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Fuse and circuit breaker coordination

Published: 14 September 2015 Category: Trade Talk

Can you please advise how I should develop current discrimination between a fuse & a circuit breaker? The fuse is upstream the circuit breaker.

Fuse and circuit breaker coordination

The purpose of discrimination (also called selectivity) is to provide a situation where the let-through power of the downstream circuit breaker is less than the melting energy of the upstream fuse.

Discrimination between devices is outlined in AS/NZS 3000 clause 2.5.7.2.

Essentially to obtain discrimination requires a comparison of the tripping curve of the upstream fuse time current trip curve and the downstream circuit breaker time current trip curve. This information is available from the manufacturers.
By comparing the curves and ensuring the two curves do not intersect, then discrimination an be achieved.

An area that can cause difficulty is the so-called "knee point" of the circuit breaker curve, where the overload trip characteristic meets the instantaneous trip characteristic. Fuses do not have a "knee point" , instead their characteristic is a smooth curve. This generally means there must be a reasonable gap between the current ratings of the fuse and circuit breaker.

Depending upon the circuit breaker involved you may have settings available to adjust the circuit breaker curve and smooth out the effect of the knee point enabling a closer fit to the fuse.

Answered by: Ian Richardson, Product Manager - Line Protection, ABB Australia
 
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