Double Pole isolator switch for Bathroom Light Pull & Extractor Fan?

[Roy22]

It allows isolation of the three fan conductors with a two-pole switch.

That might not be the most important thing in the universe but that's it.

But in the simpler circuit I've just described, flicking the FCU would also isolate those three fan conductors, and leave the pull-switch wiring dead too, when opened. Simpler, cheaper, safer.

 

[JohnW2]

Yes I do understand the diagram, thanks. What I don't understand is what this achieves.

Both EFLI and myself have told you what it achieves.

As you say, the pull switch serves merely as two single pole switches for the two loads, light & fan. No more isolation that a wall switch does a for light bulb.

True, but that switch does not, in itself, 'isolate' the neutral, which is something the manufacturer usually 'requires'.

And this arrangement carries the great gift of a permanent live in the pull isolator switch even after disconnection at the FCU, for the unwary.

Anyone 'unwary' enough to not understand that the supply side of any switch will remain live, even when the switch is off, probably should not be going anywhere near an electrical installafion.

If the FCU had instead fed a simple, single pole switch, the output could have then run to the lights and the fan's switched live input.

It could.

A permanent live for the fan could then have been run more safely/obviously from the input to that simple switch. The fan supply (permanent & switched lives) would have benefited from the FCU's 3A fuse along with the fact this is in turn powered from the light circuit, with 6A MCB back at the fuse box.

If I understand you correctly ... how would you then disconnect ('isolate') the permanent live feed to the fan without also disabling the light?

 

[Roy22]

Both EFLI and myself have told you what it achieves.

No, sorry, this is more complex, more expensive (16A isolator about 3x the cost of a 6A single pole switch); and clearly more dangerous. Remind me what the benefit is?

Anyone 'unwary' enough to not understand that the supply side of any switch will remain live, even when the switch is off, probably should not be going anywhere near an electrical installafion.

Yes but having flicked the feeding FCU, they may have good reason to believe it was then safe. Whereas it isn't.

If I understand you correctly ... how would you then disconnect ('isolate') the permanent live feed to the fan without also disabling the light?

You can't. Fan & lights both double pole isolated by the FCU with the simpler wiring. I'm sure electricians and DIYer's are well used to working with lights off, by torch light etc.

An electrician has separately told me he thinks the existing wiring circuit is 'wrong' in the sense of safety, and that a warning note should be added to the Fuse Box and/or ceiling switch. I think he's right. I'm not looking for an argument here, perhaps we should just agree to differ now.

 

[EFLImpudence]

No, sorry, this is more complex, more expensive (16A isolator about 3x the cost of a 6A single pole switch); and clearly more dangerous. Remind me what the benefit is?

In this case the pull cord switch is NOT an isolator, just a switch and a DP one because that is how the system is wired.

Yes but having flicked the feeding FCU, they may have good reason to believe it was then safe. Whereas it isn't. You can't.

It is safe to work on the fan.
Then they will have made a mistake - people often do.

Fan & lights both double pole isolated by the FCU with the simpler wiring. I'm sure electricians and DIYer's are well used to working with lights off, by torch light etc.

It is not done to work on the fan with the light on.
It is done to disconnect the fan from the electrical installation - isolate - should it become faulty and cause an MCB or RCD to disconnect the circuit so that the house lights (and possibly more items) still work until a repair is made.

An electrician has separately told me he thinks the existing wiring circuit is 'wrong' in the sense of safety, and that a warning note should be added to the Fuse Box and/or ceiling switch. I think he's right.

Add a warning note then.
There are lots of similar situations.
An electrician (albeit retired and another person with similar knowledge) has told you it is not 'wrong'. 2 - 1.

I'm not looking for an argument here, perhaps we should just agree to differ now.

If you want - but the problem is that fundamentally you are not accepting the reason for the wiring being as it is.

 

[JohnW2]

You can't. Fan & lights both double pole isolated by the FCU with the simpler wiring. I'm sure electricians and DIYer's are well used to working with lights off, by torch light etc.

If that is your view, then this whole discussion (about 'clever'/complicated wiring arrangements) becomes totally silly.

If you're happy for the light to be out when working on the fan, you could simply connect the fan to the ceiling rose and then disable (L only) the entire light circuit with its MCB/whatever in your CU - or, if you wanted 'DP isolation', use the Main Switch in the CU.