I have recently installed a staircase lighting timer (actually not in a staircase, but still a lighting setup, something for my own use). While discussing this setup with a person and later the timers themselves, i went online to show some of them, such as the Theben Elpa 8 (my favorite, although the one i installed isn't it) :
https://www.theben.de/de/elpa-8-0080002While looking through the descriptions i found out that the 3 wire circuit is not permitted in new installations in the EU. (No such limits in Israel, here both 3 and 4 wire circuits are widely used)
Any ideas what is the reasoning ?
The 3 wire circuit switches the Neutral not to the luminaires, but to the timer module somewhere in a DB, which is permanently live anyway. So the normal "circuit is open but still live" considerations dont really apply in this case
Our electrical code does require switching of L (or switching of N only together with L), but specialised control circuits are generally viewed as exempt from this requirement. In this case, the button circuit is indeed a control circuit, while the lamp circuit is not
To anyone not familiar with the staircase timers :
Those are commonly used in apartment blocks with stairs. There is a push button (momentary on) at each floor. Pushing it switches on the lighting, after a preset time (few minutes) it goes off
The function is implemented by a timing module installed in a DB somewhere in the building
Back in the day those timers were based on a slow-cooling thermal relay (Israeli "Thermion") or on hydraulic mechanisms (something that was described here by Medved in past discussions). Nowadays most are electronic, with few exceptions like the Elpa 8 which is electromechanical
The timer wiring can be one of two circuits (see attachments)
The 3 wire circuit had been used exclusively in the past with Thermions because they can only be wired this way. In the 90s the first electronic timers appeared, some of which required the new 4 wire circuit. Nowadays virtually all timers support both, sometimes requiring setting a DIP switch or similar during installation to choose the correct circuit type
The timer i used (something almost 20 years old, saved during renovations in an air raid shelter 2 years ago) is an electronic unit requiring the 3 wire circuit, and was wired accordingly
