Why does the RCD trip after putting up a light?

Because current is leaking somewhere and not coming back through the neutral. After putting up a light, the cause is almost always a wire pinched between the light and the ceiling, a neutral touching the earth, or a neutral connected to a different circuit. The moment it trips tells you which of the three it is.

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What an RCD measures

An RCD (residual current device) compares the current going to your lights through the live with the current coming back through the neutral. In a sound installation they are exactly equal. If current leaks away somewhere, through the earth, a metal shade or someone touching a wire, the sum no longer adds up. At a difference of 30 milliamps a household RCD switches off, within a fraction of a second.

That also means: an RCD does not trip because your light draws too much current. That is what the circuit breaker is for. If the RCD trips, something is leaking.

Note when it trips

Immediately when you switch the circuit on, even with the light switch off. Then the problem is on a wire that is always connected, even when the light is off. That is the neutral. The neutral is touching the earth somewhere, or a metal part connected to earth. A switch only breaks the live, so the neutral stays connected and so does the fault.

Only when you switch the light on. Then it is on the part the switch turns on: the switched live to the light, or the light itself. A live wire touching the metal shade or the earth, or a faulty driver.

Now and then, especially after rain. With an outdoor light, moisture is the suspect. Water in the light or in the box creates a leakage path to earth.

The three most common causes

1. A pinched wire. When screwing the mounting plate up, a wire gets caught between the plate and the ceiling, or a screw goes through the insulation. The copper then touches the metal plate, which is earthed.

2. Neutral and earth in the same connector, or swapped. In a full box, connectors touch each other or a strand comes loose. On a light with an earth terminal, only the green-yellow wire belongs on that terminal.

3. A neutral from another circuit. In two-way switching or in an older installation, the neutral at a ceiling point can belong to a different circuit than the live. The current then goes out through one RCD and back through the other. For both the sum is off, and that can happen just by making one connection slightly differently from how it was. That is why the photo of the old connection is so useful.

How to track it down

• Circuit off and measure, as in How do you check that the power is off?
• Take the light down and check whether a wire is damaged or pinched.
• Disconnect the light's wires and insulate each wire from the ceiling separately in a connector. Switch the circuit on.
• If the RCD stays on, the fault is in the light or in the way it was connected.
• If it still trips, the fault is in the fixed wiring. That is work for an electrician.

On a metal light with an earth terminal, the earth always belongs on it. How the connection of a wall light and a pendant light should be made is in How do you connect a wall light? and How do you connect a pendant light?

Do not

Keep resetting the RCD to see whether it holds this time. And never disconnect the earth wire to stop the tripping: the symptom disappears, but the leakage current stays, and the protection meant to protect you no longer works.

If you are fitting new spotlights, it helps to know how many can go on one circuit: How many LED spotlights can go on one circuit? New fixtures are in recessed spotlights.