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Why does this switch provide a 56 AC voltage

Discussion in 'Electrics UK' started by Jupiter01, 22 Sep 2020.

  1. Jupiter01

    Jupiter01

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    I was using my 2 pole tester on a light switch and intrigued by why the following is happening.
    Here's the light switch in question:

    upload_2020-9-22_10-14-42.png

    It's the left switch I was investigating.

    The closest cables relate to the switch in question:
    upload_2020-9-22_10-15-57.png

    The light illuminates with the operation of this switch. When the light is off, I get 50 AC on these wires and when I switch it on, I get no reading.

    Is this expected?
     
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  3. oldbutnotdead

    oldbutnotdead

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    50 volts AC relative to what? Is your tester one of those things with a series of LEDs up the side of it?
     
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  4. Jupiter01

    Jupiter01

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    Yes, it's a Fluke 2 pole tester with the LED lights
     
  5. Jupiter01

    Jupiter01

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    one of these:
    upload_2020-9-22_10-33-20.png
     
  6. bernardgreen

    bernardgreen

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    Assuming you are measuring voltage difference between the two terminals on the switch

    One probe on Live and the other on Switched Live

    The Switched Live is connected via the lamp to Neutral

    With the switch OFF the lamp will pull the Switched Live towards the Neutral potential. A filament lamp is low impedance and will pull Switched Live hard to Neutral potential. An electronic lamp is higher impedance and will not pull Switched Live to Neutral as hard as a filament lamp.

    The voltage on the Switched Live will depend on the impedance of the lamp and the impedance of the test instrument.
     
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  7. oldbutnotdead

    oldbutnotdead

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  8. JohnW2

    JohnW2

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    If you mean you are measuring the voltage across the switch then, yes, you would expect to measure a potential difference across it when the switch was 'off', but no voltage when it was 'on' (shorting out your test instrument).

    Were it a high impedance tester, you would expect to see ~230V across the switch when it was 'off'. If the tester is deliberately 'low impedance', you would see less voltage, since the 230V would then be 'shared' between your tester and the load (light).

    Kind Regards, John
     
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