hi hope somebody can help , want to connect garage to electrical supply but connecting back to consumer unit is not practical. i intend to spur off the existing house ring main which is covered by a 30ma rcd but to avoid nuisance tripping what size rcd spur which will be taken from lounge socket should i use and why ? ( can you tell me how to work this out for future reference please ) many thanks w . soppitt
A 30ma RCD is designed to trip at between 15 and 30ma so next size down would 10ma although because the 30ma does not have any time delay there is no guarantee the 10ma will trip first. Issue 16 Autumn 2005 of Wiring matters see http://www.theiet.org/publishing/wiring ... /index.cfm gives loads of info on wiring out building. Be aware of Part P.
thanks for the advice eric , so basically with a seperate rcd spur there is no guarantee that this will activate prior to the main rcd at the consumer unit so , some might say save the expense and just use a fuse spur . need to project to be completed at the lower possible expense . maybe im missing something that may bring the cost down further , any ideas ?
Further to what I said in first post.
In the main an RCD has some electronics and is therefore reliant on some voltage to work. Where there is a chance of a low voltage which may cause the RCD to fail then an active RCD is used. With an active RCD when the voltage falls below that which it requires to operate it will open anyway so fails safe!
Example a plug in RCD feeding a lawn mower has been cut with a spade at the back of the house i.e. short circuit. The mains into the house is at the front and the socket used at the back of the house is at around the mid point of a ring main fed with a B32 MCB for the MCB to instantaneously trip it needs 160 amp to flow and that needs 1.4375 ohms or less total between lawnmower and house wiring but the combination is higher that that lets say 1 ohm on flex and 1 ohm on internal house wiring so at the socket we have 115 volt at the socket which is 5 volt lower than that required for a passive RCD in the plug to work so only when the MCB thermal trip activates will the power be cut which is well over the 40ms that a RCD should operate within in fact more like 30 seconds using an active type here would reduce that 30 seconds back to the 40ms required to protect personnel.
However since the factor is time rather than amount of leakage if an active RCD plug and RCD also was fitted in the consumer unit both would trip and it would not matter what size they were.
But if the cable to lawn mower had been for example trapped in the door, exposing live conductors unseen by the user, then this damaged cable was dragged across a damp lawn. Now we are looking at how much current leaks to earth and if a 10ma is fitted in the plug and a 30ma fitted in consumer unit then the 10ma will trip first and it would not matter if the 10ma was active or passive as the voltage will have dropped very little.
So where there is a RCD fitted in the consumer unit the second one can be active or passive but where the only RCD is in the plug it should always be a passive type.
But even with a 10ma and a gradual increase in leakage because there may be already 20 ma leaking between all the white goods and IT equipment in the house it still could take out the 30ma in consumer unit before the 10ma in fused spur, socket, plug etc.
Only 100% method is feeding all RCD’s individually.