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Troubleshoot on lighting

Discussion in 'Electrics UK' started by PrenticeBoyofDerry, 30 Jul 2019.

  1. PrenticeBoyofDerry

    PrenticeBoyofDerry

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    Just been to test off a lighting circuit.

    r1+r2 fine at 1.01 ohms
    IR passes at +200 mohms
    Voltage at loop and switch line when closed is 240V
    Zs at the live loops OK
    Zs at switch lines OK

    But I am getting a voltage across the switch line when the switch is open. There are three lights on circuit two are 1way and the third is 2way. The two 1way lights are reading 19v & 17v (these two are 18w 4 pin DD 18w 4 pin DD fittings) The third 2way is reading 40v at the switch line when switch is open. Readings have been taking between switch line, referencing across both CPC and Neutral, still the same reading regardless.

    When the lamp (GLS B22) is installed to the third light lampholder, the voltage drops to zero.

    I have tested the switch line at the switch, also when open getting same readings. I have disconnected the lampholder, made no difference.

    A bit puzzled?
     
  2. Astra99

    Astra99

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    Induced voltages???????
     
  3. sparkwright

    sparkwright

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    Is this a house or flat or what?

    Does it look like a new or old install?

    Just trying to get an idea.
     
  4. JohnW2

    JohnW2

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    Great to see you - I hope all is well.
    As has been said, isn't this probably just 'induced voltage'? Have you tried putting a small load across your meter (or finding an ancient analogue moving-coil one to use!)?

    Kind Regard, John
     
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  5. PrenticeBoyofDerry

    PrenticeBoyofDerry

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    Recent rewire, 4 storey terrace house
     
  6. PrenticeBoyofDerry

    PrenticeBoyofDerry

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    I am good John, how are you?

    I would guess it would be induced voltage, not sure where from though, as the circuit is run independent of any thing else.
    Live feed in shows no sign of induced voltage, when isolated. But switch lines do, never come across anything this high.
     
  7. PrenticeBoyofDerry

    PrenticeBoyofDerry

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    Yeah but I am puzzled where from. Switch lines only, each line is run seperate from the other.
     
    Last edited: 31 Jul 2019
  8. JohnW2

    JohnW2

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    I'm also fine, thanks.
    In my experience, it's often next-to-impossible to determine where induced volltages come from - but if you do as I suggested, you could at least confirm that they are induced voltages (if they are!).

    Have you tried isolating all other circuits, just in case the induced voltages are coming from them?

    Kind Regards, John
     
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  9. PrenticeBoyofDerry

    PrenticeBoyofDerry

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    I will try a load across it later today when I return
     
  10. bernardgreen

    bernardgreen

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    On the safe side use a test lamp or some other load that can withstand 230 volts as an initial check. The "induced" voltage may not be a weak voltage

    ( WEAK a voltage provided via a high impedance source, a voltage that drops when any load is attached to it )
     
  11. DetlefSchmitz

    DetlefSchmitz

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    I'm surprised by this. I would have expected any test instrument used by electricians to have a sufficient low impedance to prevent reading these stray voltages. Mine certainly does. That is why I dislike the advice to site visitors to get a multimeter, as these are invariably very high impedance.
     
  12. winston1

    winston1

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    How about this one?

    'When the lamp (GLS B22) is installed to the third light lampholder, the voltage drops to zero.'
     
  13. JohnW2

    JohnW2

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    Well, at least that advice is 'safe', in as much as it increases the risk of 'false positives'.

    I suppose it depends upon what test instruments one is talking about. All of the current Fluke 'two pole testers' appear to have an input impedance of 200kΩ. That's probably low enough to eliminate (or, at least, minimise) detection of these 'stray voltages' in many/most cases - but Fluke are clearly not convinced of that, since the testers all have a "load button" which, when pressed, reduces the input impedance to 7kΩ.

    Kind Regards, John
     
  14. DetlefSchmitz

    DetlefSchmitz

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    Rather confirms my point.
     
  15. JohnW2

    JohnW2

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    Sort-of - in as much as it 'confirms' that at least one manufacturer is not sure that a 200k input impedance is low enough to avoid picking up stray voltages. I don't know about the input impedance of other makes of such products, nor whether they have 'load buttons'.

    Having said all that, and although I did suggest use of an 'ancient moving-coil meter', if I recall correctly most of them had an input resistance which was described in those days as 10kΩ/V or 20kΩ/V (because the commonly used 50μA meter movements) - so, on a 250V range, that would have corresponding to an input impedance of 2.5MΩ or 5MΩ - not low enough to avoid the issue we're discussing.

    Kind Regards, John
     
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