Earth leakage relay and fault currents

Industrial installations

ELR (earth leakage relay) or MRCD (modular residual current device) are industrial protective devices with adjustable leakage current and time to disconnect. In industrial setting, the leakage currents can be considerable even during normal operation. Occasional rises due to a combination of devices or machines working in a specific part of their work cycle are normal as well. Therefore the protective device should tolerate such rises for a limited period. Industrial settings often require high power availability and unnec-essary power outages can be very costly. There must however be a limit how far both magnitude and duration of leakage current will be tolerated.

The principle of operation is reasonably simple: a toroid current transformer with multiple primaries for the phase and neutral lines of the installa-tion and a single secondary, transfers current when there is a current imbalance between phases. The sensed current is electronically is compared to the user adjustable settings. If the limits have been exceeded, a signal is sent to the medium voltage switching device to disconnect the supply.

Testing effectiveness of this type of protective device has two elements: testing disconnection time and disconnection current. To thoroughly test the ELR, it should be set to each combination of its available settings. Regular testing only encompasses the actual setting at the location. Dis-connection current test uses linearly rising current until the ELR trips. It can be DC, AC or pulsed, depending on ELR type. In the latter two cases, it is the effective and maximum value that rise.

The current is injected using a flexible clamp with desired properties regarding the current shape that can be rolled into multiple turns to multiply the injected current. Smart instruments can make this part of the processing and present a realistic result. Less smart instruments will require a manual calculation.
Disconnection time tests use the value of current prescribed by the ELR and measure time to disconnection. The test current pulse is injected (as opposed to rising steadily in the current test). Minimum response times are some tens of milliseconds. The desired result is the delay that has been set.

Metrel MI 3144 is the instrument for this measurement. It comes with flex clamp for current injection in standard set. The settable parameters cover test current shape and magnitude and limits for successfully passed test. The instrument provides high performance while keeping high portability. It can be controlled using another Metrel multifunctional tester like MI 3155, or with Android app.