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🙂
2 way switching means having two or more switches in different locations to control one lamp. They are wired so that operation of either switch will control the light. This arrangement is often found in stairways, with one switch upstairs and one switch downstairs or in long hallways with a switch at either end.
Here we have a two way switching system that utilises two single gang two-way switches and a three wire control, shown in the old cable colours. It is possible to achieve a similar result using a two wire control which, although it saves on cable, is not recommended. This is the preferred approach.
Here we a have a schematic (Fig 1) which makes it easy to visualise how this circuit works. In this state the lamp is off, changing the position of either switch will switch the live to the lamp turning it on. If you now change the position of the other switch the circuit is broken once again.
The schematic is nice and simple to visualise the principal of how this works but is little help when it coms to actually wiring this up in real life!!
Fig 2 below shows how we achieve this configuration. Just like any loop-in loop-out radial circuit, the switch cable from the ceiling rose contains two wires, a permanent live and a switched live. This is cable C below, one wire connects to L1 and the other to L2 on the top switch.
Cable D (Fig 2) is a three core and earth, this is the ‘3 wire control’ that links the two light switches together. COM on the first switch connects to COM on the second switch, L1 on the first switch connects to L1 on the second, and L2 on the first switch connects to L2 on the second.
All earth wires should connect to the earth terminal in the switch back-box and if you are using metal switches there MUST be a loop from this earth terminal to the one on the switch plate (see note A on Fig 2)
NOTE: The blue and yellow wires in cable ‘D’ and the black wire in cable ‘C’ are switched lives and thus should be marked with red sheathing at each end as shown.
Switch drops from a junction box
There is a chance that if your house has these old wiring colours the switch drops may be from a loop-in-loop-out radial lighting circuit done with junction boxes rather than ceiling roses as shown in Fig 2. The switch wiring is all the same but the switch wire (cable C) leads up to a different set up.