I have just attempted to replace the ceiling light fitting in my lounge, and in doing so have ended up with a (partially) dead lighting circuit. I suspect that I have not wired the fitting correctly, and I’ve disrupted the supply to all the light fittings beyond this particular light. This is what I have done:
1) the old fitting was metal, and was connected using a pair of strip connectors – one was fed by a pair of black wires, the other fed by a single black wire. There was no obvious earthing, and no red wires. I did not expect this wiring arrangement (3 black wires only), assumed that the previous occupants had done some dodgy DIY, and decided to do some detective work…
2) …(with the circuit isolated) I connected a length of cable from the wires from the ceiling to the location of the light switch. Using a meter, I thought I had identified one wire to be continuous with the switched live, another to be continuous with the earth, and another not continuous with anything in the switch. This I assumed was the circuit neutral. I jumped to the conclusion that the three wires were somehow from the same cable, and resulted from junction box wiring. This would explain why there were only three wires in the fitting (but not why they were all black!). With much indignation, I assumed that a previous occupant had wired together the neutral and earth.
3) I wired the new fitting to the wires I thought I had identified as L, N and E.
4) After replacing the fuse holder, the whole lighting circuit was dead.
5) The fuse had blown. So I switched off all light fittings on the circuit, replaced the fuse, and switched on all fittings one by one apart from the new one. Now I have one light fitting working (which I assume to be the fusebox side of the fault), and none of the other fittings working (which I assume to be the other side of the fault).
My current theory is that the lounge ceiling fitting was actually wired up loop-in fashion. I suspect the wires that I could see were actually the circuit neutrals (the two wired together) and the switched live (the single black one, with no helpful red flag). I am guessing that the “loop-in” live connections, and the earth connections are hidden in the ceiling void, and presume that these were pushed up there because there was not enough room in the light fitting. So, I reckon that I correctly identified and wired the switched live, but incorrectly treated one of the circuit neutrals as the earth.
I now intend to treat this as loop-in wiring, and rewire the new fitting as in the previous installation. If this does not work, I may try the process of elimination described in ericmark’s useful post of 2 June.
I have two remaining concerns:
A) I really believed that one of the black wires that I now believe to be a circuit neutral was connected to the earth in the light switch. This strikes me as potentially a big problem. But, on the other hand, the old light fitting worked fine before I tried to install a new one. I’ll recheck this when I next work on this lighting circuit. But is there benign explanation for this?
B) If I install the new fitting as before it would not be earthed. On one hand, the new fitting has some metal components so should be earthed. On the other hand, I was blissfully unaware that the old fitting was not earthed. I guess the good news is that the switch cable suggests that there is an earth wire in this circuit. So to complete the job properly, I’d imagine I’ll have to lift a floorboard upstairs to gain access to the earth core and feed this down to the fitting. I can’t imagine the missus being thrilled if I need to pull up the bedroom carpet. What would you recommend?
Any advice gratefully received. And thanks for reading such a long post.
All the best,
Mark.
