Humming Balast

Had a past client call this evening about a fluorescent light mounted under a kitchen cabinet that is making a humming sound.

I told him the ballast was probably worn and needed replacing. His concern was if it was an immediate fire hazard. His specific words were “Is it gonna blow up?”

I told him that I didn’t know if it would blow up or not, but I wouldn’t feel comfortable having electricity running through **any **device that I suspected was worn. Since this light isn’t needed every day, he agreed not to use it until fixed.

I was wondering though, what does it look like when these stop working? Does the light simply not function or is there “collateral damage” that occurs.

Thanks in advance,

Loose is a better guess. I doubt it is bad. Some of the old magnetic ballasts hum their whole life

The humming is caused by loose laminate plates in magnetic ballasts.
Not likely a hazard but very annoying if it’s too loud.

They should change out the fixture or replace the ballast if they can’t accept the noise.


I’ll bet it’s loose also. If not, recommend replacement.

No danger at all in humming ballasts.

the other thing with the mangantic floursecent ballast when they get bad they will overheat and of course they will buzz like sonvagunner escpally this is true with 8 footer slimline and HO’s {highoutput}]

Merci, Marc

From / ( no, they don’t make breaker panels :smiley: )

Transformer noise is caused by a phenomenon called magnetostriction. In very simple terms this means that if a piece of magnetic sheet steel is magnetized it will extend itself. When the magnetization is taken away, it goes back to its original condition. A transformer is magnetically excited by an alternating voltage and current so that it becomes extended and contracted twice during a full cycle of magnetization.

This extension and contraction is not uniform, consequently the extension and contraction varies all over a sheet.
A transformer core is made from many sheets of special steel. It is made this way to reduce losses and to reduce the consequent heating effect. If the extensions and contractions described above are taking place erratically all over a sheet, and each sheet is behaving erratically with respect to its neighbor, then you can get a picture of a moving, writhing construction when it is excited. Of course, these extensions are only small dimensionally, and therefore cannot usually be seen by the naked eye. They are, however, sufficient to cause a vibration, and as a result noise.
The act of magnetization by applying a voltage to a transformer produces a flux, or magnetic lines of force in the core. The degree of flux will determine the amount of magnetostriction (extensions and contractions) and hence, the noise level.
The obvious question is why not reduce the noise in the core by reducing the amount of flux. Why? Because it is not that simple.
Transformer voltages are fixed by system requirements, and the amount of magnetization, by the ratio of these voltages to the number of turns in the winding. The decision on what this ratio of voltage to turns will be, is made for reasons, mainly economic. It means that the amount of flux at the normal voltage is invariably fixed, thus setting the noise and vibration level. Also, increasing (or decreasing) magnetization does not increase or decrease the magnetostriction by the same amount. In technical terms the relationship is not linear. Therefore, when we are asked, as we invariably are,– “can you reduce the noise level at the source?” – the answer is that it can be done, at a cost and for not much improvement in noise level.

When I was a kid at home having supper the ballast in the light above us blew up. Don’t remember if there was a hum before the blast or not. Have also saw lights hum and then just quit. My advice Play safe.

If I knew what was going to happen to these things I would not be working for a living.


If 1,000 guys started today replacing every humming ballast, they’d all have work for the rest of their lives.

A humming ballast can mean a ballast is on it’s last leg, but not necessarily. The catestrophic failure of a ballast, while certainly not unheard of, is completely unrelated to any prior humming.

Thanks all for the replies.

Marc, would you just wait till it goes out to replace?

The humming sound is caused by the electromagnetic effects as described above. So, why does an ‘old’ ballast hum when a ‘new’ one doesn’t? In most cases it is the result of rust or other corrosion that has built up in between the core laminations. The same phomonenon occurs on the armature of relays or contactors, making them hum as well. In those cases, you can often reduce or eliminate the hum by lightly sanding the mating surfaces to remove the corrosion. In any case, the buildup causes the metal surfaces to be unable to come completely together; if left unchecked, it can eventually reach the point that the relay coil will burn up because the coil draws much higher amperage with the armature open than when it is closed.

I would wait until it fails, unless the humming was bothersome.