Strange wiring to switch

[joniemaximus]

I've recently moved into an 80s built house and i have some questions about the wiring i've found.

I was looking at replacing the downstairs switch to the upstairs landing (2 gang, 2 way) for a white switch like for like. At this point id say the switches for this light work as expected and i'd assumed the left hand switch operated the upstairs light (it did) and the right hand switch was for a garage light that was no longer used as the garage had been converted and new lights added and the switches moved inside the new room (I'd also found what i thought was the remnants of that switch in a junction box in the loft of the converted room).

With new switch in hand, i isolated the supply to the switch, checked that i had, then removed the switch from the wall.

The wiring from the switches i found is as follows:

Landing light switch (Left hand switch):

Com: connected to red cable

L1: connected to blue cable

L2: connected to L1 of the right hand switch using single core grey cable

Mystery switch (Right hand switch):

Com: connected to single core grey cable

L1: connected to L2 of the left hand switch

L2 not connected

I thought L2 from the Landing light switch (Left hand switch) would terminate at the switch upstairs for the landing so i don't understand why it would go through the mystery switch first, but to make it worse i can find no combination where the mystery switch does anything. Why someone would do this i have no idea. However if this is the case i have no idea what is connected to the random junction box i found above the switch (albeit on the opposite side of the wall) which i assumed would go to this switch. There is no other electrics even close that would have been part of the pre-conversion build?.

Now i'm well aware i'm probably going to have to contact a competent person. Just wondering why anyone would do this or if someone could help me understand what is happening?

 

[Geoff1946]

It does sound odd, but there are various ways of wiring two way circuits.

However if you are just swapping the switch for cosmetic reasons what's the issue? Swap the cores like for like to the new switch and it will still work the same.  Be careful. If you mix them up you will need professional help.

 

[joniemaximus]

Thanks Geoff,

My concern is that i could be leaving a live somewhere without knowing where it is going? But like you say everything is working currently. Also im a stickler for understanding, even if i know i can't fix it. i'd still like to understand whats going on and how this might work.

 

[Evans Electric]

It does sound odd, but there are various ways of wiring two way circuits.

I agree  ,  drawing out the circuit as you describe   it it is definitely odd  .       Taking a guess  ...if the 2 way works up & down .... the grey in the  R/H switch must be one of the strappers  as we call them .      I'd guess you could remove the link between the two switches  & move the grey  to L2  on the L/H switch  .   R/H switch would be redundant. 

The 2 way would only work properly while R/H switch was in the correct position  at the moment .      All guesswork of course. 

 

[ProDave]

My guess is you are miss reading the existing switch and what you think is L2 for the left hand switch might be for the right hand switch.  some makes of switch put adjacent switches side by side the same way up, and some put one switch one way up and the other the other way up.

 

[joniemaximus]

The blocks on the back of the swi tch are distinctly separate so i don't think i'm reading them wrong (although i'm not naive enough to know that for sure) but i will check. Although even if one was the wrong way up it would still go through both switches (i've attached a crude drawing) My thought was the same as Evans, that if i operate the switch on the right it would stop the light working when the switch on the left was in the correct position but it doesnt appear to. I'll take pictures of the ceiling rose and both switch tonight to see if that can shed some light on it.

View attachment 20191114_121515.tif

Thank you for your help (and patience)

 

[Geoff1946]

Just a thought. Red and Blue only exist in old 3 core+Earth, (which is red/blue /yellow).

Are you sure there isn't a yellow wire that's perhaps popped out of its terminal as you pulled the unit out?   If there was, it would go in with that L1L2 link and would be starting to look like a conventional 2 way, the yellow being live feed for both light circuits.

 

[binky]

sounds possibly like 'conduit method' for 2 way switching?

 

[joniemaximus]

As geoff had pointed out there was in fact a 3rd cable which should have been in the L2 which makes much more sense now. Still doesnt explain why someone would piggy back of this for another light meaning it could only work based on another switch. Thank you for your help.

 

[binky]

live supply, not unusual.

 

[joniemaximus]

Its not a live supply when the switch is in one of the states though. If it came from the com i could understand it

 

[ProDave]

Its not a live supply when the switch is in one of the states though. If it came from the com i could understand it

There are many ways to wire 2 way switching and one of the most common is link Com, L1 and L2 on both switches.  L feed into say L1 and switched L out to light from L2.  That would give a permanent L at the remote switch which may have been borrowed to switch another light.  This is often where borrowed neutral issues stem from (though more technically a borrowed L)

 

[joniemaximus]

Looking at the ceiling rose with 1 live and 2 neutrals (and 3 Earths) something is going on, the switching cables are probably joined above the ceiling, next time i'm in the loft space i'll have a look. I work in a very logical fashion (to me anyway) so i like to know what is going on with things and why they may have been done even if i have no intention of changing everything. from what i've drawn out and imagined i believe the second live went to a light in the garage and the mystery switch was wired like that to use a 2 way switch as an on/off switch, but i'm yet to find where the cable from that switch terminates. I might have to get my stud finder out to see if i can trace it.