Energy-based discrimination for low-voltage protective devices

Discrimination technique

The purpose of this “Cahier Technique” publication is to present the new energy-based discrimination technique that ensures tripping discrimination between protective devices during a short-circuit. Both simpler and more effective than standard discrimination techniques, it has been implemented on the Compact NS range of circuit breakers used in low-voltage power distribution networks. Discrimination is ensured for all prospective fault currents on the condition that upstream and downstream circuit-breakers have different current ratings (ratio u 2.5) with a trip-unit rating ratio u 1.6.

Following a brief review of standard discrimination techniques, the authors examine the behaviour of circuit-breakers and various trip units from the energy standpoint. They then demonstrate that total discrimination is possible up to the circuit-breaker breaking capacity, over several levels, without using time discrimination techniques.

Definition

In an electrical installation, loads are connected to sources via a succession of protection, isolation and control devices. This “Cahier Technique” publication deals essentially with the protection function using circuit-breakers. In a radial feeder layout (see fig. 1 ), the purpose of discrimination is to disconnect only the faulty load or feeder from the network and no others, thus ensuring maximum continuity of service.

If discrimination studies are not or are incorrectly carried out, an electrical fault may cause several protective devices to trip, thus provoking an interruption in the supply of power to a large part of the network. That constitutes an abnormal loss in the availability of electrical power for those parts of the network where no fault occurred.

AUTHOR: Schneider Electric experts | Marc SERPINET, Robert MOREL