As long as the AC power grid does not carry too much interference, power line carrier communications (PLC) works very well in homes with single-phase AC. Unfortunately, this is not the case with a 3-phase installation. If the transmitter and receiver find themselves on different phases, they cannot communicate. The only coupling between the phases is actually at the supply company’s transformers, and as the high-frequency signals used for the powerline carrier cannot travel beyond the user’s electricity meter, they never reach the coupling point and so no coupling takes place. In this event, it is necessary to use a coupler fitted before the meter*.
Such a coupler is very easy to build; the circuit involves just four capacitors which form a high-frequency bridge between the phases.
Construction is perfectly simple, but for safety reasons it is vital to use Class X1 capacitors designed for use on 440 VAC grids (e.g. Farnell # 1166428). In theory, the fuses are not strictly essential, but they do offer additional protection in the event of a capacitor’s failing.
The PCB  fits into a case designed for use on DIN rail, which lets you install the circuit into any modern distribution box. The case to use is a 2-module wide Boss type BE350/605T (Farnell # 1171699).
Take the usual precautions when connecting up to the AC grid — after being sure to turn off the main switch, of course! The circuit will work right away. The only problem that may arise is where the AC power line carrier transmitter is connected to phase 3 in the circuit. Capacitor C3 then has an adverse effect on the high-frequency signals generated by the transmitter, as it will tend to short them out. In this situation, the simplest solution is to disconnect the coupler’s neutral terminal connection, which removes this capacitor from the circuit.