I’ve included this method of 2 way switching for reference because you may come across it in old homes but I don’t recommend you use it. If you are doing a new install or replacing a two way switch system go for the three wire control method.
You are most likely to see this 2 way lighting circuit in an industrial/commercial setting where the installation is trunking/conduit based and single core conductors are used.
Downside(s) of the two wire control system
This approach if often referred to as a ‘cable saving method’ because it only needs a two wire control. This is fine when executed correctly but here is what you need to look out for: where this is used in a stairwell where you have a switch upstair and a switch downstairs there is the danger that the neutral and the live come from different lighting circuits. See Fi2 2.
The first reason this is BAD is on safety grounds; say we are working on the light upstairs, so we turn off the upstairs lighting circuit thinking we are safe.. WRONG. The live is picked up downstairs and there are still live conductors feeding the switch upstairs and if someone flipped the downstairs switch in this diagram that live feed would extend all the way to the lamp too (you’re fried baby!!).
If you see this method used in your home ISOLATE ALL LIGHTING CIRCUITS BEFORE WORKING ON ANY OF THEM. If you are not sure ISOLATE ALL LIGHTING CIRCUITS BEFORE WORKING ON ANY OF THEM.
Note: if this method were used in a long hallway where both switches were downstairs and presumably the neutral is not borrowed from another circuit (DON’T PRESUME THIS) then this safety issue would not exist. But read on..
You see this method offered on many DIY sites but the safety issues are rarely explained adequately. Here’s another reason not to do this that I have yet to find on any DIY site:
Induction loops and RF interference
As you may be aware, any current carrying conductor ‘emits’ an electromagnetic field. The good thing about twin and earth cables is that the live and return are always in close proximity (in the same cable) so there is a cancelling effect.
Now consider Fig 2 again, the live feed leaves the consumer unit (fuse board) and runs round the house to the downstairs switch, it then runs upstairs to the second switch, up through the light. The path to neutral may well run around the upstairs lighting circuit before making its way back downstairs to the consumer unit. BINGO, we have just turned our home into a massive induction loop perfectly designed to interfere with all sorts of things:
- Induction loop hearing aid systems
- Radio receivers
- My lovely Stratt (that’s an electric guitar if you are not a muso)
- Computer networks
I though I was done there but while we are on the subject of why this method is not great:
Erroneous tripping of safety/circuit protection equipment
The practice of ‘borrowing’ a neutral from a circuit that did not supply the live may well play havoc with a modern consumer unit that has multiple RCD’s or RCBO’s.
So to summarise, if you don’t want to take a piss in the dark because your wife turned the kettle on downstairs use a three wire control method