Buzzing mains fitting

Guitarist · May 5, 2017

Hi Guys. Had an odd one yesterday... Mains light fitting with filament lamps and no dimmer switch buzzes when mounted to the ceiling. It is one of these fittings with 6 lots of wires terminated centrally, then each line and neutral feeds a separate arm. I can only assume that as I mount it onto its central threaded tube, it is creating some form of solenoid. I finally got it to stop buzzing by rotating it, which I can only assume moved the wires away from the centre. Anyone else had this?

SparkyChick · May 5, 2017

If all the conductors are passing through the same openings, then the magnetic fields created by the line and neutral should balance and cancel each other out.

Filament lamps themselves can buzz. As the mains voltage is cycling, the filament will be undergoing minute expansions and contractions as it heats up and cools down with the voltage. If something in the fitting has a resonant frequency that is a harmonic of 50Hz, it may start vibrating which will make it seem a lot louder than it probably is.

The more lamps, the more mechanical energy is being imparted into the fitting and the louder it will sound.

Guitarist · May 5, 2017

Agree with what you're saying. However, each of the LN pairs passes through a different hole, and the way the thing has been wired makes it impossible to mount it without the wires wrapping themselves around the centre threaded rod. As soon as the fitting is dropped down, the buzzing stops. As you say, could just be resonant frequency related, but be interested to hear if anyone else has come across this.

SparkyChick · May 5, 2017

I speak from personal experience with the mechanical vibration. My parents had a fitting in the lounge for years that would from time to time make noise. It actually used to be used as an indicator that one of the bulbs was about to pop.

In terms of the cable coming through different openings, if the conductors of each fitting are twisted together so they follow one another, the magnetic fields should cancel each other out for that pair. If they aren't twisted together and they follow different paths within the fitting then it may be possible for an electromagnetic affect to occur between them.

It may also be possible that an electromagnetic affect occurs between different pairs as a result of minor variations in the current flowing in each fitting. As magnetic field strength is proportional to current, there could be an interaction between the different pairs.

The other option is that simply when it's not mounted, you've not got a sounding board. When it's mounted on the bracket, and thus bonded mechanically to the ceiling, you have a much larger area that can vibrate and thus it may become perceptible, much like the sounding boards in violins and acoustic guitars amplify the vibration of the strings.