by ericmark »
Sat May 03, 2008 5:25 pm
In theory if you took 8 spurs from a ring main at the same point it would be no more loading than 8 spurs one on each of 8 existing sockets of a ring main (Although may be not to regulations) but in practice getting four 2.5mm² cables into one hole in a socket is near impossible. One way around this problem may be to change the double socket for two singles ring going in one out of other so with two spurs still only three cables into each socket. Although in domestic ring mains are common in commercial radials seem to be preferred method often in 4mm² on a 30 – 32 amp supply and I have seen where electricians have assumed it is a ring main and used 2.5mm² to extend instead of 4mm² plus of course the volt drop can easily become excessive. Always check the R1 + R2 etc. Easy on a ring Line to Line at 4% and 32 amp <= 0.29 ohms so one measurement shows if it is ring and if you can add to it keeping within volt drop. On a radial you need to find last socket. I sometimes use 230/PSC line to neutral at last socket (= total ohms to supply transformer) – 230/PSC at board = R1/2 but unless you understand keep to method your happy with. Some ELI and PSC meters measure only PSC line to earth and don’t have a switch to check line to neutral and would take longer to make test leads than measure with low ohms ohmmeter. I am assuming you’re an electrician and this is being done at work other wise it would be over the head of most DIY people. And to make out minor works cert you will need meters anyway remember as last person to work on the circuit you will be responsible for the whole circuit so testing is very important to show not only any mistakes you may make but those which may have been made in the past including if the sockets on the circuit could be used to power things outside that there is RCD protection or if missing it is noted on paperwork. i.e. watch your back.
