by ericmark »
Thu Sep 30, 2010 8:47 am
The only legal requirement is Part P with domestic and with electricity at work act commercial and although these can be satisfied by following the regulations and the regulations can be quoted in a court of law the regulations are not law.
It is a good job they are not law as there are a few mistakes. As written if you run a separated extra low voltage cable buried in the wall at less than 50mm it needs protecting with either a RCD or special earth encapsulated cable. This is impossible as it is a "separated" supply and so no earth protection system will work. Hence it requires the reader to use some common sense when reading the regulations.
For low voltage (House 230v is low voltage) then sockets less than 20A need RCD protection, (unless marked for something special like fridge) all items in a bathroom including lights, and cables buried less than 50mm deep unless one of the special cables are used. In the main that means everything.
It also says that the electric should be split into circuits.
(i) avoid hazards and minimize inconvenience in the event of a fault
(ii) facilitate safe inspection, testing and maintenance (see also Section 537)
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced
by equipment in normal operation
(v) mitigate the effects of electromagnetic interferences (EMI)
(vi) prevent the indirect energizing of a circuit intended to be isolated.
Now if one can be 100% sure nothing is likely to cause the RCD to trip without there being a fault and if it does trip there will be not danger then one could use a single RCD as is done with the supply to a caravan. However in anything but smallest flat this is unlikely to be the case and one has to make a risk assessment. In my case I decided to lose lights on the central stairs would be a problem and to lose light in the garage where the consumer unit is housed is also a problem so two emergency lights are fitted to remove the danger.
I use two RCD's and plug in MCB's which can be removed to ensure isolation when required. The house has a TN-C-S supply. Had my house had a TT supply it may have been deemed to need another form of isolation.
It is up to the designer to make this risk assessment and decide how best to comply with regulations. He must of course consider cost and I would consider items like lights which are fixed and not changed by the user are less of a problem than the sockets which the used could plug anything into.
So to protect sockets with RCBO's so they do not effect the rest of the house if it should trip makes some sense but I would consider one or two emergency lights would be far better than using loads of RBCO's. After all one can still get power cuts with main supply and no amount of RCBO's will stop that for happening.
As you can see no hard and fast rules. And without looking at your house and doing a risk assessment it would be wrong to say how many circuits the RCD protection needs to be split into. However in most houses it is considered two is enough.
