Two-way switching - not!

[Alex.]

One for the electrical wizards. My daughter's new house has a couple of two-way light switches that aren't behaving. Switch A turns the light on and off as expected, and when A is on switch B turns the light off and on as expected. But when A is off switch B doesn't turn the light on - not as expected!!
A quick look inside shows that the switches seem to be wired correctly (or what I think is correctly)., ie COM to COM, WAY1 to WAY1 and WAY2 to WAY2. Switch A is also a dimmer, which works fine.
So - faulty switch? faulty wiring?

 

[madkayaker]

wired incorrectly. the feed needs to be taken to one of the strappers and the switch wire to the other strapper. I bet feed has been taken to common

 

[Alex.]

Cheers, I'll check that out.

 

[madkayaker]

or if dimmer has been added on check that cores of cables don't protrude to far through terminal

 

[piman]

Hello MK,

while I agree it must be a wiring fault, I've always wired 2 way lamps with a feed to the common, as per enclosed diagram.

Alec

 

[Robotstar5]

This should cover all the options.....

 

[madkayaker]

Let's not forget the home owner special none of the above! Or using the earth wire as a live cos they didn't have any three core

 

[PTvor]

You'd get the effect you describe if one of the connections between the switches in piman's diagram wasn't there. Any junction boxes on that stretch of cable? They can be codged in because people don't have a single length of cable long enough. Sometimes people don't tighten the screws up properly. If it's an amateur job, there may be a connection made by doing something nasty, such as twisting the wires together and wrapping tape around it.

I've seen a couple of incredible messes created by people who really haven't a got a clue trying to install two way switches, and codging it with whatever they have to hand.

Another possibilitity is a bad switch. It should be easy to test the switches by taking them out and using a meter or battery and bulb.

The thing to bear in mind is piman's diagram, which is basically the way two way switches work.

 

[brightspark]

all modern wiring on 2 way lights are wired the 3 wire 3 plate system . a live and neutral feed to the ceiling rose. a twin and earth down to one switch live down and switch live return . and a 3 core and earth from 1st switch to the second wired as per diagram the old method used single and earth cables and a twin and earth between switches . verry inconvenient as u would need a neutral feed taken off the same circuit as the live feed

 

[madkayaker]

thats not very accurate many new builds are wired with the neutral connected behind the switch due to down lights and not wanting to wire three plate ceiling roses. that does cause issues with interference with WIFI etc but thats another issue

 

[brightspark]

thats not very accurate many new builds are wired with the neutral connected behind the switch due to down lights and not wanting to wire three plate ceiling roses. that does cause issues with interference with WIFI etc but thats another issue

the only reason down lights are wired with the live and neutral behind the switch and a switched feed out to the lights is so theres no joints in the roof space and u can only get 2 wires in the conection box on them a feed in and a feed out to the next light and so on RADIAL citcuit