Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 6 replies
  • Subscribers 32 subscribers
  • Views 1152 views
  • Users 0 members are here
Options
You may also be interested in
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

High protective conductor currents radial circuit.

wessex
Various guidance shows running an additional cpc as a ring. What are the implications of running two cpc’s, but terminate them as the radial, each cpc being terminated in a separate terminal within a twin earth accessory. Each circuit cpc then terminated separately back at the DB. This would do away with usual steal link on the accessory. Amount of terminations would be the same.
  • 0
    Are you suggesting running two c.p.c.s in parallel through each accessory - so 4 c.p.c.s to terminate at each accessory (except the last) - two into each of the two PE terminals?


    I don't see why you shouldn't do it that way if you wanted, but you would have to make absolutely sure that you were connecting one "in" and one "out" c.p.c. to each terminal, rather than both "in"s to one terminal and both "out"s to the other - as in that case you'd be back to a single loose connection could disconnect all downstream earthing (exactly what the duplicate c.p.c. and twin terminals was meant to avoid). In practice where you'd typically just have a pair of unidentified cables poking out of the wall, that's probably going to be difficult - so just taking an extra c.p.c. direct from the last accessory back to the DB is going to be simpler and easier.


    The steel link on most accessories is not only to join the two PE terminals together (which is necessary anyway if the duplicate c.p.c.s approach is going to remain effective given one loose terminal) - but (for 13A sockets at least) it also provides an earthing facility to the faceplate fixing screws (and hence to the backbox in some circumstances).


       - Andy.
  • 0
    AJJewsbury:

    ...so just taking an extra c.p.c. direct from the last accessory back to the DB is going to be simpler and easier.

    ..

       - Andy.


    Hi Andy, i see that solution being allowed in 543.7.2.201 (ii) (a) and possibly (b) but the start of 543.7.2.201 says it should comply with 543.7.1 and 543.7.1.204 says connections should be made at all points throughout the circuit, so a ring would not be allowed, more like two ladders. There seems to be a contradiction here.

    edit; 1 ladder not 2 :)


  • 0
    I can see having two cpc's per earth terminal could produce a weakness/reliability issue.
  • 0
    says it should comply with 543.7.1 and 543.7.1.204 says connections should be made at all points throughout the circuit, so a ring would not be allowed, more like two ladders. There seems to be a contradiction here

    543.7.1.204 says:

    543.7.1.204 Where two protective conductors are used in accordance with Regulation 543.7.1.203(iii), the ends of the protective conductors shall be terminated independently of each other at all connection points throughout the circuit, e.g. the distribution board, junction boxes and socket-outlets. This requires an accessory to be provided with two separate earth terminals.

    I'd don't read that as requiring both runs of c.p.c.s to visit every connection point in the same order - as long as there are two independent c.p.c. paths back to the DB, one may be 'clockwise' around a ring and the other can be 'anticlockwise' as it were.


    Likewise two PE terminals should suffice even when there are more than two c.p.c.s present. E.g. on a spur from a ring the two c.p.c.s from the spur may connector (one each) to the two PE terminals at the back of a socket on the ring along with the two c.p.c.s of the main ring. I see the basic requirement as nothing looses earthing if one terminal comes completely apart or one wire snaps.


       - Andy.
  • 0
    wessex:

    I can see having two cpc's per earth terminal could produce a weakness/reliability issue.


    Agreed, if it were required by BS7671 to have a continuous earthing (and possibly live) conductor in radial circuits it would mitigate a failed termination causing loss of earthing in another accessory. The conductor is bent and looped into the terminal, thus maintaining continuity. It is a trickier wiring job, looping cables into accessory boxes without cutting the cable, leaving enough slack, but not too much. Note that this would not be allowed by 543.7.1.204 if there was a ring, since each end needs to be terminated in separate terminals, but would be allowed by it if there was a parallel "ladder" arrangement with two CPCs
    2d31e3436a04aadf00703238516e3252-original-cpcs.png


  • 0
    Hi Andy, the ladder arrangement in 1 above could allow 2 or more points of failure and still allow all accessories to be earthed, due to parallel paths, this is not the case with arrangement 2

    It seem the acceptable arrangements for socket outlet circuits provides a lesser degree of safety than those for fixed loads supplied by FCUs, Cant think where else in BS7671 this would be the case

Community Rules & Guidelines