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Misleading test for dead

Discussion in 'Electrics UK' started by StephenOak, 8 Apr 2021.

  1. EFLImpudence

    EFLImpudence

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  3. StephenOak

    StephenOak

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    In response to "But live must connect to neutral somewhere to complete the circuit."
    which, AFAIUI, contradicts
    In response to "that connection [between live & neutral] is made at the bulb and the switch opens / closes a gap in the live wire"
    Which makes me think that we are talking at cross purposes. After all here the bulb is the load.

    As you have not explained what 'live (correct sense)' means, I am somewhat confused. There is a Live wire and a Switched Live wire at the switch and the lamp does light, so I take it one of those is not a 'live (correct sense)' wire. Presumably the Switched Live is not a 'live (correct sense)'. Is that right?

    As I said earlier, I did not mention wire colour. I did not take the wire colour to mean anything.

    So the way that light switches are wired, the switch is physically in the Line half of the circuit (e.g. at 2 o'clock).
    I was thinking that a switch physically in the Neutral half of the circuit (e.g. at 4 o'clock) would be connecting Line & Neutral.

    What I think you are saying that by definition Line ends at one side of the load and Neutral starts at the other side of the load. Is that right?
     
  4. bernardgreen

    bernardgreen

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    Live--------Switch-----Lamp-------Neutral

    The current comes along the Live wire and gets to the Switch,
    If the switch is OFF then the current goes no further.
    If the switch is ON then the current goes further and travels along the Switched Live to the lamp.
    The current tries to get through the lamp. This is because the current is wanting to get back to the power station and the best way is along the Neutral.

    The lamp doesn't like current passing through it and it limits how much current can get from the Switched Live through the lamp to the Neutral.

    If the current through the lamp is less than the fuse ( MCB ) rating then all is good.
    The lamp prevents unlimited current flow from Switched Live to Neutral.
    If someone connects Switched Live to Neutral without a lamp or other appliance to limit the current then unlimited current flows and fuses blow. Or wires over heat, melt and set fire to things.

    To be blunt if you cannot understand that there is no direct connection between Live ( or switched Live ) in the lamp to Neutral then your understanding of basic electrics is inadequate for you to do any work on electrical equipment.
     
  5. davelx

    davelx

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    Perhaps this will help

    Image-0001.jpg
     
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  6. If that's the case, how come the RCD doesn't operate?
     
  7. davelx

    davelx

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    Because the same current flows in both Live and Neutral. There is nowhere else for it to go.
     
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  8. Yes, I know, Bernards explanation wasn't correct.
     
  9. JohnW2

    JohnW2

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    In what sense was it incorrect?

    Bernard was talking about current travelling from 'Live', through switch and lamp to Neutral - such that the same current would flow through both live an neutral (so that an RCD would not trip).

    Kind Regards, John
     
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  11. He said the lamp limits the current going from switched live to neutral? By which I read the current drawn at switched live is lower than the current at neutral.
     
  12. EFLImpudence

    EFLImpudence

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    The same as your kitchen tap limits the flow of water from the supply pipe to your kettle.
     
  13. StephenOak

    StephenOak

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    Thanks for trying to help. I think the problem here is that all of the electricians are used to expressing things in a certain way (nothing wrong with that, all trades have conventions and use language in a specific way) and when I used words/phrases to mean something different there was confusion.

    As I said, I was thinking that a switch physically in the neutral half of the circuit would be connecting live & neutral, like this
    SwitchCircuit2.jpg
    In that diagram I was referring to the wire labelled B as live, in that if you held on to it and were earthed you would get a shock, but it seems that electricians would not refer to it in that fashion.

    As an example of people using words differently:
    Which (as well as being unnecessarily rude) seems to me to be nonsense. I would take a piece of wire to be a "direct connection", and in an incandescent bulb the electrical connection is just that, a piece of wire.

    Presumably Bernard means something different by some of these words.
     

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  14. davelx

    davelx

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    You should never switch the neutral, always the live. If a normal dwelling has switched neutrals something is very wrong.
     
  15. Doesn't France use switched neutrals?
     
  16. StephenOak

    StephenOak

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    I did not say I had that.

    I did ask if switches were arranged in this way for a good reason but did not get an answer.
     
  17. bernardgreen

    bernardgreen

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    So explain why the "piece of wire " in the incandescent bulb glows white hot yet the other pieces of wire do not even get warm.

    Advice that is given to try and prevent accidents can seem to be rude.
     
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