Insulation Requirements for Monitoring Distribution Boxes
This part of IEC 61557 specifies the requirements for insulation monitoring devices (IMD) which permanently monitor the insulation resistance R F to earth of unearthed a. Among all the protection methods identified by the Standard, only IT distribution systems can guarantee greater operational continuity in case of a first fault to earth: in these systems, the circuit-breaker will not trip because the fault current is limited by the high insulation impedance. It is intended to give an alert (light and sound) or disconnect the power supply when the resistance between the two conductors drops below a set value, usually 50 k Ω (sample of IEC standard. The devices recognize insulation faults as they develop, tripping as soon as the value falls below minimum set thresholds.
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