What was the fault??

[HappyHippyDad]

I have been fault finding today.

The problem was fairly common. 3 switches (on different landings) switching one light, so 2 2way switches and an intermediate. The problem was that sometimes each of the switches worked and sometimes they didn't, so sometimes you have to go back upstairs and switch one of the switches off so that the downstairs switch can switch the light on etc etc.

I enjoy these faults as it just a matter of drawing out the circuit and seeing where it has been wired incorrectly.

However, this circuit was wired correctly. Nothing fancy, just a basic twin and earth from the light (PL and SL) to the first 2 way switch and then 3 core to the intemediate and another 3 core to the last 2 way switch... and it was all correct!! So I then tested each of the switching mechanisms on each of the switches and they all tested fine. Wired it back up, thinking perhaps just a connection problem - perhaps the terminal was just on the sheaving of the conductor (long shot) and the fault was still there.

I then bypassed the intermediate swith and it worked as it should. Replaced intemediate and the fault returned. Replaced intermediate switch with one from the van and it all worked fine!!

What I cant understand is what could be wrong with the intermediate switch as when I tested between each of the terminal blocks in the switch they showed continuity (as they should) vertically (and no continuity as a cross shape) with the switch in one position and then continuity in a cross shape (and none vertically) when switched in the other position (as they should)??????

 

[R G]

Was the correct switching on the intermediate switch on the strappers of both 2 way switches and the common not switched at the intermediate ?

 

[7029 dave]

Hello Steve, with what you have explained it can only be an faulty intermediate switch, I would have said crossed up wiring maybe strappers in wrong terminals, so not true switching, but if you say all put back as was, then it can only be the switch, despite testing the old switch. Sorted anyway.:banghead:

 

[snowhead]

You tested continuity with a meter?

The switch was in use with a load.

 

[Richard Burns]

You would only need a small smear of grease over one of the contacts, or perhaps a bit of plastic broken off and being flipped about by the switch for there to be an intermittent fault on the switch, if you were testing whilst it was flat on the bench then the obstruction may have moved and then fallen back once the switch was upright again.
An intermittent fault is not usually going to be a wiring issue (except loose connections) , it would be every time you go downstairs there is a fault but it is fine going up or the like.

 

[HappyHippyDad]

Hello Steve, with what you have explained it can only be an faulty intermediate switch, I would have said crossed up wiring maybe strappers in wrong terminals, so not true switching, but if you say all put back as was, then it can only be the switch, despite testing the old switch. Sorted anyway.:banghead:

I agree Dave, It's just puzzling me! :rolleyes4:

You would only need a small smear of grease over one of the contacts, or perhaps a bit of plastic broken off and being flipped about by the switch for there to be an intermittent fault on the switch, if you were testing whilst it was flat on the bench then the obstruction may have moved and then fallen back once the switch was upright again.
An intermittent fault is not usually going to be a wiring issue (except loose connections) , it would be every time you go downstairs there is a fault but it is fine going up or the like.

Sorry Richard, my OP does make it sound like an intermittent fault. What I meant was that the fault is always there , but each of the switches will 'sometimes' switch the light and sometimes not depending on the configuration of the 3 switches at that time. The intermediate switch did have a face plate which had been glued on (rather than snapped into place as it was designed for so perhaps this was intefering with the conductivity)and there was some plaster on top of one the screw heads in one of the terminals but I tested actually on the screw part of the terminal where the conductor would have been (when testing for correct continuity on the switching mechanism).