I must say that I read it differently. When the OP wrote ....
... I assumed that he was talking about the one switch which is currently there, and that he does not know what it does - and, having not said anything about it, he may not even have considered the possibility that there was once a second switch there.
I refer the honourable gentleman to the thread title and the first line of post #1.
Do you mean the same "very dodgy things (and/or serious lacks of understanding)"? If so then yes, of course, they do - but that is expected/understandable in relation to some DIYers, but is neither expected nor desirable in relation to those who describe (and 'sell') themselves as 'electricians'.
Perhaps not, but it does satisfy my curiosity - nearly! This inline switch would be used as a secondary switch? It doesn’t need anything else? Or I guess it is an isolating switch as would break whole circuit?I have quite often put a switch in-line,like illustrated here - carefully removing a section of the sheath of the cable, leaving the neutral intact and cutting the 'live' and interposing the switch between the two cute ends ....
View attachment 381760
It looks to me as if that may well be what is going on with the cable on the right, together with the one switch you currently have. It also looks as if exactly the same may have once going on with the cable on the left, at which time there was a "2-gang" switch, with the two cut brown conductors (now in a connector block) going to the second switch.
However, none of that helps you to know what the existing switch (or the second one, if it were once there) actually does/did
Well, as I said, it doesn't help us/you to understand what the present switch 'does', now what a second one there in the past may have done!
It's not the normal way in which light switches are arranged, but it can be a simple and convenient way of doing things in some situations - if the power come 'in' in one of the cables and the other cable is connected to the light, it will 'work' - and, indeed, is, electrically speaking, no different from any other way of wiring in a light switch - in all cases the neutral from the supply goes straight to the light and the 'live' from the supply goes to the light via a switch (so that the light can be switched in and off).
Huh. I never realised that thank you!
Hence the encountered 'borrowed neutrals' as found from time to time.
Possibly, but I think it's much more common for DIYers to 'discover' the presence of pre-existing 'borrowed neutrals' (usually when they install RCDs) and then don't necessarily understand the problem or what needs to be done about it - rather than that they necessarily create the 'borrowed neutrals' in the first place.
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