70:(approximately) evenly reduced. An unsymmetrical fault in insulation occurs when the insulation resistance, e.g. of one conductor is reduced much more than that of the remaining conductor(s). Relays, which use the unbalance voltage (3 voltmeter method), are not insulation monitoring devices according to this standard. Under special network conditions, a combination of several measurement methods may be necessary to fulfill the monitoring task, including an asymmetry monitoring.
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According to 61557-8: Insulation monitoring devices must be able to monitor both symmetrical and asymmetrical insulation faults according to their specified measuring principle. A symmetrical fault in insulation occurs when the insulation resistance of all conductors in the network to be monitored is
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to earth of unearthed a.c. IT systems, of a.c. IT systems with galvanically connected d.c. circuits having nominal voltages up to 1 000 V a.c., as well as of unearthed d.c. IT systems with voltages up to 1 500 V d.c. independent from the method of measuring.
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61557-8:2014, "Electrical safety in low voltage distribution systems up to 1 000 V a.c. and 1 500 V d.c. - Equipment for testing, measuring or monitoring of protective measures - Part 8: Insulation monitoring devices for IT
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61557-9:2014, "Electrical safety in low voltage distribution systems up to 1000 V a.c. and 1500 V d.c. - Equipment for testing, measuring or monitoring of protective measures - Part 9: Equipment for insulation fault location in IT
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IEC 61557-8 Electrical safety in low voltage distribution systems up to 1 000 V a.c. and 1 500 V d.c. – Equipment for testing, measuring or monitoring of protective measures – Part 8: Insulation monitoring devices for IT
40:(sample of IEC standard for medical applications). The main advantage is that the ungrounded or floating system allows a continuous operation of important consumers such as medical, chemical, military, etc.
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For the monitoring device different terms are used: "Insulation monitoring device" vs. "ground fault relay" vs. "ground fault detector" vs. "LIM" (Line
Isolation Monitor)
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Most monitors work by injecting low level DC on the line and detecting. Some manufacturers use a patented AMP-monitoring principle (Adapted
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This part of IEC 61557 specifies requirements for insulation monitoring devices (IMD), which permanently monitor the insulation resistance
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Some manufacturers of monitors for these systems are capable of handling VFDs (
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between the two conductors drops below a set value, usually 50 k
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Insulation
Monitoring Devices (IMD) according to IEC 61557-8
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Terminology differs by region and by market segment.
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