Type B MCB Query

[monkeyblaine]

I have been having trouble understanding breakers... for example a 16A breaker will only start to trip at around 25 amps, why is this? why does it not trip at its rating 16A it makes designing circuits a bit weird when you really look into tripping times and cable current capacity tables? I'm pretty new to all of this so any help would be appreciated...

[JohnC]

Hi Monkeyblaine

I dont have my books to hand at the moment so i cant quote specific figures but simply its the same reason in essence that a 30mA RCD doesnt not trip at exactly 30mA and that 30mph in your car is not actually 30mph. There has to be a tolerance, and in regard toi MCB's its primarily due to inrush currents and short duration overloads. For example an MCB has to be able to operate at xx% overload for 1 hr (i'll dig out the exact regs ect when im home). Inrush currents form Flourescent lighting and motors also play a part which is why you have Types B C ad D.. lets take a motor rated at 10Amps it could have an inrush start current of 25Amps and a running current of 8Amps. It actually helps with design becuse you dont end up over designing circuits. you wouldnt want to install 6mm cables on a 32A MCB for a 10A motor just becuse it hits 25A for a second or so.

[monkeyblaine]

Ok, thats much clearer now. Thank you... I have just finished my level 2 at college and I have started my apprenticeship, but i'm constantly facing new questions and things that confuse me, so it helps to have this as a resource.

Thanks again JohnC

[JohnC]

A little extra info.

A BS88 fuse has to be able to carry 1.25 * its rating (25%) for 1 hour but must fuse within 1 hour at 1.6 * its rating (60%). For a BS60898 Its must be able to carry 1.13 * its rating for 1 hour but must trip at 1.45 * its rating. So a "small" overload for a BS88 would be upto 1.59 * its rating and for a BS60898 it would be 1.44 * its rating.

These fiqures are for fuses and Circuit breakers rated upto 63A

Hope that helps a little more

Well any questions just ask and we will try to answer them as best we can..