MCB choice

[barnies]

Hello,

Just a quick question that i was thinking about,

Why is a mcb chosen higher than the current taking in a circuit say the circuit was pulling 18amps you would a 20amp breaker wouldnt this 20amp breaker not be to high to trip because the circuit is pulling 18amps which less than rating of the MCB? how does it work why are they chosen higher?

 

[londonboy]

Basically... If there happened to be a fault/overload, then that 18amps will increase dramatically. If theres a short circuit (live/line/phase to neutral) then thousands of amps could flow, which would blow/disconnect the fuse, within the permitted time (generally 0.4 seconds).

 

[Spark123]

Also you don't want the breaker to trip all the time as it will be a nuisance.
A 20A breaker will not trip at 20A too, from the graphs in BS7671:2008 a 20A breaker will allow 30A to pass through it in excess of 1000s before tripping. However we do not use this leeway when selecting a breaker.

As a rule we look at the design current, select the next size breaker up and then select a higher rated cable (as installed including correction factors) for the circuit.

 

[barnies]

so no matter what the calculated amps is for a circuit, e.g 18amps, there is always gonna be more than that under fault conditions yea? is that why the nearest breaker is used to prevent it from getting to extreme amps??

Thanks.

 

[barnies]

Also you don't want the breaker to trip all the time as it will be a nuisance.
A 20A breaker will not trip at 20A too, from the graphs in BS7671:2008 a 20A breaker will allow 30A to pass through it in excess of 1000s before tripping. However we do not use this leeway when selecting a breaker.

As a rule we look at the design current, select the next size breaker up and then select a higher rated cable (as installed including correction factors) for the circuit.

why does a 20 amp breaker allow 30amp to flow through doesnt that defeat the purpose of a 20amp mcb??

and why is the cable that is choosen its current carrying capacity bigger than the mcb current capacity?

 

[Spark123]

Not really, an MCB has a fusing factor of 1.45 which means it will allow 1.45x its rated current for up to 1 hour to pass through it. Fortunately the current carrying capacities of cables in the appendixes of BS7671 take this into account.
A cable has a greater CCC than the breaker so that the breaker protects it and the cable doesn't get too hot (and possibly catch fire).

It would be a nightmare if a 20A breaker tripped every time you pulled 20A, for example every time you switched your vacuum cleaner on which for a split second will pull in excess of 20A.

 

[ban-all-sheds]

barnies - this is all in the Wiki.

//www.diynot.com/wiki/electrics:fuses-and-miniature-circuit-breakers

//www.diynot.com/wiki/electrics:mcbtocable