by ericmark »
Sun Oct 17, 2010 1:52 pm
The rules tell us anything in a bathroom must have RCD protection since 2008 so any supply will have an RCD which will trip at 30ma within 40ms. We are also told maximum current for cable and for a 6mm cable using Reference Method 100# (above a plasterboard ceiling covered by thermal insulation not exceeding 100 mm in thickness) the maximum is 34A and for a 10mm cable with same reference method 45A. Then we are told that the earth loop impedance (ELI) for a B45 type MCB must not exceed 230/(45 x 5) = 1.02 ohms and with the ELI from DNO often around 0.35 ohms so with 10mm cable being about 0.0044 ohms per meter there is a limit on length as well. Since a RCD will be used it is not so much earth loop impedance as neutral line loop impedance which we need to look at. However although most electricians will not work out each item due to doing so many installations we realise when we are close and will measure where required. Most of the measurements are required as due to being a bathroom we will need to submit the installation certificate or minor works certificate to the Local Authority Building Control (LABC) either direct or if member of a scheme through our clearing house.
Very likely the unit you have seen will do the job. But to just fit without testing would be wrong and since most Electricians can get discount there is no point in you getting it for him. The thing to remember with all jobs is where the tradesman supplies and fits if it goes wrong he has to replace it FOC. However if he only fits then if it goes wrong you pay him all over again.
The LABC charges and the hire charges for test equipment mean it is an expensive option to DIY. Also where mini-consumer units are fitted they often go to a Henley Block which can't be isolated without drawing the DNO's fuse. Even drawing the fuse it is not truly isolated and one has to be very careful.
Of course people will ignore all the rules and will DIY anyway but ionisation of the atmosphere although rare is very nasty and without training you are taking your life in your hands. Just been looking at pictures of hole in floor where my son works and it went wrong. Lucky no one hurt. But be aware any sparks can start ionisation. It has killed people since man arrived on the planet and will continue to do so to my mind where we can avoid it we should. I don't walk the golf course in a thunder storm and I do wear goggles and no nylon etc. working on any live circuits.
