mcb discrimination

ianphilip007 · Apr 13, 2012

hi looked at a job today 32a mcb feeding outbuiding CU. out building cu has 20a mcb for radial sockets is this ok re discrimination (mcb's in series) im sure it is but thought there was a specific ratio

Darkwood · Apr 14, 2012

La Poste said:
You could always investigate a C type MCB for discrimination.

Just had this covo in a different thread no domestic mcb will give discrimination against any other under short circuit and to note the type b,c or d is inrush allowance and nothing to do with the large current that will flow in a short, i could give you the table list of partial discrimination between different size mcbs but most incoming supplies exceed this in fault current so not worth it, just have to realise its not achievable between standard mcb's.

Guitarist · Apr 14, 2012

If you are feeding an outbuilding with an MCB and a fault caused it to open rather than the MCB in the outbuilding, then other than the slight inconvenience of walking to the house, no other circuit would be affected. It's a different ballgame when an outbuilding fault can take out the RCD in the house and create a potentially dangerous situation.

Darkwood · Apr 14, 2012

Good design would put the outbuilding on its own rcbo, but if not ust make sure its not on the same circuit that has the stairs lighting on, but domestic really shouldn't fall under the concerns of a potentially dangerous situation.

Guitarist · Apr 14, 2012

Was a bit tongue-in-cheek darkwood. We keep being told how dangerous it is to have an RCD trip when someone is potentially walking downstairs.....
I was talking about discrimination for an outbuilding that has an RCD in its board being fed from an MCB.

Darkwood · Apr 14, 2012

Guitarist said:
Was a bit tongue-in-cheek darkwood. We keep being told how dangerous it is to have an RCD trip when someone is potentially walking downstairs.....
I was talking about discrimination for an outbuilding that has an RCD in its board being fed from an MCB.

Arhh! no worries but tongue in cheek is a sore point at mo! ..... i did the decent thing trying to break up a fight between 2 ppl i know last night and them been ex army it wasnt easy but i did end up with my tooth punturing through my cheek, lots of claret everywhere but feel like ive just been in the dentist chair lol