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Two way+intermediate domestic light circuit extraneous voltage head scratching

Colin Stares

Hi my house is supplied via T-T system, the pole mounted mains transformer is 100mts away,

the measured Ze is 120ohms

on a two way+intermediate circuit I’m measuring 60-90vac when the light is in any off position,

I have disconnected all circuit wires and it all measured clear, the circuit is blowing led lamps and fluorescent lamps are pulsing when in off positions, any ideas!!!

Parents

  • what is the input impedance of the meter you are measureing to 60-90v with - note that 3c/earth cable has 50-100pf/ metre of core to core capacitance, and that is in effect in shunt with the off switch, so the voltage never falls to zero.

    If a conventional resistive filament lamp causes the voltage to fall to zero, you could consider adding 0.1uF or so in parallel with the lamps so that in the off state there is ~ 100:1 capacitive voltage division.
    Mike.

Reply

  • what is the input impedance of the meter you are measureing to 60-90v with - note that 3c/earth cable has 50-100pf/ metre of core to core capacitance, and that is in effect in shunt with the off switch, so the voltage never falls to zero.

    If a conventional resistive filament lamp causes the voltage to fall to zero, you could consider adding 0.1uF or so in parallel with the lamps so that in the off state there is ~ 100:1 capacitive voltage division.
    Mike.

Children
  • in reply to mapj1

    Hi Mike, I received my capacitor today and fitted it as suggested and the voltage has dropped to 3vac, I think it’s a result and all is working ok.

    regards

    colin

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