MCB's and current capacities

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taffyg1409

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Hi all, I am one of these '5 day' sparkys whose done a month on a DI course but has not alot of experiance and wondered if anyone can clarify something for me with reference MCB's!!

I'm happy with the old Ib > In > Iz for current carrying capacities. But a C type breaker is designed to break 5 to 10 times the normal full load capacity whis for a C16 would be 80a to 160a. how does that work when the 1.5mm conductor carries less that 30a.

I'm obviously missing something big here!!! Please help me to understand how these type C & D mcb's work.

Rgds:(

 

the doctor

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hi, good question

the secret is that the large current is only present for a small time during disconnection so it wont be harmed-as long as your disconnection times are ok

dont worry about been a five day wonder-the ones who scream w****r are usually the biggest w*****s of all in my experience... ?:|

 

the doctor

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oh and yes

b trips in 5 times max

c trips in 10 times max

d trips in 20 times max

the spin off from this is if you check the maximum loop impedance figures for say a b, c, d 16A you will see that;

loop for c16 is half of a b16 and

loop for d16 is half of a c 16

in lay terms twice the current requires half the impedance to work so ohms law does work thenB)

 
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