Discuss Supplementary bonding in the UK Electrical Forum area at ElectriciansForums.net
Where do we measure to if on a mcc can I measure from the chassis stud to the extraneous part?Is the barrier at risk of contact with any other cable or extraneous parts? I guess it is tricky to know if rebar in the concrete is externally earthed, etc.
It is probably not required, but at Mike puts it, if in doubt bond it! Worked fine for the 15th edition after all...
I think the more scientific approach is the check its impedance to the system Earth, if it is below 0.05 ohms or above 22k (I think, need to check) then it is not needed as in the first case can be considered already well-bonded, and in the 2nd case can be considered as not having enough of a potential extraneous path to allow more than a 10mA fault current.
What do we class at the MET of An MCC. I know there’s a resistance between exposed conductive part and extraneous part ( current causing the breaker to trip in 5s divided by the resistance measured should be less than 50v. Just struggling with determining if it’s extraneous or not.As already said, the correct place is the MET (especially for the low bond impedance check), but for the 22k test then anything you know is on the CPC circuit.
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If far from the MET then you should allow a reasonable increase in the 0.05 ohms test. For example, what might be expected of X meters of 10mm cable as a good indication of a solid bond.
What do we class at the MET of An MCC. I know there’s a resistance between exposed conductive part and extraneous part ( current causing the breaker to trip in 5s divided by the resistance measured should be less than 50v. Just struggling with determining if it’s extraneous or not.
So testing if supplementary bonding I could just test from a bonded part of the circuit to the parts of question and then from the values determine if it’s extraneous or not? But main bonding is always measured from the MET is that what you’re saying. Thanks for your help!Testing to establish the requirement for supplementary bonding doesn't need to be from the MET and should be done with all other bonding connected, which is somewhat opposite to testing to establish whether main bonding is required.
If in doubt do it.
So an insulated and sheathed cable requires break down of both inner insulation and the outer sheath, how does that happen then?I would totally disagree - with respect you think exposed conductive part is metal part being used as CPC - Exposed conductive part is metal part that is part of the electrical installation but Not the circuit that could become live under fault, just like short circuit I.e from brake down of insulation.
Ageing is a poor excuse unless it’s VIR cables I have never seen Modern pvc cables break down because of ageing.ageing? doesn't matter does it
BS7671 page 30 Fault.....
but that's just one example
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