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I installed a couple of GU10 wall-mounted lights above my bed recently and it all went fine.

Wall mounted light

Tonight, I noticed a perceptible noise coming from one of them while it was off. Call it a hum or buzz, hard to describe, something you might expect from a low-quality transformer, for instance.

I switched the light on and off and the noise disappeared. I couldn't reproduce it. I tried to set the switch in middle position where the light would flicker and even then, no noise.

What would be the reason for this noise?

I don't think the light bulb can be responsible since the switch was open. Could it be a loose wire connection? It is easy enough to open and check the connection, but how could a loose connection explain that noise?

Notice the switch is part of the lamp. Any connection issue around the switch itself would probably be the maker's fault. I only had to plug wires into a connector ("recent" one, no screwdriver needed, like WAGO terminals, I don't know how these are called).

Obviously, once the reason for that noise is identified, I'm interested to know whether or not it is dangerous.


Edit: It occurred again this morning with no apparent trigger action. Switch open / light off.

This time I could set my ear right against the black box to confirm the noise comes from inside it.

It is definitely an electric noise, with a stable frequency.

I moved the arm of the lamp and it stopped and then again I couldn't reproduce.

I'll double-check the connections.

Still, I find it strange. If there was a shortcut, I assume the breaker would open the circuit.


Edit: I opened the box and noticed that a cable on a connection of the switch was a bit worn off, which could be an explanation. I "fixed" this with a bit of duct tape. When I put the current back, I noticed the noise again... but from the other lamp. Acting on the switch didn't silence it but it stopped when I moved the arm of the lamp. I checked the box and there is no worn off cable in this one. I also checked with a multimetre that all connections are fine and there is no short-circuit.

Also I noticed that phase and neutral were switched. In fact consistently switched on all the wiring I did that day (lamps, sockets,...). I just fixed this, wired everything right.

I'll see if the noise occurs again.

I did a bit a research to learn why switching phase and neutral is bad and I don't understand why it would result in that noise. The phase was not cut by the switch so it could reach the light bulb but that shouldn't be an issue unless another part of the circuit has an issue, right ?

(FWIW, I didn't mention this in the question as I assumed it wasn't relevant. The lamps are not mounted in the wall but in a headboard and the wiring inside the headboard converges to a plug that I plug in a wall socket. It was easier this way and there will be no work when I remove the headboard. From a practical perspective it means it is easy to check the connection from the plug to the lamps. It also means I got everything in the headboard (power sockets, USB power sockets, and lamps) plugged to a wall socket. I suppose ideally lamps should be wired to a separate fuse but I assumed it wasn't worse than former situation where all devices, phone chargers, lamp,... were plugged in a power strip.)

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    A bit odd to have a noise with the light off, usually it happens when the power is flowing, light is on. I imagine this just a simple on/off light and no wifi/smart device. I would turn off the power, unplug or breaker off, and check the connections for tightness including the bulb. Off hand I would suspect the switch, or maybe just a flying bug got into it.
    – crip659
    Mar 13, 2023 at 11:11
  • Indeed simple ON/OFF. If it was light on, I'd suspect the bulb. But it was switch open and no light was coming out of the bulb. Also weird it only happened once. Could it be the switch was not perfectly closed and a little energy was passing, enough for the bulb to produce that noise but under the threshold needed for light to be produced? Still, it's weird that it happened by itself during the night without anyone acting upon the switch. And the box is closed almost hermetic so I don't see how a bug could get in. I can double-check the connection.
    – Jérôme
    Mar 13, 2023 at 13:16
  • For a single time with the light off a bug almost makes the most sense. The sound could have been close to the light instead of inside. Electrical hums/buzzes usually are repeatable till fixed and they should be fixed right now. It might be that the light picked up on some odd vibration at the right frequency. Sound can travel and bounce off of surfaces and can be hard to find the exact source.
    – crip659
    Mar 13, 2023 at 13:39
  • @crip659 it happened again. Definitely an electrical issue. I edited the question.
    – Jérôme
    Mar 14, 2023 at 8:13
  • Did the noise return after you corrected the phase/neutral swap? Mar 19, 2023 at 14:11

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I find it strange since it is an GU10 lamp (direct 230V connection, no switching power supply).

Check if you connected the wires wrong (switched hot with neutral)

Update:

Since the issue got resolved by switching the wires: The problem was that upon switching the lamp off, you disconnected the neutral, and not the phase (the hot wire). In consequence the led-lamp, that contains electronics, received marginal power (the 50Hz), even when switched off. The 50Hz AC made the humming sound.

In case of an incandescent GU-10 lamp you wouldn't heard something due to the lack of electronics (the capacitor to be precise).

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  • Indeed, phase and neutral were switched. Thanks for the heads up. Could this explain the noise by itself or is there another issue? (Question edited.)
    – Jérôme
    Mar 19, 2023 at 12:06
  • I just noticed the update. I still find it strange the noise would appear randomly and disappear just by touching the lamp.
    – Jérôme
    May 23, 2023 at 13:31

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