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Lighting circuit fault?

Discussion in 'Electrics UK' started by kirbett, 12 Feb 2005.

  1. kirbett

    kirbett

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    I've just tried a 9W energy-saving bulb in one of my house light fittings, and discovered that when it is switched off, it flashes briefly at about 2 second intervals. A normal bulb exhibits no malfunction (seemingly).

    The light in question is a 2-way switched light on a landing. The cable from the 2-way switches connects to the mains power and to the cable to the bulb at a junction box in the loft. There is no "gizmo" (dimmer, security timer, etc) on this particular light, although I do have such on some other lights in the house.

    With the power off, I have measured the (normal) resistance of the bulb circuit and the switch circuits at the junction box and at the switches. All circuits that should have been open-circuit were. There was no indication on my multimeter of any continuity across the cores of the lamp or switch cables or to earth.

    However, with the power on, with the switch-live-return disconnected at the junction box, and with the switching circuit open, I can measure a voltage (of about 20V AC) between the switch-live-return and both the live-to-the-switch and earth.

    Suspecting the switches, I completely disconnected the "outermost" switch and replaced the "innermost" switch (which was a bit old) with a new switch in 1-way mode. With the switch-live-return still disconnected at the junction box and the new switch open, I could still measure about 20V across the switch connections.

    Suggestions? Any chance this could be due to some harmless "induced" voltage? I'm hoping so, because we were changing the light bulb (and lamp shade) only after having just finished more than a couple of months work stripping, sanding, filling, repairing, and repainting the landing and stairwell ...
     
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  3. fido

    fido

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    Have you tried the bulb in a table lamp to see if it still does it?
     
  4. Damocles

    Damocles

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    Digital meter? There have been several posts recently about flickering fluorescent bulbs due to capacitative coupling between cables. Does your switch cable have an earth, and if so was it connected when you made the measurements? you are measuring the voltage on a floating wire closely in parallel with one at 240V ac and one at 0. So good knows what reading you might get using a very migh impedance meter.
     
  5. kirbett

    kirbett

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    I've tried the bulb in other upstairs light fittings and it doesn't seem to flash. Also in a table lamp attached to a long extension lead.

    My meter is a trusty old analogue Micronta Multitester. The switch cable does have an earth, but the earth is only connected at the junction box (not at the switch).

    Hypothesis - the switch cable in question is the longest switch cable in the house when measured from the junction box to the "outermost" 2-way switch - at least 11 metres. Maybe the low wattage bulb uses a rectified circuit to build up a striking voltage? In which case, could the capacitative coupling across the open switch-cable have enough umphh in it to build up enough voltage in the bulb over two seconds to cause to it to try to strike?
     
  6. kirbett

    kirbett

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    OK ... I've gone back over the forum looking for other references to capacitive coupling. They seem to confirm my hypothesis. To quote but one (from mapj1)

    "I'd like to add to the comment by Felix, this is absolutely true, as far as it goes, except for 2 way switch circuits where depending on the internals of the three core and earth, the capacitance to ground can be ~half that of the capacitance between the two cores that are both on the same side of the earth.
    The attack of the flickers on a properly earthed house are then usually only seen on the landing light circuit where the 10m of three core plus earth is about a nanofarad to live, and 500pF to deck..."

    Only question remaining is shouldn't the packaging on my Philips Ecotone be updated to read "not suitable for dimming, electronic switching, photocells, timing devices, dusk to dawn sensors, and two-way switched landing lights"?

    Panic over ... on with the decorating ... :D
     
  7. felix

    felix

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    No doubt about it; it's capacitive coupling. If it wasn't a two way switched light I would say one of your earths was faulty. Here's why.

    In a flat twin and earth cable there is virtually no coupling from live to switched live because the earth wire is in the way - UNLESS THE EARTH IS FAULTY. Check this first.

    In a three core and earth cable this doesn't apply. Only one core is shielded by the earth wire. The other two are coupled. This is of no consequence with filament bulbs but if the cable is very long you could get flickering with a small fluorescent bulb.

    There are two ways to wire up a two-way lighting circuit. (Actually, there are three but one of them isn't safe.)

    The modern system connects all three terminals of one switch to the matching terminals in the other one. Live power goes to L1 and switched live comes from L2 - or the other way round, it makes no difference. There is no way to eliminate the capacitive problem from this set-up.

    The old system had live power going to COM in one switch. L1 and L2 were connected to matching terminals in the other switch and switched live came from its COM. You can convert your switches to this arrangement with the addition of a single piece of choc block. Remove both live feed and switched live from their terminals. Connect live feed to COM instead after removing the wire that was in there. Use the choc block to connect switched live to the wire removed from COM.

    To eliminate capacitive effects from this circuit make sure that the wires between L1 and L2 are on the same side of earth in the cable. Switched live will then be shielded from them by the earth wire.

    PS: In case anybody's interested, that third two-way circuit has both L1's connected to live and both L2's connected to neutral - or the other way round if you prefer. The light goes between the two COM's.
     
  8. mapj1

    mapj1

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    The way to stop the flicker, without wasting energy in a parallel resistive load, is to fit a mains rated capacitor in parallel with the lamp, reducing the 'divider ratio' of the leakage capcitance to below the lamp striking voltage. A polypropylene of 100nF or so should do - I'd look in maplin or the CPC catalogue. Consider class X rated devices as these will be OK accross the mains indefinitely. (at a push a motor suppressor type would be an alternative.)
    regards, M.
     
  9. slimwire

    slimwire

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    This is a very old thread.
    Anyway, can anybody help post a diagram of the 'third way' of wiring a 2 way switch that is unsafe?
    I read the above but don't quite understand why it's unsafe.
    Thanks.
     
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  11. bernardgreen

    bernardgreen

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    I found that two way system in a couple of houses in the distant past. More recently I came across it when the house owner mentioned that the light switches on the stairs needed replacing frequently and often tripped the MCB. When the switch was operated the arc between the fixed contact and the moving contact continued until the moving contact landed on the other fixed contact. The arc was now crossing the 3 mm gap directly between Live and Neutral and maintained itself until the MCB tripped or the switch burnt out

    In the old bakelite and ceramic two way switches there was a large gap between the two sets of fixed contacts. Often they were two separate make switches with a common toggle. The moving contact was far enough away from the fixed contact that the arc collapsed before the moving contact reached the other fixed contact
     
  12. JohnW2

    JohnW2

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    Do you think it was done deliberately by someone who believed it was an appropriate way to do it? On the face of it, quite apart from the problem you go on to describe (which, as you say, may not have been a problem with the switches of yesteryear), it seems no less dangerous (at least, on 50% of occasions) than having a switch in the neutral.

    Kind Regards, John
     
  13. bernardgreen

    bernardgreen

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    The danger is that both sides of the lamps can be Live when the lamp is out. This could be a hazard to anyone who assumes that because a known good lamp is out then there is no Live in the fitting.
     
  14. JohnW2

    JohnW2

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    Of course - hence my question!! As I said, it appears just as dangerous as switching the neutral which presents the same hazard.

    Kind Regards, John
     
  15. John D v2.0

    John D v2.0

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    yes it's not as dangerous as exposed live parts, but it's up there with reversed polarity and switched/fused neutral.

    Also as it turns out, through the various switch states you can have a borrowed live and neutral with this setup, ie the light could be powered off downstairs live and upstairs neutral, or vice versa!
     
  16. JohnW2

    JohnW2

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    Indeed - which is why I was wondering (hoping not!) whether it was ever regarded as an acceptable practice. I have to say that until endecotp mentioned it a while ago, it had never occurred to me that anyone would arrange (or ever would have arranged) 2-way switching in that manner!

    Kind Regards, John
     
  17. slimwire

    slimwire

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    Thanks for the diagram. I can see why it's not safe. It pays to be careful to switch off the breaker even if one is changing bulbs.
     
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