The DIGITAL MICROPROCESSOR BASED RESTRICTED EARTH FAULT RELAY (REFR), is a
universal protection device for high, medium and low voltage networks. It is aimed
primarily at high impedance differential schemes such as restricted earth fault and
generator differential protection.

In this type of protection scheme, based on circulating current measurement,
heavy through fault currents during a fault condition can cause current transformers to
approach or exceed their saturation level and allow sufficient unbalance current to
operate the relay. This is due to the asymmetrical current of extreme magnitude which
has dissimilar effects on seemingly identical CT's.