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Double pole switches for lighting

Kenneth Odam

I replaced an old fluorescent light fitting with an LED version - it had an emergency backup facility which required a separate unswitched live.

I took a live feed from an existing emergency light to my new fitting.

Everything seemed to work until I turned off the light - the battery charging light went off and the light came on at low power.

It turned out that the original light circuit had a double pole switch.  When it broke the neutral line the emergency light came on.

I bypassed the neutral side of the switch and the lights all work as expected.

I have never come across double pole switches used for lights.  Is there any good reason for this?

There is a double pole cct breaker upstream for maintenance work, so I don't think I'm creating a hazard.

In the unlikely event that the live side of the double pole switch breaks in the on position, the neutral switch would turn out the light but leave it all floating at live potential.  I can therefore think of good reasons not to use a double pole switch.

Thanks

Ken 

Retired MIET

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  • Also note that unless you can be sure the neutral always makes first and breaks last, that such arrangements can cause spurious RCD tripping, due to that brief transient where the neutral is broken and the line is not, and the capacitance between N and E  charges up to line voltage. The clue is that it is as likely to trip on switch off as switch on, unlike a real fault which is more likely to trip while already switched on.

    In an installation of this type I suspect there is no  RCD to shock into tripping but I have heard of a few domestic funnies where a switched neutral seemed to lead to erratic RCD operation but only at times when it was really hard to verify, but then changing to switched live stopped the problems.
    Mike.

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  • Also note that unless you can be sure the neutral always makes first and breaks last, that such arrangements can cause spurious RCD tripping, due to that brief transient where the neutral is broken and the line is not, and the capacitance between N and E  charges up to line voltage. The clue is that it is as likely to trip on switch off as switch on, unlike a real fault which is more likely to trip while already switched on.

    In an installation of this type I suspect there is no  RCD to shock into tripping but I have heard of a few domestic funnies where a switched neutral seemed to lead to erratic RCD operation but only at times when it was really hard to verify, but then changing to switched live stopped the problems.
    Mike.

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