2330 exam

[kmatthews]

quetion: the earth fault loop impedance measurement on a socket protected by a type c circuit breaker is over the limit permitted by bs 7671 tables. there is no fault with the wiring, suggest two methods which may be used to overcome this problem?

 

[ezzzekiel]

change to type b which has higher zmax or provide additional protection by rcd

 

[kmatthews]

i though type c had a higher rating than type b. can i look it up in the regs

cheers

 

[GT1]

Increase the csa of the cpc. Thereby lowering the resistance.

 

[ezzzekiel]

can i look it up in the regs

cheers

dunno can you?

32a type b = 1.44
32a type c = 0.72

gt1 answer about cpc size would prob be what there looking for, rcd should not be used as a fix all solution (although it gives a zs of 1667 ohms ish)

 

[GT1]

The other option would be to break the circuit into two.
And split it over 2 mcb's.
This will also lower the resistance of the cpc due to the shorter length of the circuit.
Obviously these aren't necessarily practical solutions, but purely theory.

However it demonstrates knowledge of the effects of increasing csa or decreasing the length of the cpc.

 

[kmatthews]

yea that would work but im my question is suggests that there is nothing wrong with the wiring.

thats what got me a little confused.

if its not a problem on the wiring it can rally only be the protective device or the load applied?

 

[the_mouse]

The C&G answers are, change to type b mcb and install a RCD


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[GT1]

Agree with swopping to type b mcb as it has a higher Zs tolerance.

But how is fitting an rcd part of the answer to this question ?
How does fitting an rcd lower the Zs ?

The other answer could also be to increase the csa of the main bonding conductor. This will lower the Ze value and therefore the Zs for this circuit.

 

[the_mouse]

That's the answer C&G are looking for the question does ask how to overcome the high zs problem not how to lower it, we had this in a practice paper last week.


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