Q & A of the Day - Why is there 126V between the two CPCs in a light fitting?

Question: During the installation of some additional emergency luminaires, the electrician concerned split two (in and out) circuit protective conductors (CPCs) in an existing fitting. After a 'tickle', he measured 126V between the two CPCs. The circuit was isolated. I was informed that this was carried out again on another lighting circuit supplied from a separate distribution board.

The local regional electricity company (REC) has examined its system and reports no faults. We have a monitor on the intake but nothing looks untoward. The premises is a public house and draws about 150A/phase with some hefty ventilation and cooling kit. The installation is less than two years old.

Current Zs is less than 1 ohm, Ze not taken yet. We are about to embark on serious intrusive testing and inspection. Has your team ever encountered this situation/ It's an interesting one...

Answer: This is likely to happening because of luminaires fitted with electronic filters/capacitors. If the circuit was isolated before the CPCs were separated, any potential should have been discharged.

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