Workshop wiring question

This topic has 12 replies, 10 voices, and was last updated 14 April 2016 at 01:58 by Robin Graham.

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13 April 2016 at 00:42 #234396
Robin Graham
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@robingraham42208
When I had my workshop wired up over a year ago I asked for a dedicated 32A line from the (new) consumer unit in case I needed to run anything current-hungry. The sparks just left the wire taped but unterminated on the understanding that I would wire in a socket as and when needed. Today I tried to do that, but when I cut the wire to length for the position I wanted the socket, the RCD on the (mainly) workshop side of the CU tripped out. Obviously I'd ensured that the MCB dedicated to the circuit was off before starting work. It turned out that there was a steady 9.7V between live and earth with the MCB off. I guess that's more than enough to trip the breaker if I made a short to earth while cutting the wire.

My questions are (a) where has this voltage come from (it seems a lot for coupling to the rest of the wiring, tho it is bunched up) and (b) should I just crack on and ignore it or get the sparks back in? Clearly 10ish Volts isn't going to kill me, but if it's indicative of a fault in the CU I need to sort it.

Regards, Robin
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13 April 2016 at 00:42 #8100
Robin Graham
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@robingraham42208
13 April 2016 at 01:18 #234399
Colin Whittaker
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@colinwhittaker20544
Could be so many things …

My first guess would be that the live and neutral have been confused and it's 9.7V on the neutral that got shorted to earth.

Second guess would be some load after the MCB that is connecting the neutral to live and thus allowing a neutral to live to earth to trip the RCCB.

In either case I'd check whether shorting the neutral to earth trips the RCCB. If it does then everything is most probably working normally.

(Do remember an RCCB compares the live and neutral currents and is normally set to trip if the currents differs by 30 mA. If the neutral to earth voltage is big enough to pass 30mA then the RCCB can trip without the need for any live power.)

By the way … the neutral normally gets grounded at the local sub station. The further from the sub station and the more current you and your neighbours are drawing the larger will be the neutral to earth voltage.
13 April 2016 at 06:40 #234402
andrew briggs
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@andrewbriggs24687
If you short neutral and earth as you cut the wire to lenght it will trip.

You are better turning the power off by the main switch before starting any wiring and using a torch ofif you have a split load consumer unit turn off the half your working on

Andy
13 April 2016 at 08:12 #234410
Zebethyal
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@zebethyal
If your 32A circuit is protected by an MCB, then when switched off, this is only disconnecting the Live (in effect this is just a single pole single throw switch).

When you cut through the cable you most likely created a connection between the Neutral and Earth which is what caused the RCD to trip.

Remember that AC flows both from Live to Neutral as well as from Neutral to live and these can also be used as 2 legs of a 3 phase supply.

If you want to be sure that a circuit is truly off, either switch off the RCD that protects that part of the Consumer Unit or have the relevant circuit protected by an RCBO that will disconnect both the live and the Neutral wires (double pole single throw switch)
13 April 2016 at 08:14 #234411
David lawrence 3
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@davidlawrence3
You sometimes get a small voltage induced into mains cables when they are on a run and lying next to each other, Worse in metal conduit, I had 60 volts once between a dead live wire and earth in a tv studio I was working in, no current just a few volts but possible enough to trip a rcd. Its a bit like audio cables picking up hum from a close mains lead but in your workshop case you cant here it. I have just had a new kitchen installed and the rcd tripped 4 times in 1 morning by the builders just moving cables around which were turned off but earth touched the neutral now and then. I put all my workshop wiring in plastic round trunking, cost about £ 30.00 but it looks great, well worth the bother. regards David
13 April 2016 at 09:14 #234418
Martin Kyte
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@martinkyte99762
Turn all your florescent lights off and see if it (9.7V) goes away.

Martin
13 April 2016 at 10:16 #234441
Ajohnw
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@ajohnw51620
Multimeters often have an input impedance of 10 meg ohms so it's possible to work out how much current would give a voltage of 9.7v other that they can give iffy readings in circumstances like this. The only way to check for shorts is a megger really. Maybe you can hire one but domestic mains testers usually use a lower test voltage and everything needs to be disconnected from the circuit. Even light bulbs.

Neutral can often have volts on it in comparison with earth, might juts be due to voltage drop caused by the current drawn while it's getting to you. The is why live feed and neutral return currents are compared in RCD's.

If it's tripping disconnect everything and see if it still happens. Then connect one thing at a time.

RCD's were available with differing current trip levels as well. I vaguely recollect down to 5ma, which should be ok actually but some domestic items might cause them to trip.

John

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13 April 2016 at 12:19 #234463
Mike Lightfoot
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@mikelightfoot72419
Hi as has been said its a neutral earth thing as you cut the cables, turn off the mains it's the only way, as for induced voltage I used to work in an electric traction shed under 25Kv wires the scaffold we used for changing wiper blades used to register some quite high voltages all scary stuff and to make it worse you can't see the damn stuff
13 April 2016 at 13:13 #234470
Robin Graham
Participant
@robingraham42208
Thanks for your replies, all much appreciated. I hadn't thought to check neutral to earth, but as it turned out there's only a few tens of mV between them. Turning off the workshop lighting reduced the voltage on the live by only 500mV or so. I guess it must be induced voltage I'm looking at though, the cable does run in a whole bunch of stuff supplying the rest of the house. Presumably the reason nothing appears on the neutral is because the MCB is SP (I now know!) so live is floating but neutral is (somewhere) tied to ground potential.

I tried 'shorting' live to earth through 50 ohms, that didn't trip the (30mA) RCD, so there's no signifcant current leakage I guess.

Anhow, I wired the socket up, and it seems OK – the welder for which it was intended runs fine off it.

Thanks again, Robin

Edited By Robin Graham on 13/04/2016 13:13:37
13 April 2016 at 18:45 #234515
Martin Connelly
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@martinconnelly55370
Any use of electricity in your house will result in a current in the neutral wire. It only needs a small resistance in this cable for a voltage to show up between the earth's zero and the neutral. V=IR applies. This small voltage is able to produce a current in excess of the RCD tripping current when the neutral is connected to earth (low resistance). This small current flowing through the earth cable means the current in the live is different from the current in the neutral so the RCD trips. Avoid cutting the earth and neutral at the same time and there should be no problem.

Since you often share a neutral with neighbours their use of electricity means that even switching off all items in your house may not stop this nuisance tripping. As suggested earlier switching off all power with the double pole switch that isolates the live and the neutral at the consumer unit is the safest thing to do anyway.

Martin
13 April 2016 at 23:35 #234546
Phil Whitley
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@philwhitley94135
Something a bit odd here! Go to your incoming mains supply and see if there is a PME (protective multiple earthing earthing) sticker on the cut out. If there is, neutral and earth are bonded together in the supply authority cut out in every house, and therefore neutral and earth will be at exactly the same potential in all parts of the installation, UNLESS there is something wrong with the earth to your workshop. If however you are working on a cable earth which means that your earth is connected to the lead sheath or the armouring on the incoming supply cable, these are well known for picking up stray voltages and not being a particularly good earth, so you would expect some potential difference between neutral and earth. As others have said above, cutting across neutral earth will cause tripping. If you get it all done and still get nuisance tripping it is possible you are getting cumulative leakage from several appliances, which individually are less than the tripping current of the RCD, but when they are all on together, the RCD trips.
14 April 2016 at 01:58 #234552
Robin Graham
Participant
@robingraham42208
Thanks Martin and Phil for further advice. Phil, there is indeed a PME sticker on the incoming mains – I had (vaguely) wondered what that meant, so thanks for explaining. Off this topic, but thanks also for your many suggestions for drying my cellar workshop out – it's now nearly free of damp – an almost miraculous transformation from when I moved in 18 months ago.

But back to the electrics. I think the suggestions that it was a neutral to earth short causing the RCD to trip have somehow become a certainty. I'm not sure that is what happened though. I've just re-measured with a (true RMS) DVM and the results are:

Circuit live – Live-Neutral 246V. Live-Earth 246V. Earth-Neutral 9mV.

Dedicated MCB off, everything else on – Live – Neutral 9.6V, Live-Earth 9.6V , Earth-Neutral 7mV..

The negligible potential between negative and earth fits with Phil's explanation of the PME sticker, so given that it's a single uninterrupted run of cable from CU to socket the voltage between live and earth/neutral must be either induced or due to a fault in the MCB. As far as I can figure it out! Probably doesn't matter, but i'd like to know what's going on.

Robin.

Edited By Robin Graham on 14/04/2016 02:04:59
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