Earth Fault Impedence

Loyal · May 17, 2010

Can anyone please explain what is the maximum earth fault loop impedance permitted for moulded case circuit breaker (MCCB)?
Also, BS 7671 says a disconnection time of 5 sec is allowed for breakers in distribution circuits. What is the difference between a distribution circuit and final circuit?

johnboy6083 · May 17, 2010

it depends on the rating of the protective device.

a distribution circuit is one that supplys another consumer unit or distribution board.
A final circuit is one that would supply the final equipment ( lighting, motors, appliances ect)

iceman · May 17, 2010

distribution circuit is a circuit suppling a c/u sub mains etc
final circuit is a circuit supplying like a light - socket - cooker - motor circuit it does exactly wot it says on the tin final circuit from c/u where it usualy has an end point

Loyal · May 17, 2010

Thanks for help about the distribution circuits.

I undrstand that the values of Zs as mentioned in table 41.3 of BS 7671 are the same for MCB and MCCB, is it correct?

Also, Table 41.3 provides Zs for breakers up to 125A only. I need Zs for larger breakers (150A, 200A, 300A, etc). Where can I find Zs for the larger breakers?