Two phases in one light switch... regulated against?

[itsonly3wires]

just completed an installation condition report on a commercial property...

I've come across a situation where there are two phases present in one light switch and not sure how to handle it. Here is the background:

Building has been subdivided and present wiring dates to 1980. Previously the building was occupied by two separate tenants. One a drama school on ground and first floor and the other a (flat) private dwelling above on the second floor.
The drama school has since taken over the lease of the flat and the two have historically shared a single stairwell.

Building has 3 phases. One phase serves a ground floor retail premesis adjoining the property. One serves the drama school the third phase serves the former flat.

What I have obsevered is a lighting circuit serving the stairwell is wired from the second floor flat's consumer unit. It is switched from two locations, one location in the flat, the other in the drama school.

The switch on the drama schools landing also houses switches for two other lighting circuits that are wired from the drama schools consumer unit.

So we have a location where one switch unit (3 gang, 2 way switch) that has two different phases from two separate CU's.

My question is this...

Should that switch be labelled with "more than one source of isolation" and "415V"?

Should that switch be there at all?

I am in a position to put into place a best practice solution so I'd be interested to know what everyone thinks the best solution might be.

Labelling the switch probably makes this comply but I am curious as to whether a switch rated to 240V is rated to switch 240V or rated to insulate 240V and therefore my 415V phase to phase is outside specification.

 

[Wilko]

Good morning IO3W.
I am uneasy with this as identification of the phases and separate supply may surprise someone. Do they both have their own neutrals - i.e. 2 separate circuits wired to that one light switch ?

 

[Richard Burns]

Multiple phases within a light switch bank arrangement is not a problem, you might also think of any three phase piece of equipment that will be in a similar state.
Adequate identification of the different circuits would be sensible.
There should be no part of the switching where two phases are wired to the terminals of one physical gang as the switch is probably not rated for that, but otherwise it should be OK. A 415V label would only be applicable if the voltage to earth were 415V.

 

[Pete999]

Makes safe isolation a bit difficult in this case doesn't it?

 

[dmxtothemax]

If your worried about the safety aspects ? then affix a label "caution" - two sources of supply ! Such signs are used in Australia.

 

[Pete999]

just completed an installation condition report on a commercial property...

I've come across a situation where there are two phases present in one light switch and not sure how to handle it. Here is the background:

Building has been subdivided and present wiring dates to 1980. Previously the building was occupied by two separate tenants. One a drama school on ground and first floor and the other a (flat) private dwelling above on the second floor.
The drama school has since taken over the lease of the flat and the two have historically shared a single stairwell.

Building has 3 phases. One phase serves a ground floor retail premesis adjoining the property. One serves the drama school the third phase serves the former flat.

What I have obsevered is a lighting circuit serving the stairwell is wired from the second floor flat's consumer unit. It is switched from two locations, one location in the flat, the other in the drama school.

The switch on the drama schools landing also houses switches for two other lighting circuits that are wired from the drama schools consumer unit.

So we have a location where one switch unit (3 gang, 2 way switch) that has two different phases from two separate CU's.

My question is this...

Should that switch be labelled with "more than one source of isolation" and "415V"?

Should that switch be there at all?

I am in a position to put into place a best practice solution so I'd be interested to know what everyone thinks the best solution might be.

Labelling the switch probably makes this comply but I am curious as to whether a switch rated to 240V is rated to switch 240V or rated to insulate 240V and therefore my 415V phase to phase is outside specification.

If I'm reading your post correctly, that you have 1 phase at 1 position and another phase at another switch position, controlling the same light, then it's a wonder there hasn't been a big bang somewhere along the life of this circuit, looks to me as though during the work to make different uses of the building, someone has pulled a fast one withe the alterations, maybe?

 

[telectrix]

think OP meant 2 phases in a 2 gang switch, 1 gang is 2 way on 1 phase and the other gang is on a separate phase for another light.

 

[Pete999]

think OP meant 2 phases in a 2 gang switch, 1 gang is 2 way on 1 phase and the other gang is on a separate phase for another light.

Right thanks Tel

 

[dmxtothemax]

a possible trap for the unwary !
But if he follows correct proceedures then there is no danger,
Only if you don't follow proceedure is there danger !
TEST TEST AND TEST
never assume anything

 

[Deleted member 26818]

In the past when I dealt with multigang switches with different phases, I used to place a 415V label on the switch plate.
As I understand things, this is no longer required, though the practice is still carried out by some.

 

[The Ghost]

C3

 

[Darkwood]

In my opinion there is absolutely no need for any measures to be taken, multiphase in switching is common practice in commercial and industrial, there is no voltage to earth greater than 230v so it does not need a warning label and part of any electricians testing routine would quickly identify 2 circuits so it poses no risk unless they are incompetent or presumptuous ... I have wired plenty a lighting circuits in the manner discussed here, it is the norm and tends to only come up as a question when someone with mainly domestic experience or just learning the ropes comes across multi phases in a light switch.

 

[ruston]

I agree , it is common in commercial and industrial installations.
It is one of the reasons that these short courses fall short.

 

[westward10]

No notice, label required whatsoever.

 

[Stspark]

As others have already stated, i'd have no issues with 2 different phases in the switch.

I'd perhaps however consider putting a simple dymo label to indicate the switch has 2 supplies from different DB's. DB1-2L1 DB2-2L3

 

[Toneyz]

To have different phases in a light switch in a 3 phase installation is common and although the regs state a warning notice if voltages exceed 230v to earth (514.10.1) it is good practice to install label inside switch in fact MK provide one with grid yokes. My issue is with the two point of isolation eg fed from two distribution boards. Therefore I refer to Reg 514.11.1 .

 

[itsonly3wires]

Thanks chaps...

I will need to get the switch labelled up due to the isolation issue.

The problem is particularly apparent due to the switch being located right next to a consumer unit where one of the phases can be isolated.

Obviously there are many instances of 3 phases being switched in one light switch but I've rarely seen an instance where two consumer units in two separate areas of a building supply one light switch.

Thanks again for the words of advice!

 

[westward10]

Doesn't need a notice anyone working on it should undertake safe isolation procedures.

 

[itsonly3wires]

I've always treated 7671 as a minimum requirement.

No harm in safeguarding a fellow electrician... after all that install could test as isolated and no end of locking and tagging would prevent the other consumer unit from being switched on.

I could see myself incorrectly thinking I had isolated the supply.

I think in this instance it would really be necessary to see first hand why there could be confusion.

 

[Pete999]

Doesn't need a notice anyone working on it should undertake safe isolation procedures.

Providing they are proper Electricians Westy.