Wiring of télérupteurs
The French answer to the mechanical two way switch is the electronic télérupteur. A relay that does the switching, inside your consumer unit, or hiding in a boîte de dërivation somewhere in your house. Activated by two - or as many as you wish - push buttons.
How to wire these télérupteurs? Two diagrams: 3- wire and 4-wire. The obvious advantage of the 3-wire method is less wiring, and you can do it "local": in the neighboorhood of the switches - boutons poussoir - and the lighting.
But personally I prefer the 4-wire approach, for several reasons. Not for the first time the telephone rang: Robert, il-y-a un problème ici avec l'éclairage. Went over, and finally discovered, after constructing a pathway in the attic with beams and floorboards, an open boîte, et le voila: a suitable case for treatment, a télérupteur with a burnout. The familiar block type, for use in boîtes that take you sometimes hours to discover where they are hiding. But looking at this picture: they have been lucky there, could have started a fire. And they would have noticed it much too late: no early warning smell of something going wrong. That's one of the reasons why I prefer the more expensive 4-wire method, with télérupteurs in the consumer unit. Easier access, and with the benefit of an earlier feedback (smell) if something goes wrong.
Why did it go wrong in this case? Perhaps overheating because of a too high contact resistance, almost loose connection. You should not try to overtighten, but some force required. And in this case possibly a combination of factors: there was also a prise commandée connected to this circuit, not anymore now. Only for lighting purposes, but not everybody knows. It is easy to overload such a télérupteur of only 10 amp, feeding such a special socket. Only for lighting and not more than 300 watt, and not for electric heaters.