Earth Fault Impedence

[Loyal]

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]

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]

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]

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?

 

[GT1]

Blimey. Big !
If they are not in BRB (dont have mine to hand) i think you will have to get in touch with manufacturers...!?

 

[Guest123]

I need Zs for larger breakers (150A, 200A, 300A, etc). Where can I find Zs for the larger breakers?

You'll have to contact the manufacturer for this information.

 

[Loyal]

If I have a circuit breaker with a frame size of 100A and a trip of 32A, then shall I have to use the Zs values for 100A or 32A?

 

[ezzzekiel]

good place to start
Earth Fault Impedence - DIYnot.com - DIY and Home Improvement

 

[Sintra]

good place to start
Earth Fault Impedence - DIYnot.com - DIY and Home Improvement

Interesting.

 

[pevvers]

If I have a circuit breaker with a frame size of 100A and a trip of 32A, then shall I have to use the Zs values for 100A or 32A?

If the thermal trip is set to 32 then you would use 32. Get so touch with the manufacturer like everybody else has said...