Changing light fitting with red/black uk wiring

Question

I have attempted to replace my bathroom light fitting. It has 2 red wires and 2 black. In the original standard light fitting, it has one black in the live and one in neutral. Then the two red loop wires were in the middle fitting see photo:

Now see below photo of the new light fitting. It worked when one of each the black and red were in both the neutral and live. But when I turned the light off, the electric trips, and won't come back on. What am I doing wrong? Note the naked wires are earth.

Answer 1

It worked when one of each the black and red were in both the neutral and live. But when I turned the light off, the electric trips, and won't come back on. What am I doing wrong?

It sounds like you have wired the switch across the light. When you "turned off" the switch you were actually turning it on and creating a short circuit.

The two red wires need to connect to each other, but not to the light fitting. Traditional UK ceiling roses had a dedicated terminal for this, but sadly most fancy fittings don't, so you have to add your own connector. The tranditional option in the UK would have been a peice of terminal block, but push-in or lever connectors from the likes of wago are increasingly becoming a popular alternative. Personally i'm a fan of the wago lever terminals.

The black wire from the switch should be sleeved red or brown and connected to the live terminal on the light, while the black wire from the supply should be connected to the neutral terminal on the light.

There are various ways to determine which is the supply cable and which is the switch cable, but I suspect the easiest for you is to connect the cables to the light one at a time while capping off the other cable temporerally with terminal block or wagos. Then turn the power on and if the light comes on you have the supply cable.

Answer 2

Well, the old junction block was the standard 8-void splice block provided with nearly every BSI-approved UK light. It was wired correctly for what we Yanks call a "switch loop" or switch leg (since UK people do different things with their colors than US).

You seem to have correctly replicated the wiring (in your photo, by the way +1 for a "before" photo)... except the red wires appear to be orphans. The manufacturer of this lamp appears to have "left you hanging" when it comes to splice blocks. Are you sure this light is BSI approved and/or sourced from a domestic EU supplier, such that the CE marking would be genuine?

But yes, once you figure out a Code legal method for splicing red-red, Bob's your uncle. Or should be.

Now, if it is tripping the circuit breaker, the next question is, "Is this an RCD breaker?" Because it may be tripping in RCD mode - meaning it's not >100A of current flow, it's just >30ma of current flow from hot or neutral into ground. Given that you seem to have wired it correctly, that would indicate a faulty lamp.

Temporarily lift the ground wires off the splice block (and make sure they're not touching any metal part of the lamp). Does the RCD stop tripping? If so, that confirms a faulty lamp. Back to the shop it goes. Don't even think about running the lamp ungrounded, it has a ground fault for Pete's sake!