Changing light fixture - How do I confirm the power is off using a multimeter?

Discussion in 'Electricians' Talk' started by Kramer85, Jan 21, 2022.

  1. BiancoTheGiraffe

    BiancoTheGiraffe Screwfix Select

    Agreed Tony...

    If the big red switch is flipped and the lights have all gone off, what's the chances that something is still live?!
     
  2. Tony Goddard

    Tony Goddard Screwfix Select

    Not much, unless the previous owner had tapped into next door through the party wall - I've seen it done!!
     
  3. CeSparky1

    CeSparky1 Active Member

    I bought a house where a builder had run a supply to a conservatory, bypassing the consumer unit, just coming in from the meter... Turn off the whole house and the conservatory was still live, only way to isolate it was pull the main fuse, thats before we even get into the overcurrent protection etc...

    Those lights though need connecting to the T+E in a junction box above the ceiling or via wagos if they will fit in the recessed bit.. Had to fit a few of them recently a customer provided, horrible cheap things..
     
  4. Adamfya

    Adamfya Screwfix Select

    What would you say is a decent tester?
    For diy rather than pro.....as the op in this case?
     
  5. The Happy Builder

    The Happy Builder Screwfix Select

    I have been caught out, I was going to work on a light fitting that I could see from the consumer unit, I turned the light on and turned a light circuit MCB off, the light went out so I started work.

    The light fitting had a permanent live from a different circuit to the switched live and I received an electric shock.

    There was absolutely no logical reason as to why it had been wired like that, but it had and I didn’t prove dead resulting in receiving an electric shock.

    That day I was lucky, these days I have every type of voltage proving device, I can fill a tool pouch with them, the most commonly used are a Megger two pole tester with single pole voltage indication and a Megger non-contact tester.

    But those are not foolproof, because there is the danger of getting an electric shock from the neutral which is normally at nearly zero volts, but is carrying current and will come up to full mains voltage when disconnected, possibly due to an interconnection between two or more circuits commonly known as a borrowed neutral.

    So again really you should retest individual conductors after you have disconnected the individual conductors having avoided touching them.

    I was taught at college by an ex-electricity board electrician if there’s any doubt to short the conductors as the final test as it’s better to sacrifice a pair of long nosed pliers than yourself, on the big cables the electricity board guys use a proving gun that fires a steel bolt through the cable to short it out.

    As a DIYer you should never get yourself into a situation where there should be any doubt as you can turn the whole installation off at the main switch coming into the house, however many installations do not have a single point of isolation and have more than one main switch, so if in doubt turn them all off.

    The danger then comes from people trying to turn the electric back on whilst you are working, twice I have had teenagers trying to turn the electric back on at the main switch because the internet was off as well as the devices in their bedrooms, they were both actually standing on my hop up trying to turn the partially dismantled consumer unit on whilst I was in another room, never underestimate what idiots may try to do.
     
    BiancoTheGiraffe and Adamfya like this.
  6. The Happy Builder

    The Happy Builder Screwfix Select

    Extreme proving dead, for the domestic electrician the equivalent is cutting through the cable with insulated wire cutters, unfortunately I have blown a hole in a pair of wire cutters cutting a live lighting cable, it should not have happened, but it did.

     
  7. The Happy Builder

    The Happy Builder Screwfix Select

    It’s not until you actually blow a hole in a pair of steel pliers or melt a screwdriver you actually realise how much energy is present in the cable, it’s scary when the steel melts and splatters, even in a small cable such as a lighting cable.
     
  8. Comlec

    Comlec Screwfix Select

    It’s always the lighting circuits that get you in the end.
    We seem to have the irrational belief that they are somehow less dangerous.
    But we know it is a tiny current that kills you, so a chamber busbar is just a dangerous a bit of 1mm and a dodgy choc block.
    For me I follow what I was taught decades ago.
    • Isolate (fuse in your pocket, now a fancy lock off kit)
    • Test for dead
    • Alway know where your free hand is (so not holding a steel frame or a water pipe)
    • Search with a closed fist (especially under floors)
    • Don’t touch bare conductors
    And I suppose now we can add
    • Use the PPE
    Like many other still have had to odd shock but nothing serious. Usually as a result of carelessly ignoring one of the above.
     

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