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7 posts • Page 1 of 1
Am replacing a spur and the socket outlet it is connected to. Plugged lamp into it before starting work, flicked mcb = lamp goes out. However, both of my AC voltage detectors still registered. Multimeter showed no voltage. So just to be sure turned off the consumer unit main switch. What could be causing the detectors to react?
There are a number of factors. First question is what earthing system are you using? With a TN-C-S turning off with just the MCB is likely good enough but with most other systems double pole isolation is required. (TN-C-S is also called PME)
Most single connection voltage detectors and also those working with no connection are relying on the surrounding earth to measure against. So if the neutral is not bonded to the earth in a very positive way then one can pick up the neutral which technically is called a live wire.
There are three common earthing systems TN-C-S, TN-S and TT the latter in the main needs a RCD to auto disconnect should there be an earth fault.
There are other factors as well. One is the lights. In old houses they use to use two instead of three wires between two way switches. This can cause an induction loop with could cause your voltage detectors to light.
Thanks for reply. The earthing system is TNS. Additionally, on same circuit, there is a single outlet which I want to move and double up. As cable is not long enough, is it ok to remove socket entirely and use a 17th edition 32a junction box to connect existing cables to block and then run spur from said block to new position (only about 1 foot) in much the same way as running a spur from an existing socket?
The rules are only one un-fused spur from any point on a ring main and it can supply only one outlet be that a single or double socket using 2.5mm cable.
Also the maximum lenght of the spur should be under 3 meters although this is disputed by some depends how one reads the regulations. However you do need to ensure the loop impedance is within the limits. The limits vary according to type of overload type 1 to 4 or B to D according to age of MCB or fuse.
The problem with DIY is people rarely have the meters required to measure loop impedance so just take a chance. That of course means it is impossible to say if what you have in mind would pass. So to keep within the 3 meter rule would make sense.
Also there are permitted routes for buried cables within a wall. Basically vertical, horizontal but not a combination and diagonal is never allowed. And for any new cable and socket RCD protection is required unless you use special cable and socket is marked for one special item like freezer.
Because of these limitations one common way it to replace the existing socket with a RCD fused connection unit and then you can have as many sockets as you like from that. Also the loop impedance will only matter line - neutral and line - earth can be so high with RCD protection it's unlikely to not comply. Also the line - neutral goes from 1.44 ohms using a B32 MCB to 2.42 ohms using a 13A fuse so is also unlikely to be exceeded. Clearly it should still be tested but being in the real world and knowing that's unlikely with DIY then I would say that's a good compromise.
Thanks. Just had a new consumer unit fitted in Dec 2011, mcbs and rcd's 17th edition. Tests (prior to my DIY) all within limits. The extended cable is only about a foot. What I'm not sure of is: having replaced the single socket with an Ashley 32amp junction box and then run the new cable extension from that to a newly positioned double socket, whether the extension is actually a spur? That is: is the junction box (which now has the ring's 2 live, 2 neutral and 2 earth connected to one side of the block and new 2.5mm cable's 1 live, 1 neutral and 1 earth to the corresponding opposite side of the block) just the same as if I had left the socket in position and spurred of it or could I spur off the new socket with its single live, neutral, earth???
p.s. I'm not actually intending to do that, just trying to get my head round the logic of the circuit.
Thanks in anticipation
The English I will admit is confusing as it seems the same thing is sometimes called a radial and sometimes a spur. Also there are fused and unfused spurs. I will try to explain.
The final ring (which may not be the final circuit but is still given that name) goes around in a big circle and so all outlets on that are feed by two cables each rated around 21A and although in theroy if one was to draw the full load very close to the consumer unit it could be over loaded that would be rare and we are allowed to protect to 32A.
From that ring we can branch out taking only one branch from each place the ring is terminated and this must go to just one outlet. This outlet could be a double socket, single socket or fused connection unit. In all cases it is then the fuse in the FCU or plug which then takes over protecting the circuit after that point.
So we can plug in an extension lead with 8 sockets on the end and that's OK as now protected but 13A fuse in plug or we could terminate in a FCU then supply another 8 sockets from the FCU again like the extension lead it's protected by the 13A fuse in the FCU.
A circuit is all after an overload protection device so in theory fitting a FCU means your making a new circuit but it seems the writers of regulations and laws seem to have overlooked that and only consider when it is protected in a consumer unit should we call it a new circuit.
As I say it's the English which causes the problem it's not clear. I call a branch which can take the full load of the protective device a "radial" and one limited to 3 meters with the protective device at the end rather than at the origin a "spur" however that's not how the regulations define it.
As to installation certificates and minor works certificates often they don't have figures written for every sockets but a general figure for whole circuit. So with a loop impedance of 0.35 ohms on the DNO supply and a ring with a loop impedance of 1.1 ohms the total is just on the 1.44 allowed for a B32 MCB. In the centre of the ring you would not be able to take a spur but near to the consumer unit you could take one with full 3 meters.
Since one does not know the route without a loop impedance meter one has little idea as to if you are well within limits or outside the limits so I would say play safe and use a FCU instead of the junction box. Also the FCU looks neater than a junction box both need to be accessible for maintenance only the special maintenance free type can be hidden.
Also if you use a FCU you can add more than one socket but with a junction box your only allowed one. So when ever I can I use FCU rather than a JB.
7 posts • Page 1 of 1