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Post by willU2 » Sat May 23, 2009 1:20 pm

Can someone please explain what CPC readings should be expected in a domestic situation & why?

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Post by sparx » Sat May 23, 2009 8:16 pm

CPC stands for circuit protective conductor ie the earth wire in each final circuit.
I am not sure how to answer this as it's a very wide subject & I don't know your understanding of things electrical, or the reason for your question.
An individual circuit in a domestic system will have a CPC smaller than its L&N conductors eg a 2.5mm cable will have a cpc of 1.5mm, a 1.5mm cables cpc will be 1.0mm thus it's resistance will be higher than a live/neutral.
An installation I tested yesterday for example had a cooker circuit whose 6mm line/neutral resistance from consumer unit to outlet read 0.058ohms each & the 4mm cpc read 0.092 ohms. (known as r1+r2).
So you can see the order of resistance is very low, the lower the reading the more fault current that will flow under line to earth short circuit conditions, the higher the fault current the quicker the protective device will operate to cut of the supply.
There are tables published which give max allowable L+cpc readings for each size/type of protective device in order for them to operate in less than 40milli-seconds if a short circuit occurs,
in my example above, using a 32A type C mcb the max is 0.58 ohms , this reading has to include the external part of the system going between the sub station and the consumer unit (called Ze) which in this case read 0.06 ohms
so taking that from my max of 0.58 gives me a circuit allowance of 0.52 ohms so my circuit reading 0.15 was well inside limit.
Normally we would test whole circuit in one at far end and I got a reading of 0.21ohms which we call Zs and is still much less than the allowed 0.52ohms.
does this mean anything to you? I hope so,
regards SPARX

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Post by kbrownie » Mon May 25, 2009 5:17 pm

as well as the resistance reading r1+r2, Ze, Zs, as so well put by sparx, you could also measure the insulation resistance between your CPC and Live conductors (Neutral and Line).
In a domestic, the test would be made using an insulsation resistance device/meter set at 500volts DC and must measure at least 1 Mega ohm, but depending which meter you use the reading will read well above 200 Mega ohms in a healthy circuit.
The on-site guide provides tables on resistance values and explains the test methods

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