High integrity socket earthing

[oscar21]

2nd fixing some sockets in an office today and we were talking about high integrity earthing, I've not got a clue what the sockets are going to supply, I'm guessing PC's, monitors, that sort of thing. There aren't any specs for the job so we just wired them as you normally would, I was trying to explain the earthing system that's sometimes used when you expect high leakage currents by running an additional earth or using a 10mm but it suddenly dawned on me that I couldn't really explain it as it didn't make much sense to me.

Whilst its true that a 2nd earth/CPC acts as redundancy whats so special about computer type sockets, yes they leak current but so does a faulty kettle or anything else plugged in, a loose earth on that circuit is just as bad. And as for the CPC in separate terminals on the socket, whats all that about, surely if you have 20 sockets wired normally then that's 20 earth terminal screws that run the risk of coming lose where as in separate terminals you have 40 chances of a screw working itself lose and breaking the circuit.

I did learn about this I seem to remember but it was about 25 years ago or more, we wired some computer sockets in a school and they tripped the RCD when in use despite the circuit passing the testing with flying colours. Someone explained earthing for PC sockets to us after that.

 

[nicebutdim]

2nd fixing some sockets in an office today and we were talking about high integrity earthing, I've not got a clue what the sockets are going to supply, I'm guessing PC's, monitors, that sort of thing. There aren't any specs for the job so we just wired them as you normally would, I was trying to explain the earthing system that's sometimes used when you expect high leakage currents by running an additional earth or using a 10mm but it suddenly dawned on me that I couldn't really explain it as it didn't make much sense to me.

Whilst its true that a 2nd earth/CPC acts as redundancy whats so special about computer type sockets, yes they leak current but so does a faulty kettle or anything else plugged in, a loose earth on that circuit is just as bad. And as for the CPC in separate terminals on the socket, whats all that about, surely if you have 20 sockets wired normally then that's 20 earth terminal screws that run the risk of coming lose where as in separate terminals you have 40 chances of a screw working itself lose and breaking the circuit.

I did learn about this I seem to remember but it was about 25 years ago or more, we wired some computer sockets in a school and they tripped the RCD when in use despite the circuit passing the testing with flying colours. Someone explained earthing for PC sockets to us after that.

There's nothing special other about them other than having two earth terminals.

 

[pc1966]

Basically the idea is you have no single point of failure for the CPC system so if a fault occurs there is not a risk of high-ish current leaking via some class I item's body to a person, etc.

The sockets recommended have double earth terminals so you wire CPC in to one and out of the other eventually forming a ring (either RFC, or separate CPC just to complete this for a radial). That way any single CPC disconnecting is not leaving either its socket, or the remainder, floating.

There is a 10mm CPC option but really you are better to go RFC or radial + extra CPC. Of course the RFC need not be 32A, you could put it on 20A if desired.

It ought to be less of a risk now as any socket circuit leaking a lot is going to trip RCD protection, but 10mA is the threshold of not being able to let-go and your RCD won't trip below 15mA, maybe more like 25mA. There may be other reasons why the risk is considered unacceptable, but that is the most obvious.

 

[pc1966]

Example of such a socket:

13A DP 2 Gang Switched Socket with Outboard Rockers White

Dual earth terminals for high integrity earthing

www.cef.co.uk

13A DP 1 Gang Switched Socket White

Dual earth terminals for high integrity earthing

www.cef.co.uk

 

[westward10]

Second earth bar within the distribution board as well. Merlin Gerin supplied these as a bespoke order.

 

[pc1966]

Also to add the guidance was the CPC ends should not go in the same screw terminal at the CU, again so both cant come loose on a single fault.

Beaten to it by @westward10 !

 

[cliffed]

Also to add the guidance was the CPC ends should not go in the same screw terminal at the CU, again so both cant come loose on a single fault.

Beaten to it by @westward10 !

Do believe that was a thing of the past… no need to do that now

 

[pc1966]

Do believe that was a thing of the past… no need to do that now

I guess it depends on the level of need for integrity. There was talk of the whole high-integrity aspect being dropped in AM2 but seems it was not.

Also I guess the new MK rapid fix sockets with Wago-style terminals would also comply with the dual-CPC arrangement, assuming folks don't put both CPC in to the same hole...probably with the same sleeving...

 

[cliffed]

I guess it depends on the level of need for integrity. There was talk of the whole high-integrity aspect being dropped in AM2 but seems it was not.

Also I guess the new MK rapid fix sockets with Wago-style terminals would also comply with the dual-CPC arrangement, assuming folks don't put both CPC in to the same hole...probably with the same sleeving...

Thinking more @ the DB than the power point

 

[pc1966]

Thinking more @ the DB than the power point

Still, if integrity matters you really want two CPC attachment points.

In some cases the 2nd CPC might be conduit, etc, so connected separately via DB case. But if you can then keeping both CPC apart in the DB helps integrity, just not easy of test/discover-ability if not labelled clearly.