Discrimination with LV power circuit-breakers

Discrimination techniques

The purpose of this “Cahier Technique” is to set out the discrimination techniques which apply specifically to low voltage power circuit-breakers. These devices are characterized by their high rating (800 A to 6300 A), and their location at the head of the LV installation, generally directly downstream of an MV/LV transformer. This location justifies the strict discrimination requirements which apply to them.

This article begins with a resumé of discrimination techniques, followed by an explanation of the links between discrimination and general circuit-breaker characteristics. Finally, some practical examples will be provided on selection of devices to be installed.

Breaking capacity

This is the usual name for the ultimate breaking capacity (I_cu). Icu is the highest short-circuit current intensity which the circuit-breaker is able to interrupt. It is defined for a given rated operating voltage U_e.

Cascading

Using the limiting capacity of the upstream circuit-breaker to increase the actual breaking capacity of the unit downstream. Enables use of circuit-breakers with a lower breaking capacity than the prospective short-circuit current downstream of a current-limiting circuit-breaker.

Current limiting circuit-breaker

Circuit-breaker which, when interrupting a short-circuit current, limits the current to a value considerably less than the prospective current.

DIN

“Déclencheur INstantané”: Instantaneous self-protection release. By assimilation, the corresponding threshold.

DINF (or MCR)

“Déclencheur INstantané à la Fermeture” or “Making Current Release”, instantaneous release intended for self-protection of the breaker during the closing operation.

Electrodynamic withstand (EDW)

Capacity of a unit to tolerate, by nature of its construction, the electrodynamic effects of a short-circuit current, in particular without repulsion of its main or plug-in contacts.

Isc

Short-circuit current, given as a peak value, actually crossing the circuit-breaker, taking account of any limitation.

Icw

Short-time withstand current. This is the maximum short-circuit current (as an rms value), which the circuit-breaker can withstand for a defined period (0.5 or 1 or 3 s) without alteration of its characteristics.

AUTHOR: Schneider Electric expert | Jean-Pierre NEREAU