conductor (s). So-called earth fault monitoring relays, which use the unbalance voltage (displacement voltage) as the sole measurement criterion when an earth fault occurs, are not insulation monitoring devices within the meaning of this standard. Under special network conditions, a combination of several measurement methods may be necessary to fulfill the monitoring task, including asymmetry monitoring.

See also

Norms

  • DIN EN DIN EN 61557-8 (VDE 0413-8): 2015-12, "Electrical safety in low-voltage networks up to AC 1000 V and DC 1500 V devices for testing, measuring or monitoring protective measures - Part 8: Insulation monitoring devices for IT systems "
  • DIN EN DIN EN 61557-9 (VDE 0413-9): 2015-10, "Electrical safety in low-voltage networks up to AC 1000 V and DC 1500 V - devices for testing, measuring or monitoring protective measures - Part 9: Equipment for insulation fault location in IT Systems "
  • IEC 61557-8: 2014, "Electrical safety in low voltage distribution systems up to 1 000 V ac and 1 500 V dc - Equipment for testing, measuring or monitoring of protective measures - Part 8: Insulation monitoring devices for IT systems"
  • IEC 61557-9: 2014, "Electrical safety in low voltage distribution systems up to 1000 V ac and 1500 V dc - Equipment for testing, measuring or monitoring of protective measures - Part 9: Equipment for insulation fault location in IT systems"

literature

  • Gerhard Kiefer: VDE 0100 and practice. 1st edition. VDE-Verlag GmbH, Berlin / Offenbach 1984, ISBN 3-8007-1359-4 .
  • Günter Springer: Expertise in electrical engineering. 18th edition. Verlag Europa-Lehrmittel, Wuppertal 1989, ISBN 3-8085-3018-9 .
  • Wolfgang Hofheinz: Protection technology with insulation monitoring. VDE series standards understandable. Vol. 114, 2nd edition. VDE-Verlag, Berlin / Offenbach 2007, ISBN 3-8007-3026-X .

Individual evidence

  1. ↑ DIN EN 61557-8 VDE 0413-8: 2015-12 website VDE Verlag, accessed on August 6, 2017.
  2. ↑ Functional principle of an insulation monitoring device website Bender GmbH & Co. KG, accessed on August 6, 2017.