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Restricted Earth Fault Relay Description The relay uses a type B61 attracted armature element energized via a low pass filter circuit and a full wave rectifier. The relay has a pre-set setting of 15V. Other resistors are introduced into the circuit to provide the voltage setting range up to 270V in increments of 5V using heavy duty DIL switches.
Included within the relays, are the essential non-linear resistors to limit the peak voltage output from saturated C.T.s, these resistors protect the C.T. insulation and secondary wiring.
Current Transformer Requirements
Experience has shown that most protective C.T.s are suitable for use with the high impedance relays and where the C.T.s are specially designed for this protection, their overall size may be smaller than that required for an alternative current balance protection.
The basic requirements are
- All C.T.s should have identical turns ratios, if possible.
- The knee point voltage of each C.T.should be at least 2xVs.
- The knee point voltage is expressed as the voltage applied to the secondary circuit with the primary open circuit which when increased by 10% causes the magnetizing current to increase by 50%.
- C.T.s should be of the low leakage reactance type.
Most modern C.T.s are of this type and there is no difficulty in meeting this requirement. A low leakage reactance C.T. has a jointless ring type core, the secondary winding evenly distributed along the whole length of the magnetic circuit and the primary conductor passes through the approximate centre of the core.
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