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Posted By Topic: 2.5.7.2.3 (c) Fuse and Circuit-breaker Discrim

OctaneOutlaw
Nov 04 2019 20:18

I had a question the other day relating to service protection devices that has sparked me to look into this one

I always thought discrimination below 250A was a should and not a shall which reading over this section today I have seen is correct for MCB-MCB only

Reading this section I'm lead to believe that I must discriminate with the service protection device because it's a fuse

I've tried to find some info online but haven't had much luck. Can anyone give me some more information or advice relating to this?

The supply fuse is a 63A gG HRC, I've read that a 32A MCB doesn't give the right level of discrimination, if so, what does?

Does this mean using a 32A MCB downstream is non-compliant?
   

bjem85
Nov 04 2019 23:11

See figure 2.14.

If you have a C curve circuit breaker (instantaneous trip <= 320A), and the fuse time-current curve is above the circuit breaker curve for all times >0.01s, then there should be no problem.

According to some data I have, a 32A C-curve circuit breaker must trip within 1 second at 10 times its rated current, or it goes into instantaneous tripping (10ms or less). A 63A gG fuse will blow in about 3.5 seconds at that current.

I think you should be in the clear.
   

OctaneOutlaw
Nov 05 2019 12:25

I've tried to have a good look over the clause and to try work out and track down some curves to work it out myself but like I said I haven't had much luck, will have to keep looking around