bs7671 reg.543.7.2.1 Ask questions and find answers to many subjects relating to electrics and electrical work

[kbrownie]

Hi all,
Need a little clarification on this regulation (543.7.2.1)
A couple of points;
Q1) Does a spur now need to have two cpc cables if so does this mean returning a single cpc back to the socket the spur was taken from?
Q2) Is it also true that a radial circuit most have a cpc ring circuit.
and what this about connecting the cpc across two radial circuits to create a cpc radial?
Sorry I don't see the logic, am I missing something?
KB

[ericmark]

Only reading regs the same as you but I would say:-
Q1 yes it does need two cpc cables or equivalent (ii)(a) and no does not need to return to same socket. (ii)(c)
Q2 I am assuming that if for example wired in SY cable then one earth terminal would connect to SWA brade and other to Copper earth wire.
I would consider they are looking for two earth wires in some way so if one earth wire becomes disconnected there is always a second route.
Eric

[kbrownie]

Hi Eric, thank's for quick reply.
So now we need to use both earth terminals on socket-outlets, refering to your reply; That if you was installing T+E in a ring circuit spur you can connect an earth in pvc singles to any other socket-outlet on that circuit. and if you was connecting a radial power circuit in again in T+E that you now need to create a ring circuit with the return cpc again being in pvc singles.
I can see the sense in it, but as for connecting a cpc across two radial circuits would this not have safety issues, as far as isolation of one of these circuits and the potential of this circuit becoming live through a fault on the other circuit, which has not been isolated?
Your thoughts are appreciated
KB

[ericmark]

On a ring main only real change to normal is one terminal used for earth from one cable and other terminal used for earth on other cable. Theory if either fails the other will maintain the earth. Using socket where it comes from ring with two earth terminals on a spur you need four core cable as if not in the cable min size of 4mm so really needs 4 core two normal Line and neutral and from one earth terminal of host socket to one earth terminal or slave socket and other earth terminal of host socket to other terminal of slave again two independent earths.
That is how I read it pity can't get triple and earth in 2.5mm of course if coming from a fused spur 1.5 triple and earth could be used.
That's the way I read it. Maybe I am wrong?
Eric

[kbrownie]

Thanks again for reply eric, is much appreciated. Maybe easier just to extend the ring!
Thanks again
KB

[TOPSPARK]

After checking out the reg book i have to concur that a separate cpc has to be taken back from the spurred device to the existing socket on the ring.This seems a little extreme however not all sockets have two separate cpc connetions on the back
regards
Topspark

[kuzz]

Can this be right? I've never wired in this way, has anyone else been doing it? i can see there is an argument for it, but surley with goodworkmanship a cable won't drop out or snap. but if we did work on "what if we loose an earth" why stop there? how about if we loose the second earth? maybe 3 or 4 earths to our srurs and radials just to be on the safe side? and then why stop at socket radials. how about if a lighting circit looses an earth or maybe a wire down to a swith. I have seen an earth broken in a red/red t&e by a picture nail, so separate earths to every swith now? comments?

[kbrownie]

Thanks again for all replys,
and as Topsparks says not all socket-out lets contain two earth terminals so if guess these would need replacing if to comply to 17th eds,
Kuzz this topic did not really appear to me untill I received the publication of the 17ths OGS. Where it clearly shows a diagrams of the cpc in spurs, rings and radials and cross references to the reg (543.7.2.1)
That is why I thought to post this inquiry.
again thanks for all views and replys.
KB

[sparx]

Hang on fellas lets not get carried away here!!!
this reg is for special circuits supplying 5 or more PC's etc that have filters which leak to earth, this is not your standard domestic ring, & yes we have installed a number of such circuits in Dado trunking & cross connected cpc's between 2 radials to give both 2 cpc's
this whole sub section of the red book comes from reg 543.7 and can't just be picked out in isolation (no pun intended)!
regards SPARX

[kuzz]

And now it makes sense. Cheers for en-lighting us. Unfortunately i don't have my own regs book, so was just going off the interpretation of others.