Old light switch wiring

[Kelrich]

Hi guys
i have an old switch and wish to put a new 1 gang switch in its place, when removing the old switch i had 3 unmarked terminals 1 had red cable in 2nd had black, the 3rd had another black cable innit but this was a seperate cable on its own, is this a switch wire and do i reconnect new switch ie
red- com
black- L1
where does the seperate black cable go
any help thanks in advance

 

[PrenticeBoyofDerry]

Is this a two way switch, does another switch also operate the light or maybe it did once?
And are you sure there are no markings on the switch?
You say the third cable is on it's own, so do we assume the first and second cables are twinned in PVC sheathing?
Also no mention of an earth cable, so what material type of switch are you installing? If metallic and no earth cable this should not be done!

 

[Kelrich]

Hi
yep the red and black come out of a grey sheath no earth and a single black cable, the switch is at bottom of stairs but upstairs switch has never worked, is the seperate black been used as a connection to upstairs switch at some point, switch is plastic thanks

 

[riveralt]

Hi
yep the red and black come out of a grey sheath no earth and a single black cable, the switch is at bottom of stairs but upstairs switch has never worked, is the seperate black been used as a connection to upstairs switch at some point, switch is plastic thanks

You could always buy a single gang two way switch and replace like for like - are you sure there are no markings on the old switch - must be very old if there arn't?
While you have the opportunity you could of course fix the upstairs light at the same time.
If you can provide clear photographs of both the downstairs and upstairs switch wiring that would be very helpful.

 

[Space cat]

If you have a meter, check for continuity between the terminals of the old switch (all wires removed but PLEASE make a drawing first). You should find two terminals that are never connected whichever way you flip the switch. These are equivalent to L1 and L2 on the new switch; the third one is COM.

If you don't have a meter, you can do this test with a battery and torch bulb - but if you're planning to do any more electrical work a simple meter will be a good investment.

PS: The lack of an earth is worrying. Is there nothing else in that two-core cable?

 

[PrenticeBoyofDerry]

As above check inside the two core cable for a bare copper conductor it may have been snipped back and you may be able to reinstate it, but it would only be worth doing if you have earth continuity of the CPC (earth cable) throughout the circuit and back to the board. It is possible that all CPCs in that circuit have been snipped back. If you can not reinstate continuity of the CPC you must avoid using metallic fittings on the light circuit.
It sounds very much like this circuit was/is a two way circuit and you could easily have a loose connection at the second switch or the switch maybe faulty.
So a little more investigation is needed at the second switch, again mark the cables and connection before removal, take a note/diagram for reference.
Also some photos would be handy if you could upload them. So we can have a look.
If there is some confusion over what the cables represent and how the circuit works, it would be wise to purchase or borrow some test equipment, such as multimeter or voltage indicator and continuity tester.