Harmonic restraint earth-fault protection
RAISB is a sensitive, time-delayed earth-fault protection for use in solidly earthed power systems. It is typically used to provide a sensitive earth-fault backup protection for a transformer or for a power line as a phase overcurrent or earth-fault relay having a second harmonic current restraint.
The relay is inherently single-phase. Residual earth-fault current via the summed CT phase-currents or a CT in the neutral may be used for the earth-fault relay application.
RAISB is available in a basic variant with definite time delay, flag indication and test switch, and in other variants in combinations with inverse time overcurrent relays and a counter for elapsed time.
Main Features:
- 2nd harmonic restraint for stability during transformer inrush
- Settable operate current
- Reset ratio > 90 %
- Rated frequency 50 or 60 Hz
- Variants available with definite or inverse time delayed tripping output