Switch off the circuit in the fuse box, then measure with a two-pole voltage tester between every pair of wires: live and neutral, live and earth, neutral and earth. Test the tester just before and just after on a socket that does have power. Only when every reading is zero is it safe.
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Why the light switch is not enough
A light switch only breaks the live wire to the lamp. The other wires at the ceiling point stay connected to the circuit. If the switch has been wired the wrong way round, it breaks the neutral instead, and the live is still on the lamp while it is off. You cannot see the difference from the ceiling.
That is why every job on a light starts at the fuse box. Switch off the circuit the ceiling point is on. If you do not know which one that is, switch off the main switch. A sticker or a note on the breaker stops someone in the house from switching it back on while you are up the ladder.
The five steps professionals follow
For electrical work, installers follow the procedure in NEN 3140, the Dutch standard for working safely on installations. It has five steps. Three of them apply directly to a light at home:
- Isolate completely. The circuit off at the fuse box, not just the switch on the wall.
- Secure against switching back on. Make sure nobody switches the circuit on again while you are working.
- Verify that it is dead. Measure, do not assume. That is the step this article is about.
The other two steps, earthing and short-circuiting, and shielding parts that stay live, belong to work on larger installations and high voltage. You do not need them for a ceiling light.
Which tester you need
You probably know the screwdriver with a light in the handle. That kind of single-pole voltage detector measures through your own body to earth. On an insulated ladder, with thick soles or on a dry wooden floor, the light sometimes stays off even though there is voltage. For NEN 3140 it therefore does not count as a way to verify that a circuit is dead.
Use a two-pole voltage tester to IEC 61243-3. It has two probes and measures the voltage between two wires. It does not give you a false sense of safety just because you happen to be well insulated.
How to measure at a ceiling point
A ceiling point usually has three or four wires. Brown or black is live, blue is neutral and green-yellow is earth. A second brown or black wire is often the switched live going to the switch. Measure every combination:
- live to neutral
- live to earth
- neutral to earth
- switched live to neutral and to earth, if there is one
Measure even while the wires are still in a terminal block or connector. You do not have to undo anything to know whether there is voltage on them.
Test the tester, before and after
A tester with a flat battery or a broken lead shows zero, even at 230 volts. So first measure on a socket on a different circuit: it should show voltage there. Then measure the ceiling point. Then measure the socket once more. If it still shows voltage, you know the zero at the ceiling point really was zero.
It sounds fussy, but it takes thirty seconds and it is the only way to be sure your reading is right.
What if there is still voltage
If you measure voltage while the circuit is off, another circuit is feeding this ceiling point. That happens with two-way switching, or with a neutral that is shared with another circuit in an old installation. Switch off the main switch and measure again. If it is still unclear, have an electrician look at it before you go on.
If everything reads zero, you can connect the light. How that works for recessed spotlights is in How do you connect recessed spotlights?. For a replacement spotlight or fixture, see the recessed spotlights or the wall lights.
Is a screwdriver with a light in it enough to check that the power is off?
No. That kind of single-pole detector measures through your body and can stay off while there is voltage, for example when you stand on an insulated ladder. Use a two-pole voltage tester to IEC 61243-3.
Is the power off when the light switch is off?
Not for certain. The switch only breaks the live to the lamp, and with a wrongly wired switch it even breaks the neutral. Always switch off the circuit at the fuse box and then measure.
Why should you test the tester before and after measuring?
A broken tester shows zero even when there is voltage. By testing it on a live socket just before and just after your measurement, you know it worked while you were measuring.