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Neutral voltage?

Workshopshed

I recently tripped my RCD when changing a socket face as the live was disconnected via the MCB but the neutral was still connected so it tripped when I unscrewed that. I'm guessing that with the thick mains cables that even a small voltage could still produce enough current to trip the switch.

 

Obviously I'll turn it off at the RCD in future for this kind of job but it did raise a couple of questions.

 

I appreciate that the neutral floats in the middle of the 3 live phases but I thought it was close to earth potential. What kind of voltage difference would there be between earth and neutral?

 

Do the electricity companies have some kind of standard for neutral to earth voltage that they have to meet?

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  • 0

    Andy

    RCD's work by detecting an inbalance of current flowing in the phase and neutral wires.

    Depending on the brand/type it ranges from 10mA (workbenches and medical) to 30mA for normal use.

     

    If the phase is not powered, then there should be nothing flowing in the circuit.

    I'd be suspicious that you don't have a second circuit with the neutral tied or even swapped at the board.

     

    You can check the voltage by adding a second earth outside in a wet portion of earth.

    You should then be able to measure the neutral and earth voltages at the switchboard and even your socket.

     

    The definition of a circuit is a phase and neutral wire feeding a load or loads.

    Disconnecting a neutral (or phase) wire anywhere on that circuit should not trip the RCD.

     

    Mark

  • 0 in reply to mcb1

    Yes, I understand that the RCD works by looking at the difference between current between live and neutral. In my case there is a current flowing neutral to earth and nothing flowing live. I have read that the neutral potential can be as high as 20v above earth depending on the installation.

     

    http://wiki.diyfaq.org.uk/index.php?title=Mains_Voltage

    http://wiki.diyfaq.org.uk/index.php?title=Earthing_Types

     

    Based on that 30mA value I can work backwards. My circuit is at the end of an armoured cable which is approx. 3m long. Estimated resistance from the links below 12.1mOhms/m so 72.6mOhms.

     

    V = IR = 0.0726 * 0.03 = 0.002V

     

    So I've at least this voltage but it could be higher.

     

    http://www.electriciansforums.co.uk/inspection-testing-certification-electrical-installations/57023-resistance-1-5mm-flex-per-meter.html

     

    This site suggests a rule of thumb of 2v or less.

    http://electricalnotes.wordpress.com/2012/07/28/impact-of-floating-neutral-in-power-distribution/

  • 0 in reply to Workshopshed
    In my case there is a current flowing neutral to earth

    That is possible, however the RCD would be tripping all the time.


    You found it tripped when you disconnected the neutral??

     

    the live was disconnected via the MCB but the neutral was still connected so it tripped when I unscrewed that

     

    Is the RCD before or after the MCB.



    It is entirely possible to have current flowing in the earth cables, and is usually when there are multiple switchboards each with an earth stake (house, garage, etc) and the neutral and earth linked in the sub-board.

    It is also possible to have poor neutral connection at the board, but again I would expect the RCD to be tripping all the time, not just when you disconnected the neutral wire.


    The one scenario I know can cause the RCD to trip with no mains, is if the neutral and earth wires are joined after the RCD.

    There is enough current imbalance to cause it to trip (especially on a three phase house), since they expect the sum of the 3 phases to be equal, and therefore no current flows in the neutral wire from the transformer.

    Any phase imbalance will result in neutral current flowing, and since neutral = earth at the supply point, another earth appears as a neutral as far as electrons are concerned.


    It used to be common for electricians to join all 3 wires of a 'dead' circuit together, which caused the fuse/breaker to trip if it was livened.

    The problem was the exploding fuse/breaker, so they resorted to wiring just the neutral and earth.



    The "extra earth trick" will let you measure the neutral and earth voltages, and maybe help pinpoint where your issue is.



    Mark

  • 0 in reply to mcb1

    mcb1 wrote:

     

    In my case there is a current flowing neutral to earth

    That is possible, however the RCD would be tripping all the time.


    You found it tripped when you disconnected the neutral??

    Yes, I was using an inappropriate screwdriver with an uninsulated shaft and believe I shorted the neutral to earth through either the metal casing or perhaps through myself. The RCD does not trip under normal conditions only when I do stupid things. I'll be getting myself an insulated screw driver for future use and also turning off the isolator switch in future.

     

    mcb1 wrote:

     

    the live was disconnected via the MCB but the neutral was still connected so it tripped when I unscrewed that

     

    Is the RCD before or after the MCB.

    At the house end there is a RCD followed by an MCB. At the workshop end there a distribution box with another RCD used as an isolator (which I'd not turned off as I needed the lights on) followed by one MCB for the lights and one MCB for the sockets. The outdoor section is SWA in a 19inch deep trench, an electrican put that in for me and I was adding an extra socket in the shed using metal sockets and more armoured cable.

    Trench.JPGimage

     

    The one scenario I know can cause the RCD to trip with no mains, is if the neutral and earth wires are joined after the RCD.
    There is enough current imbalance to cause it to trip (especially on a three phase house), since they expect the sum of the 3 phases to be equal, and therefore no current flows in the neutral wire from the transformer.

     

    Yes, that's exactly what I believe has happened in this case although it's single phase.

     

    I don't believe that there's actually anything wrong with my circuit just my incorrect assumption that the circuit was isolated when only the MCB in the workshop was turned off.

  • 0 in reply to Workshopshed

    Andy

    Great that you've worked it out and really nice drawing.

     

    While your house is not 3 phase the street is, and conceivably your neighbor might be on another phase, while the transformer is closer to you.

     

    NZ is moving away from the multiple Neutral/Earth situations. Each sub switchboard will be fed with phase, neutral and earth, and tied to the single earth at entry point. (Consumer unit)

    Depending on ground conditions and the supply route, it has been found that the domestic neutral current prefers to flow via the earths back to the transformer, and this can cause electrolysis in metal buildings.

     

    We tend to assume the mains loads are a clean resistive (or inductive) load, but switchmode supplies, Variable Frequency Drives (VFD) and the like are not and are common now.

    Hence what we assume is a 50Hz current could have many high frequency components which have vastly different characteristics.

     

    mark

  • 0 in reply to mcb1

    The diagram is credit to the IET wiring regulations magazine

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