Circuit breakers and 'tripping characteristics'

[DIY]

Can someone explain the difference between 'C' and 'B' type circuit breakers?

I appreciate that they have different 'tripping characteristics' and hence one (B?) will trip faster than the other - but which is most suitable for a normal domestic consumer unit?

I want to replace my old fuse box with a modern consumer unit but find that consumer units 'pre-loaded' with mcb's tend to have far too many circuit breakers for my needs. (My present fuse box just has 5 fuses - upstairs sockets, downstairs sockets, upstairs lights, downstairs lights, and one extra which used to be for the cooker socket but which now just feeds a small group of ordinary sockets)

Toolstation sell a 6-way consumer unit - but you have to purchase the circuit breakers separately and they come as either 'C' type or 'B' type with no explanation as to which is the more suitable for normal domestic use.

DIY

[ARWadworth]

You would normally fit a B type in a dometic CU.

Read this as it saves me typing:)

Adam

[Andrew Gabriel]

That's good. Only thing I would add is that I normally use Type C for lighting circuits on the basis that it might prevent some nuisnance trips when a filament lamp arcs across. I have no envidence that it does (and it certainly doesn't solve the problem completely), but it generally does no harm, with lighting circuits being way over-protected at 6A anyway.

[john]

Just look at the length of the 17th edition consumer units with 2 RCD's / main switch and 10 mcb's !

[DIY]

Many thanks for your response and for the link Adam - exactly what I needed!

DIY

[DIY]

That's a good tip - I know of several people who have consumer units that trip out every time a bulb fails.

DIY

[DIY]

Indeed! Who on earth has that many circuits requiring protection in a normal residential home?

DIY

[ARWadworth]

Me. When I finish.

Although it meets the 16th edition regs which still apply at the moment.

Non RCD side Cooker (I do not have an electric cooker but I laid the cable for one) Lights Up Lights Down, Loft and Landing Lights outside Fridge and Freezer sockets, burgular alarm, and the central heating Outside sockets (fed by a RCBO) Computer room sockets Shed

RCD Side Sockets Kitchen Sockets Downstairs Sockets Upstairs

[Doctor D]

It really helps.

I fitted a new CU with type B's for my in laws a few years ago and as their two lounge light fittings (total 10 x 40 watts) consume a lamp almost weekly they were getting frequent nuisance tripping. I changed the lighting MCB's to type C and the problem has all but disappeared.

I've got them on CFL's almost everywhere, but they refuse point blank with the lounge lighting. Watching TV there involves sunglasses as you squint under 400 watts of ceiling lighting, 160 watts of wall lighting and two standard lamps now fitted with 18 watt CFL's. My father in law has sight problems, but won't accept the argument that the TV is actually clearer without simulating studio lighting to watch it!

[Ian_m]

Type C can stand an overload current for longer before tripping. I have two "C" in my lighting circuits, one for kitchen with 11 50W transformered downlighters and other for hall lights. This was done as the "B" type tripped occasionaly when the kitchen lights were turned on and putting a "C" on hall lights prevented trips when one of the hall bulbs blew, which plunged hall, external light and garage (with CU) into darkness, making reseting the trip quite dangerous.

[Ret.]

Thanks for that - I'll use "C" mcb's for the lighting circuits,

DIY

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