Black cable for 110v site extension leads?

[bernardgreen]

A Circuit Protection Conductor ( CPC ) can exist and perform its function without it having any connection to the earth ( ground ).

The circuit protection function will occur as long as the CPC has a low impedance connection to the supply at any point other than the live and neutral conductors of the circuit it is protecting. Most commonly this is a tap on the transformer.

As to two or more RCDs on the same circuit. They do provide that extra bit of safety to cover the 7% chance of one of them failing so there is no reason to ban having two on a circuit.

Testing one with its test button should not trip the other one as the imbalance created for the test is local inside the RCD and is not a leakage to earth.

 

[JohnW2]

A Circuit Protection Conductor ( CPC ) can exist and perform its function without it having any connection to the earth ( ground ).

Sure, but I don't think that's in dispute. The potential confusion being discussed is really all to do with terminology/semantics. The functional purpose of a CPC is often/usually described as 'earthing', even in the absence of any connections to 'ground'.

The circuit protection function will occur as long as the CPC has a low impedance connection to the supply at any point other than the live and neutral conductors of the circuit it is protecting. Most commonly this is a tap on the transformer.

Is it not all a bit of a vicious circle? CPCs afford protection against faults between L and the 'earthing point' of the supply (e.g. the CT of a transformer). It would therefore seem that if that 'earthing point' is not distributed (via CPCs), and is not connected to ground, then there is no real possibility of such a fault, hence no need for CPCs.

As to two or more RCDs on the same circuit. They do provide that extra bit of safety to cover the 7% chance of one of them failing so there is no reason to ban having two on a circuit.

Quite. As I said, in other situations, a 7% (or probably even 0.7%) risk of failure of a safety device to operate when needed 'in anger' would not be acceptable; some 'back-up' would not only not be 'banned', but would undoubtedly be required!

Testing one with its test button should not trip the other one as the imbalance created for the test is local inside the RCD and is not a leakage to earth.

Yes, that makes sense. On thinking, there's no way that what goes on when one presses the test button can result in an L/N imbalance either upstream or downstream of the device being tested, since there is nowhere else for current to go!

Kind Regards, John

 

[bernardgreen]

Is it not all a bit of a vicious circle? CPCs afford protection against faults between L and the 'earthing point' of the supply (e.g. the CT of a transformer). It would therefore seem that if that 'earthing point' is not distributed (via CPCs), and is not connected to ground, then there is no real possibility of such a fault, hence no need for CPCs.

The CPC provides a means for cable damage to be detected and, depending on the type of damage, maybe operate a protective device, MCB or RCD before the damaged cable becomes a hazardous item.

 

[JohnW2]

Is it not all a bit of a vicious circle? CPCs afford protection against faults between L and the 'earthing point' of the supply (e.g. the CT of a transformer). It would therefore seem that if that 'earthing point' is not distributed (via CPCs), and is not connected to ground, then there is no real possibility of such a fault, hence no need for CPCs.

The CPC provides a means for cable damage to be detected and, depending on the type of damage, maybe operate a protective device, MCB or RCD before the damaged cable becomes a hazardous item.

I don't really agree with that, and I certainly would not have put it like that.

In the absence of a CPC, the only thing that could damage the cable would be an L-N (or L1-L2) overload or fault within the load, and that would be dealt with by an OPD even in the absence of a CPC.

It is the introduction of a CPC which both opens up the possibility of "live-'earth'" faults and facilitates operation of an OPD (or RCD) should such faults arise. The primary purpose of having the CPC (and 'earthing') is to bring about operation of a protective device if an exposed-conductive-part becomes 'live', before anyone has a chance to touch it - nothing to do with 'protecting cables'.

Kind Regards, John