Why do self-contained luminaires glow in BS, which are switched via the L'?

Due to technically unavoidable line capacitances, interference voltages can be coupled into the L' line from 
neighboring current-carrying conductors. The electronics built into our luminaires are insensitive to common 
disturbances. However, if the line capacitances/interference voltages become too high, the luminaires may glow.

This behavior can be avoided by installing an interference suppression capacitor of type X2 with a capacitance 
of 100 nF between N and L'. (see connection diagram below)
Capacitors with connecting leads for direct installation, e.g. in a switch box, 
can be obtained from specialist dealers. See e.g. at Conrad