Double pole switches for lighting

It is not common to use a double pole switch for lighting. Reasons to do so include.

The supply once had BOTH poles live, for example between phases of a 3 phase,4 wire system at 127/220 volts, or between outers of a three wire 120/240 volt system.

The polarity of the supply is unknown or liable to vary. Esp on mobile or transportable buildings etc. Or provision for a portable generator.

The neutral was looped via the switch position and someone believed that a double pole switch switch was required.

A history of switch failures, perhaps due to high wattage fluorescent or discharge lamps, alleviated by fitting a double pole switch.

Thanks for the suggestions.

In this case we have a single phase supply to a permanent building built in the early 1980's.  Each switch ran 4 8ft fluorescent tubes - about 400 watts per switch.  As far as I know nothing has been changed since the building was built

The neutral and live all go through the same switch panel, Your comment "someone believed that a double pole switch switch was required" therefore looks likely.  Local folklore suggests that it was built by the apprentices.

I'm tempted to bypass the neutral sides of the switches so that they are effectively single pole.  This would remove the possibility of the live switch failing ON, the neutral being switched OFF and the fitting floating at live.  Can anyone think of a reason why I shouldn't do this?

In the circumstances described I see no need to retain double pole switching of the lights. It is only required in relatively rare circumstances, none of which seem applicable here.

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.

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

In days of yore during my 5 year, student apprenticeship; we spent 6 months in each Scottish steel, tube plant. I was sent on my Jack Todd to fix a circuit in the laboratory of a dated plant; the lighting and power circuits were 2 phase. I remember when the lab manager asked why I was taking so long; I replied that "The fuse is OK but I can't find the neutral". This was my first experience with such 2 phase.

The same plant where they had a working, coal fired, steam driven, mobile crane and a low pitched roof which collected smoke from the numerous coal fires around the shop floors. 

Nostalgia ain't what it used to be!

Jaymack         

Given the updated rules on switching off for mechanical maintenance (which includes changing a lamp) - i.e. just about full isolation is required - I can see an argument that something more than just an ordinary 6A plateswitch might be needed these days if you want to avoid plunging the whole area into darkness just to change one lamp. That said a lot of 20A DP switches aren't rated for isolation either (BS EN 60669-2-4).

   - Andy.