The 5B3 relay uses a type B61 attracted armature element energised 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.