Zones, Overlap and Backup Philosophy

Designing for the fault you cannot predict

A protection engineer does not design for one fault. The design must clear any credible fault, anywhere in the system, while one of its own components has also failed. That second condition is what turns a collection of relays into an engineered system.

This course is written for engineers who already set and test relays and now need to design, review and defend protection schemes. It assumes the basics of overcurrent, earth fault and differential protection. The focus is on judgement: which arrangement, why, and what happens at the edges where the textbook answer stops working.

Zones of protection

A power system is divided into zones: a generator, a transformer, a busbar, a feeder, a motor. Each zone has protection that is responsible for faults inside it. Unit protection defines its zone exactly by the location of its current transformers (CTs). Non-unit protection, such as overcurrent, has a zone that is set by its settings and is only approximately defined.

Every point in the system must be inside at least one zone. To guarantee this, adjacent zones overlap, and the overlap is arranged around the circuit breaker. This is done by placing the CTs for each zone on the far side of the breaker from the zone itself, so that the breaker sits inside both zones.

The blind spot between CT and breaker

If the CTs for both zones are on the same side of the breaker, there is a section between the CT and the breaker contacts that one zone sees and the other does not. A fault there can be detected by one zone, which trips its breaker — and the fault is still fed from the other side. This is known as a blind spot or dead zone.

Worked example. A busbar protection uses CTs on the busbar side of an outgoing feeder breaker, and the feeder protection uses CTs on the same side. A fault between the CT and the breaker is outside the busbar zone and inside the feeder zone. The feeder relay trips the feeder breaker, but the fault is on the busbar side of it and continues to be fed by the busbar. Only slow backup clears it. The design fix is overlap; where that is impossible, schemes such as end-zone or dead-zone protection detect current still flowing after the breaker has opened and trip the busbar.

Worked example — reviewing a backup claim

A design for a 33 kV incomer claims 'main and backup protection'. The review finds a differential relay and an overcurrent relay, both fed from the same DC distribution fuse, tripping the same trip coil. A single blown fuse or an open trip coil removes both. The reviewer asks for separate DC supplies, a second trip coil on the breaker, trip circuit supervision on both, and breaker failure protection tripping the upstream breaker. Each change addresses a specific single failure, and the design report should say which.