DIY Doctor

Why am I Getting a Slight Current on a Circuit Switched off at the Consumer Unit?

Postby duncanrennie » Wed Apr 23, 2014 11:26 am

    I am replacing some broked (cracked) 13amp plug sockets. I have turned off the power by flicking the circuit switch on the consumer unit. The circuit is then dead and I have checked by pluging in a lamp and by checking each wire with an electrical test screwdriver. No glowing light, However when working on the wires a mild electrical shock is occasionally delivered, not even as much as a static seems to come from the earth cable. If I also turn off a seperate supply to the imersion heater (or kill the power to the whole proprty) this stops it happening? Why is this circuit doing this and do I need to be concerned??
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    Postby proptech » Wed Apr 23, 2014 6:28 pm

    Hi duncanrennie

    No doubt what you are finding is induced by other live cables being in close proximity to the circuit cables you are working on. This is a very common situation and it's surprising just how high a voltage that can be produced in this way.
    So,it's very unwise to work on a circuit that is only switched off in this way. Always turn off the main switch.
    Ditch the neon screwdriver, they are unreliable, and can be dangerous. Get yourself a proper two probe tester.
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    Postby duncanrennie » Fri Apr 25, 2014 7:20 pm

    Thanks proptech,

    I'll take your advice, and yes the other cables all run alongside each other.
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    Postby kbrownie » Sun Apr 27, 2014 10:22 pm

    I suggest the neon test screwdriver, is not used as a safe method of testing for voltage and purely used as a screwdriver, they are unreliable and therefore dangerous.
    Purchase and use a two probe voltage indicator, that actually gives you a numerical measurement, this would then offer a better idea of what you have and whether it is induced or if you have a serious issue that requires immediate attention!
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    Postby ericmark » Thu May 01, 2014 4:05 am

    Testing for dead is quite complex it involves using a proving unit and even then many proving units do not ramp so a tester with a 50 volt lamp may well show it lighting with a 500 volt proving unit but could fail with 50 volt and although 50 volt AC may be the point between low and extra low voltage to me it's still too high.

    So in real terms at home we switch on something like a standard lamp and then turn off "Isolator" and check it has gone out then use a neon screw driver in case you have made a mistake a second string to the bow.

    Not correct method but reasonable.

    I like the neon screwdriver as it will alert one where faults like a borrowed neutral exist but I don't rely on it. To be frank if you use a Martindale tester and hold one end of lead and probe with other end it works just like a neon screwdriver yet this is an accepted test instrument. And the contact-less voltage tester will light buy rubbing on a jumper. So nothing wrong with the neon screwdriver as a tool it's only the way some people use it which is wrong.

    But for your question the answer is single pole breakers don't isolate. You need to use a double pole device to isolate and in a house that means turning of all or half the power as only the isolator or RCD will isolate the MCB does not switch the neutral.

    There are some single width RCBO's which do switch the neutral although do not monitor neutral for overload but these are quite rare.

    In our houses we have three types of earth TN-C-S, TN-S and TT with a TN-C-S supply and also to some extent a TN-S supply the earth and neutral are that close to the same voltage that switching off a MCB is normally good enough. However with a TT system you can get up to 50 volt between neutral and earth so it does need an isolator.

    To me any voltage tester should indicate any voltage over 12 volt and have no battery or switch so can't be switched off however the Martindale tester only starts at 50 volt.

    So it's down to a risk assessment does one use a meter which needs a battery which could be flat and also has to have correct range selected or a tester which required quite a high voltage before it works?

    Without a proving unit which ramps up there is always a risk so I would assess using a meter and a neon screwdriver would give one a reasonable chance of not making a fatale mistake. And still be within a reasonable price range.

    There is two ways you could get a shock when the single pole breaker is open. One is induction and capacitive link to adjacent wires the other is more serious something with an earth fault or some borrowed neutral causing either the earth wire or neutral wire to have a voltage above earth potential.

    Where the isolator is used i.e. duel pole this does not switch off the earth so earth faults can be transferred one house to another. Very rare but items like gas and water pipes can transfer faults which is why we bond.

    The whole idea of bonding is not to get everything to earth potential although often it will but to ensure all items which can be touched at the same time are the same voltage what ever that voltage is. Like the birds landing on power cables if everything same voltage then no shock.

    But main point it is NOT a test screwdriver. It's not to test with you use a meter for that. But it is good as it may alert you when you disconnect a borrowed neutral.

    Should explain borrowed neutral I suppose. If for example the upstairs lights are on one MCB and down stairs lights on another when some one wanted to make the stairs lights two way they could take the line from upstairs and the neutral from down stairs.

    With the neutral connected a meter will show no voltage on both line and neutral down stairs but as you disconnect the neutral the line from upstairs vie the bulb will make the neutral line voltage with a 60W bulb 0.25 amps will flow through your body. It is considered 0.030 amps is safe limit so around 10 times safe limit.

    Borrowed neutrals are against the rules but it does happen most common cause is two way lights on stairs. However switching off isolator would stop one getting a shock so idea is always use main isolator rather than just the MCB.
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