What a feeder protection relay does

It measures current and voltage on a feeder and trips the circuit breaker when it detects a fault condition. Unlike a low-voltage breaker with an integral trip unit, a relay is a separate device configured for the specific protection scheme the feeder requires.

Protection functions typically applied

Overcurrent and earth fault protection

Directional overcurrent where fault direction must be distinguished

Distance or differential protection on more complex feeders

Breaker failure protection

Under- and over-voltage, and frequency protection where relevant

Trip circuit supervision, so a broken trip coil is detected before it is needed

What the configuration depends on

The network earthing arrangement

Whether the feeder is a single source or part of a ring or parallel arrangement

The fault levels expected at that point

The coordination scheme with the relays upstream and downstream

The communication requirements for SCADA or substation automation

Commissioning

A relay installation is commissioned by injecting test currents and voltages, verifying each protection function and its trip time, and checking the trip circuit end to end. The settings are recorded and handed over.