Tuesday, November 04, 2014

Selective coordination for breakers and fuses

In simple words, coordination or selectivity is 'how we design which breaker to trip first if fault occurs without disturbing unrelated breaker upstream/downstream'.

I've learned this in Power System Analysis last time but that's on substation scale.
Here in smaller scale, we decide for selectivity of breakers/fuse in swithcboards/MCCs.

Time-current characteristic for breaker

If the two curves overlaps it shows that both breakers do not coordinate for fault current greater than the crossing point.

Time-current characteristic for Fuse

Load side have to be more sensitive so that when fault occurs at downstream, the load side breaker will trip first.

Methods of selective coordination

Interlocking may be apply so that if fault occurs at any of the level, the breakers on the other level wont trip.

But now I wonder, what if the upstream is a breaker and the downstream is the fuse. and the fuse is much high rating than the breaker?
How can I determine the coordination?

Full slide show can be viewed at the link below.
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