is this correct?

[wreck-it-steve]

If you have a cable feeding a sub board (protected by MCB) and a fault happens on a circuit fed via the sub board then it will trip the breaker for that circuit in the sub board but not trip the breaker feeding the sub board?


Thanks

 

[flameport]

That's how it should work.

Whether it does depends on the characteristics of the circuits and protective devices.

 

[mikeyd]

Yes. or no or maybe.

If the MCB's are identical with the same rating including trip curve (type B, C etc) then definitely, it's a maybe.

 

[wreck-it-steve]

so if one set was "C" type and one "B" type then it may be OK?

 

[PrenticeBoyofDerry]

discrimination/selectivity should be in.

 

[wreck-it-steve]

discrimination/selectivity should be in.

??

 

[PrenticeBoyofDerry]

discrimination/selectivity should be in.

??

read this it will help you understand
http://www.tlc-direct.co.uk/Book/3.8.6.htm

 

[wreck-it-steve]

ok

 

[riveralt]

I thought your electrician was fitting this board - or did you fit it yourself and are now having problems?
http://www.diynot.com/forums/electrics/sub-board-query.302687/

 

[333rocky333]

If the main mcb was running near max capacity, when the fault occured there is a good chance that it would trip before the circuit mcb, we get it a lot in pubs, thats why hrc fused supplies seem to be preferred to feed sub boards

 

[Adam_151]

MCBs don't often discriminate very well with one another, if you have a current large enough to trip the mag of the submain breaker then it will trip as well as the final circuit MCB, with fuses if your downstream device total I²t is less than the submain pre-arcing I²t then it'll discriminate. There is no real such thing as pre-arcing I²t with MCBs, if the current is above the trip level then it unlatches and its going to trip the fact that the other device is clearing the fault is irrelevant now, you're going to have to reset both!