Why does my ceiling fan make little intermittent lightning flashes?

We have a ceiling fan in our bedroom. I think it’s a Hunter, but I’m not positive. Anyway, the controls for the fan itself and the lights are separate. We usually run the fan at night to cool off the room, but obviously the lights are off.

Sometimes, very intermittently, the fan will suddenly make a little flash, almost like a quick lightning flash. It doesn’t come from the lights (at least I don’t think so–since I can’t predict when it will happen, I usually see it out of the corner of my eye). It’s very faint, and very fast.

Any idea what it might be?

I wonder about the setup here. So you’re in bed, presumably trying to fall asleep, with the fan (but not the light) on. Why are your eyes open to see this?

Are you rubbing your eyes when you notice it? I discovered phosphenes in just that way…dark room, where is that light coming from? Scared me, I’ll admit.

The blades do build up a static charge. That’s why dust likes to stick to them so much. I’ve never heard of them building up enough to discharge a spark though.

Are the blades reflective ie glossy or are they dull (eg unpolished wood)? It could be that every now and again a light somewhere else from say a passing car or something - outside the room - glints off a blade.

Are you able to tell if the flash is from the center (motor) or a blade?

A more worrying possibility is that something is sparking. Is the flash accompanied by a click or tick sound?

It seems to be up near the ceiling. No click or tick. Just a really quick flash of light.

They’re slightly glossy.

Nope, I know what you mean by that, and this isn’t it. It’s definitely a flash from somewhere around the fan.

Motor or blades?

Up near where it’s attached to the ceiling, I think. It’s really hard to say for sure because I haven’t gotten a good look at it. It’s really fast, like a lightning flash.

That sounds a bit worrying to me. The cables pass through there. It could be they have worn their insulation and are intermittently shorting and causing a spark. A spark will usually make a tick or click but an intermittent brief flash in an area where there are cables is potentially disastrous and I would get it checked, noise or no noise.

Yikes. I won’t be able to get it checked for a while, and I doubt I could reproduce the problem with an electrician present. Would it be safe if I didn’t run the fan until I can get it checked?

BTW, I have noticed this intermittent flash for quite some time, though it’s very rare. I only recall seeing it a handful of times over the last couple of years.

Hmmm, I think it would be safe if left switched off, assuming power is cut to the fan when it is switched off at the wall. That is highly likely but not a complete certainty. I’m not familiar with how fans are wired in the US.

I don’t think your inability to reproduce the problem with an electrician is present is likely to be an issue. On opening it up, either the insulation will be intact or not. If it’s not intact then the cable should be replaced and if it is intact then it wasn’t the problem. Flash or no flash.

There are people on these boards who know far more about this than me. I hope I’m remembering correctly that @CookingWithGas is an electrician or similar?

I should add, I might well be jumping at shadows here. It’s pretty rare for a mains short to develop, and for it to be so intermittent. But I can’t think what else would produce a very short bright flash at the point where a ceiling fan attaches to a ceiling.

Not me. I do household electric work and have a clue about code, but not a pro. There are folks here who are experts but I can’t bring a name to mind at the moment.

Do you have pets? Pet hair can build up on the motor winding and cause such shorting to occur. My living room fan shorted out and had to be replaced due to what appeared to be a build up of dust and pet hair over a span of ten years.

Haunted. Try a EVP recorder.

Good thought. We have several cats. Is this something that can be cleaned (by a professional) if it occurs? I hope we don’t have to replace the fan. It’s a nice one and I really like it.

It would be interesting to vacuum the fan near the ceiling and see if you pick up cat hair.

I wonder what kind of motor your fan had.

A problem motors have to solve is keeping in contact (and conducting electricity to) with something that’s rotating. One way we’ve solved this problem is with a brush motor. The rotating part of the motor is in contact with flexible brushes made of a conductive material that touch the rotating bit.

Over time the brushes can get worn down leading to intermittent gaps in contact; when such a gap occurs, electricity can jump through the air and over the gap, leading to tiny arcs and flashes.

My understanding (and I’m certainly no electrician) was that brush motors are generally used in things that aren’t on for super long periods of time, like power tools (and it’s quite common to see arcs fly when using a power drill or Sawzall) because something like a fan (that could be on for hours or days or even weeks at a time) would wear down the brushes too quickly leading to arcs; but a quick Google search reveals that brush motors are common on fans.

Whether these sparks are normal in your type of fan or a sign of worrying deterioration I will leave to the experts, but the worry would be that these arcs damage the brushes even further, and if the contact gets bad enough you could have more intense arcs that lead to more serious damage or even fire. Again I don’t know how likely this is in your case, though.

The motor casing for certain ceiling fans has little open slits to let air circulate. My first thought was something sparking inside the motor casing that can be seen through the vent slit in the dark. Could be worrisome, could be harmless, but probably best to keep it turned off until it can be professionally examined.

If it turns off by a wall switch that would be fine.

Interesting? Cat hair gets everywhere. I have to clean my fan blades at least once a month to clear out the car hair. I’d be more than surprised if no cat hair was found.

Brushed motors are universal motors:

These are typically used for situations which need to deliver high torque when they move away from their target speed. You find them in a lot of consumer-grade power tools and appliances (shop vacs, corded handheld drills, handheld mixers, wood routers, etc.). Two disadvantages is that they are noisy because of the brushes riding on the commutator, and of course as you noted, the brush wear.

Ceiling fans have used induction motors for a long time, but lately brushless DC motors are being used more.

Neither uses wear-prone brush/commutator systems, both types are quiet, and neither produces arcs during normal operation.