Light Switch Problem

I have a problem that I believe there is a solution to but I'm lacking in the knowledge on how to do so.

There two switches in a building. Both are single pole single throw switches and cannot be changed or replaced.

I need to make these operate like a standard 2 way switch you'd find at opposite ends of a hallway to control a single light.

There are cables for live, neutral and ground, with two travelling wires between both switches.

I've seen somewhat similar questions answered by suggesting the addition of a flip flop switch or a relay, but I'm not confident on how they work so don't want to risk a short circuit.

Would anyone here be able to advise on this? I would gratefully appreciate any input.

• you're out of luck if you cannot replace the switches ... are you even legally allowed to be working on this? Commented Dec 12, 2023 at 2:14
• We'd need a diagram showing the wiring in each switch box and the lamp. 1. Presumably the circuit originally used 2-way switches? 2. And the light works if both switches are in the ON position? 3. And one of the travelling wires is unused at present? Commented Dec 12, 2023 at 8:54
• I can't provide a diagram, but you can safely assume it was a standard 2 way switch original wiring. Commented Dec 12, 2023 at 9:52
• Regards to the legal question, this is not for commercial or residential. Please think of this as just a theoretical problem. I just want to understand a solution before requesting any professional to get involved. Commented Dec 12, 2023 at 9:54
• Effectively I'm looking for an XOR logic device if it even exists? Commented Dec 12, 2023 at 12:11

It's not impossible, but might not be a good idea.

Figure 1. Original, current and future(?) wiring.

Option (c) converts the lines to DC. SW4 will pass positive half-cycles and SW5, negative half-cycles. RLY1 will energise on positive and RLY2 on negative. The relay contacts perform standard 2-way lighting function.

Your technical problem now becomes finding a device that will switch on with half-wave DC current and not chatter violently. Opto-isolators are another option, would be silent and integrate easily with a "smart relay" to do the switching logic.

Your compliance problem is whether or not this is legal and meets local electrical standards.

• Actually I found this item yesterday after searching for the device you suggested. Salus RM16A Hard Wired 16amp Relay. Would that work? Commented Dec 13, 2023 at 9:33
• I have never heard of it. Commented Dec 13, 2023 at 10:42
• Can't you just wire each switch to an SPDT relay, and then treat the relays as 2-way switches, like (a)? Commented Dec 13, 2023 at 11:56
• That's what I'm thinking this would achieve. (Salus RM16A Hard Wired 16amp Relay). Looks to be the right rating. Commented Dec 13, 2023 at 12:10
• @SimonB, only if there are two wires from the second switch to the lamp - and that's unlikely. That's why I asked for a diagram. Commented Dec 13, 2023 at 15:41