Protection Against Intermittent Earth-Faults - ABB REJ 527 Technical Reference Manual

1MRS 750616-MUM
4.1.4.4.
Directional or Non-Directional Earth-Fault Relay
Technical Reference Manual

Protection against intermittent earth-faults

An intermittent earth-fault typically occurs in an insulated cable where the
insulation has a crack and water leaks into the cable. The earth-fault dries up the
crack in the cable, extinguishing the fault, but reappears after a short time as water
leaks back through the crack. The process repeats, resulting in a succession of fault
current pulses.
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When the resetting time of the overcurrent unit is shorter than the interval between
the fault current pulses, the relay will be continually reset and not able to trip. A
resetting time longer than the start time, but short enough not to interfere with
normal operation of the protection and control system, will help to eliminate some
less common health and safety problems.
Set value
≥100ms
Trip level
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Momentary peak
value of neutral
Momentary
current Io
value of
neutral
voltage Uo
Fundamental
amplitude of Io
≥100ms
REJ 527
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