Last week I've put lights in the garden. There was already a wire coming out from a switch in the house so it was an easy job. Since there is a door at the other end of the garden I though it would be nice to have an extra switch there. So I've bought a cable with two wires to use as L1/L2 and wired it to the 1st switch, following this schema from here
I managed to connect everything and tried a few times and both switches were working fine.
After a couple of hours, I've switched on a light in the kitchen and the box power went off. And there started the madness. I found that there was power on the ground wire running to the lamps. I've checked all connections thinking I had mixed it up somewhere. I've disconnected ALL ground wires and there is still power: between two lamps there is a standard twin & ground cable and I've left only L and N connected. If I probe the ground there is power. (I only have a Neon Screwdriver for testing). This is for ALL the wires. I thought (although improbable) that all the wires melted shorting L and G but N is fine and I would expect such an event to mess everything.
Even more weird, I've found there is power on L2 when both switches are on L1. Thinking of a problem in the new switch, I've disconnected the L2 wire running from one switch to the other and found there is still power on it! It's not connected to anywhere, neither end, but when I put the switch to L1 that wire has power.
Tomorrow I'm going to buy a tester to check if the wires are actually shorted. Anything else I should check?
UPDATE: I've just checked the wires and there is no short-circuit. Yet when the circuit is closed, if I switch on another light the RCD trips. The current on the "disconnected" wire is around 45V, so definitely not a short-circuit. Probably just capacitive coupling as suggested by ThreePhaseEel. Would this be enough to trip the circuit?
UPDATE 2: I've put everything back as it was and now works fine. I tried all switches in the house and the circuit doesn't trip. I've ensured no naked wire pokes out of the junctions. Maybe it was just that.