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Strange Reading

Former Community Member
Former Community Member
Good evening all,


I’m currently working on a domestic new build, it’s a TT system with the meter, Time Delay RCD and Fused isolator installed within a road side GRP kiosk. From the kiosk there is a 50.0mm 3 Core SWA (approx 45mtr run) which terminates into a Sarel enclosure externally and then short DI tails to the DB (split/split) internally. Due to the number of additions a second DB was required which is fed from the first (approx 5mtrs of 10.0mm twin and earth). After testing/livening up the sub-main to the second DB yesterday I used my Fluke (T5-1000) tester as a double check that polarity etc was all ok .... L-N 235V, L-E 234V, N-E 0V .... I have absolutely no idea why I did this next step, but I did, and this is the cause of my confusion. With the main switch on the second DB in the off position I tested between the incoming live and the neutral bar ... 208V, the main switch is definitely double pole and operating correctly so in my head I initially thought that maybe I had a neutral pinched to earth somewhere .... but it was time to go home so I just disconnected the supply to the second DB. Returned to site today, re-tested the five circuits from the second DB, all good ... with the main switch turned off I used the Fluke to measure between the incoming live and each neutral individually, the three circuits that are local to the DB all 0V, the two circuits that leave the room 72V on one, 150V on the other (yet these two circuits have now been tested twice and additionally the cables traced through their entire routes). I then went to the first DB .... (only one circuit connected to one side of the split board, the supply to the second DB), turned off main switch, measured between live incomer and neutral bars 0V on the one with nothing connected, 208V on the one with the neutral to the other DB ... retested the sub-main to second DB ... all fine !!! I’m getting frustrated again just writing this down, anyone experienced similar or have any suggestions please?
  • 0
    Are you saying any potential issue is now resolved, or, if not, are there any RCD devices in use that have a functional  earth connection ( small FE lead from RCD to MET) at any DB and,  or Origin ?
  • 0
    I tested between the incoming live and the neutral bar ... 208V

    I suspect what you're seeing is just capacitive coupling. With the incomer open the outgoing circuits from that DB won't be completely floating, but will be slightly coupled to earth or indeed live conductors of circuits fed from the other DB, by capacitance. Just two wires running next to each other for a few metres is enough to make a tiny capacitor - a few pico Farads - and as as the supply is a.c. a tiny current could potentially flow from one wire to the other even though they're perfectly insulated. Normally it's nothing to worry about - as the capacitance is so small, the currents that could flow will be tiny - normally microamps - so no danger of shock, but with modern high-impedance voltmeters can still see the resulting voltage - which can seem quite high if there's little or nothing on the 'other side' as it were of the potential divider to pull the voltage down. Old analogue volt meters were largely immune to that effect as they had a relatively low internal resistance so would cause the voltage difference to collapse before it could be measured.


    Some meters have a facility of introducing a shunt resistance of a few k Ohms when measuring voltages - exactly because of this effect.


    There's a similar effect of low-energy lamps flickering or flashing when switched off - again due to the capacitive coupling between the L and SL wires.


    Do be careful however - it is possible (if unlikely) that some real (and dangerous) faults could produce the same initial effect (anything from cross connections to other circuits to supplies coming in from other buildings - so be cautions until you're satisfied that this really is the cause.


       - Andy.

  • 0
    This is a case  where a 15 watt pygmy lamp between the 208 volts and earth could prove if its a real voltage or as ajjewsbury says is capacitive coupling
  • 0
    Plus one for the 'floating voltages' probably being an artefact of capacitive coupling, and nothing to worry about.

    Presumably somewhere, the energised and the floating cables run parallel for a bit.

    The impedance of the cable's mutual capacitance is quite high (tens of megohms equivalent at 50Hz) and so the give away is that the volts all vanish when any realistic  load is connected (or if you are very brave, you can see the voltages change at the application of a wetted finger, using the body resistance as a part load Do not do that on anything more than  about 50V.)


    The safer test, as has already been said to measure with a test lamp in parallel  with the meter

    Typically twin and earth cables introduce  between 50 and 100 picofarads per meter of length, between L and E and N and E (varies with cable cross-section and brand of cable)

    A typical modern multi-purpose meter is either 20 or 10 million ohms input resistance, and a capacitance of 200pF is about 16 Megohms at 50Hz , so a couple of metres of T and E is enough to fool a modern meter into reading about half the mains voltage or so, and a longer length of cable gives you progressively nearer 230V There is nothing magical about T and E, and any pair of conductors that are the same sort of size and spacing will give comparable capacitances, and pick up AC voltages one from the other unless there is an earthed shield in the way.

    Mike.

  • 0
    Try a voltage tester that dumps phantom voltage when you press the two buttons or vibrates if it is real.

    https://www.fluke.com/en-gb/product/electrical-testing/basic-testers/fluke-t150

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