How to Achieve Satisfactory Short circuit Performance

Achieve Satisfactory Short circuitPerformance

 

Miniature circuit breakers (MCBs) are
widely used for protection from
electrical faults in domestic and light
industrial installations. These devices
protect against short circuit faults of
typically 1–16 kA. During a short circuit
fault an electric arc is drawn between
opening contacts. The arc is then propelled
by electromagnetic forces into splitter plates
which divide the arc into a number of
smaller arcs. This results in a high arc
voltage, which counteracts the supply
voltage to limit the peak fault current. The
energy released by the fault is reduced and
damage to both the circuit and the circuit
breaker is minimized.

To achieve satisfactory short circuit
performance rapid contact opening and a
high contact velocity are usually necessary.
At low contact opening velocity, the arc root
stays on the contacts for a longer period of
time. Heat is generated in the arc chamber
and the contacts are eroded. This has
resulted in a range of devices where the
opening mechanism is driven by a solenoid
energised by the short circuit current. The
solenoid plunger not only trips the spring
driven opening mechanism, but also directly
drives the opening of the contacts. This
“hammer action” provides a very rapid
acceleration of the moving contact during
the initial contact opening.

In addition to the high current event the
circuit breaker must also trip on moderate
over-current. This is usually achieved with a
bimetallic strip. Increasingly, circuit
protection devices provide additional
functionality such as residual current or arc
fault detection.

Reference: Piezoelectrics in Circuit Breakers: Design & Test Paperback – 24 Dec 2014 by Dr Kesorn Pechrach Weaver PhD (Author)