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Unusual voltage measurements

vantech

Hi, 

Just moved into a new build and have 2 questions which have me scratching my head. 

The electrician has used through crimps (so he says on the phone), to join a 25mm armoured and meter tails in a metal junction box. He stated this was inside of the meter cupboard on the outside wall, but it isn‘t. I think this is OK as a setup, however the builder has whacked the joint into the cavity of the wall behind the built in meter cupboard so it isn’t accessible - this of course isn’t acceptable. 

The second finding is that, if the main switch in the DB is turned off, I can measure 230V between L&N and L&E (as expected, PME system) on the input side of the isolator. 

What i am unsure of, is that a measurement between the live side of the isolator and the Neutral (output side of the isolator), shows a voltage of 230V. This neutral output, should be isolated by the main switch. My thought is that perhaps voltage is flowing back to earth through electronic devices that naturally leak to earth?  

The isolator is absolutely working as it should, so no issues there. The voltage must be running back somewhere else . 

I haven’t seen this before, so before I call the original installer i wanted to raise it here first. 

Thanks. 

  • 0

    Subject to the usual caveats regarding proper workmanship and materials, there is nothing INHERENTLY wrong with use of crimp connectors on larger conductors.

    I would perhaps have concerns as to whether the SWA is correctly terminated and as to the correct earthing of the armouring.  Also have the meter tails been extended to a length greater than permitted.

    As regards the voltage readings, I do not find the description entirely clear. Are we talking about a main switch built into a consumer unit, AND a separate isolater.

    If a double pole switch is turned of, there should be no significant voltage between any conductors down stream of  that switch.

  • 0

    broadgage: 
     

    Subject to the usual caveats regarding proper workmanship and materials, there is nothing INHERENTLY wrong with use of crimp connectors on larger conductors.

    I would perhaps have concerns as to whether the SWA is correctly terminated and as to the correct earthing of the armouring.  Also have the meter tails been extended to a length greater than permitted.

    As regards the voltage readings, I do not find the description entirely clear. Are we talking about a main switch built into a consumer unit, AND a separate isolater.

    If a double pole switch is turned of, there should be no significant voltage between any conductors down stream of  that switch.

    The tails/armoured is at least 6-8 metres long, protected only by the DNOs fuse. This is wrong, I know, but a 100A fuse in a separate isolator is offering the same protection as the DNOs fuse, also rated at 100A. 

    Regarding the measurement. Imagine a DB. You have the incoming supply on top of a main switch, and the outgoing on the bottom. 

    Board is turned off, voltage between L&N and L&E on the input side is 230V and the output side 0V (as expected). 

    If I measure from the live of the input and the neutral of the output, I get 230V. This is what is throwing me. There is a potential between the L input and what should be an isolated N output. The only logical explanation I can think of is N-E leakage on out going circuits, however all are protected by RCBOs that do not nusience trip.  

  • 0

    The reported voltage readings may be fine. With a 2 pole switch turned off, there should be no significant voltage between any conductors downstream of this switch. This is as reported.

    Testing between the still live INPUT terminal of the switch, and the isolated downstream neutral, should in theory and in a perfect world, show no voltage.

    Back in the real world, there will be capacitive coupling and a little leakage between the isolated downstream conductors and true earth. A sensitive test meter will show this.

     

  • 0

    The rule about needing a separate fuse (even if it's the same rating as the DNO's fuse) may not seem to make much sense, but it's a DNO's rule, and they may get grumpy if it's broken.

    As for the N voltage issue - have you measured the continuity (or otherwise) between N and E on the load side of the DB isolator switch?

  • 0

    wallywombat: 
     

    The rule about needing a separate fuse (even if it's the same rating as the DNO's fuse) may not seem to make much sense, but it's a DNO's rule, and they may get grumpy if it's broken.

    As for the N voltage issue - have you measured the continuity (or otherwise) between N and E on the load side of the DB isolator switch?

    Not yet but i will tomorrow. I would presume if there is no continuity, the voltage would be due to capacitance and electronics connected downstream?

  • 0

    It sounds rough, just for starters I would assume there should be a fused switch between the tails and the SWA.

    Presumably you cannot see how the SWA armour is terminated and earthed?

  • 0

    Sparkingchip: 
     

    It sounds rough, just for starters I would assume there should be a fused switch between the tails and the SWA.

    Presumably you cannot see how the SWA armour is terminated and earthed?

    No, can’t see the joint and this is the issue. I called the electrician who did the install who said “the joint is in the meter cupboard“ but there wouldnt be enough room hence why it’s been stuffed into the cavity. 
     

    Unsure if I should take their word for it, or report to the NIC and get it rectified. 

  • 0

    Basically you need a switch Fuse fitted at the meter end the SWA joint should be accessible for at least the gland. There’s absolutely no excuse for not fitting the switch fuse, the maximum tail length apart from being common knowledge would have been in the provided paper work for the new connection. How is the SWA made off at the consumer unit?

  • 0

    You need to ask the builder first to rectify the hidden joint and missing switch fuse and if that fails involve the warranty provider.

  • 0

    Colin Haggett: 
     

    You need to ask the builder first to rectify the hidden joint and missing switch fuse and if that fails involve the warranty provider.

    Consumer unit end lands into a metal clad box, and then the internal affair through trunking into the DB. This part I am happy with. 

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