Mornin' All,
Imagine this scenario. You have just installed a new R.C.B.O. consumer unit in a 1970s bungalow. The supply appears to be TN-S. But may be TNC-S in reality. There are two lighting circuits each supplied separately by an individual B6 30 mA R.C.B.O. The hallway is an "L" shaped design with two ceiling pendant lights. There are two two way switches at the ends of the hall way, but between them are two intermediate switches as well. Four switches in total control the two pendants together. All cables are in good condition and the insulation resistance is very high between L and N and all Lines. There is evidence of rodent infestation in the loft.
When just the front door lamp is fitted the light works and switches correctly from all four switches.
But when the second (remote) lamp is fitted as well and turned on by any switch, both R.C.B.Os trip off.
Why?
Z.
