OK, as professionals, we know that you aren't supposed to use a dimmer on a fluorescent lamp. Problem is, that the general public does not know this. With CFLs being all the rage, it's just a matter of time before we start seeing more and more stories like this:

http://www.wtop.com/?nid=598&sid=1395955

I'm sure that the package containing CFLs states that you should not use them with dimmers, but honestly, who reads the packaging for a light bulb? This could be a huge problem.

Not only could be ... it WILL be. With luminaries such as Al Gore making blanket statements about there being dimable fluorescents, I expect we will see a lot more of this.

I have to wonder quite how this could happen, and if it indicates a basic design shortfall in the item involved.
There is no indication whether the dimmer or the lamp was the source of ignition, but in either case a safe failure mode should have been designed in. (eg thermal fuse)
After all, if the device has the ability to overheat to ignition point there could be other faults leading to this, without someone mixing incompatible items.

Geoff, I believe harmonics are to blame. The dimmers are designed to power simple resistive loads and contain transistors that switch on and off very rapidly to dim the lights. The CFLs, on the other hand, contain capacitors which are designed to operate at 60Hz, but are now subjected to much higher frequencies, at which they offer a far lower resistance- essentially becoming a short circuit to ground. These components are super-cheaply designed already, but with the dimmer, they overheat and blow even quicker. The extra current through the dimmer would cause it to overheat quickly, too. A thermal fuse would help immensely in the dimmer, but I don't believe it would do much good in a CFL. Better solution would be an inductor designed to trap the higher frequency harmonics. But this might only give the illusion of being able to work on a dimmer, and still cause problems.

Honestly, consumers have every warning- it's clearly labeled on the CFL package, and clearly labeled on the dimmer package, and 99.9999% of the time, the CFLs just die quickly with no fire, and the consumer learns his/her lesson, and spreads the word to their friends. The Darwin awards exist for a reason.

All the labeling in the world may reduce but not fix it. As electricians, do we read the packaging and the safety warnings before installing something?

I agree. I've put in thousands of panels and GFIs and assure you that I've never read the instructions or warnings. The sad thing is that I just realized that in my own bedroom, we have CFLs in both lamps since they have flimsy paper shades. I did this because I was afraid that the heat of a standard "A" bulb would scorch the shades. One of the three switches in my 4-way setup is a dimmer! We never dim them, but I honestly never even thought about it. I sure will now.

Good point,
This is why it is important for electricians to keep up with the latest and greatest.

There are fish-hooks in installing any new item to the market in somebodies house, etc.

I had an instance of these CFL's nearly catching fire a few years back in the house next door to where I was living at the time, but these weren't on a dimmer.

Just on the subject of dimmers vs CFL's, I recently installed some CFL's for a guy down the road, I told him that I would have to remove the dimmers from his lounge lighting circuits, because the two do not agree with each other.
The guy could not seriously believe that you could not dim a fluorescent lamp!
(I realise that they can be dimmed, but the control gear price is really only for Commercial installations)

There are a number of CFLs that are advertised and marketed as being useable with dimmers. As they are relatively expensive ($15 each) and I don't have any dimmers in my house, I haven't tried any of them.

I am carious what their listing states about them dimming.

Here's an example of one from GE, that says it is dimmable on a standard incandescent dimmer:

http://genet.gelighting.com/LightPr...adCrumbValues=&ModelSelectionFilter=

Knowing full well that dimmers and CFLs don't mix, I tried one on our 13-lamp chandalier because, well, 13x40W is a lot more electricity than 13x15W for less light and I wanted to convince my wife we should be using CFLs, which she normally hates. Also, it's a cheap 6 for $10 wal-mart CFL so I didn't mind if it burnt out in a week as has been my experience with our other dimmer and the occupancy sensor in the den.

Inexplicably, it's not only worked, but worked GREAT. 12x40W incandescents with 1x15W CFL- the CFL is brighter, dims better, consumes less electricity, and has lasted over a year now. With the dimmer the whole way down, the incandescents are useless, but the CFL is giving off a niec bright glow still. And it gives much better color of light (it's got these faux candle covers that normally makes the room look orange.) I'm hesitant to try it with more than one CFL without yanking the dimmer, especially now that I've actually looked at the circuit and what I did here, but it really does surprise me.

British Gas, (a major retailer of electric power, - due to the UK "free market"), is presently advertising that it is about to deliver four CFLs to each of its customers.
Who knows what make or type these may turn out to be, or whether anyone will read any instructions before using them.
That could stoke the flames a bit!
I'm actually with BG, so I'll report back when mine arrive, if this thread is still running.

Be careful though. Just because it works does not mean it is safe. You can put a fire out with gasoline but...

It is like call a DYI'er an electrician. "hey it works. What the problem?"

I've taken apart a CFL and they contain a rather crude switching power supply.

Three things or combination of things could be happening:
Capacitor is shunting excessive harmonics
Main switching transistor is overburdened
Main rectifier is overburdened with the harmonics (cheap ones cannot handle much more than 100Hz)

Switching PSUs will attempt to maintain output regardless of input...and dumb ones do it until the switcher fails due to high current! Such failures are often violent with the transistor actually exploding and leaving in some cases just some half melted leads on a scorched board!

The above poster is correct, filtering the harmonics will not work unless the power supply is designed to properly respond to a low supply voltage. It will still fail, it might take longer to fail, but I'd be surprised if it lasted longer than a day or so.

All of these potential failure modes could be prevented by simply adding a thermal fuse and then mounting it to the main switching transistor. Since the thermal fuse will also blow if subjected to excessive current, you can protect against the diode and cap failure mode too as they will typically fail shorted and pop the fuse.

Two related notes:
At the theatre I work for we use CFL's for the house lights on an old CD80 Strand dimmer rack (well, used to, the Iowa floods killed the rack and the switchboard). They dim quite nicely, minus the minimum dimming percent (cathode heating). We plan to flip them out, once we get back into the building full time, as the old R-40's burn out. For those that don't work with them, the Strand rack is a semi-standard wave-chopping dimmer system.
An odd use I found for dimming a non-dim is breaker locating. In the same building as above we have WAY too many panels (it was built in 1928 and remodeled four times). I needed to find a breaker to switch out two dead dimmers with three-way switches. I found the breaker by dimming the CFL's, which caused the panel to hum. Once I found the panel I killed breakers until the lights died. (I obviously verified no power before I worked on it, I don't trust this building).

The reason that dimmers don't work with CFL's is because of the way they modify the phase angle of the input voltage.

Having said that, a standard fluorescent lamp requires a strike initially to get the gas in the tube to ion-ise.
This takes a bit of current to achieve, more than what a light dimmer made for incandescent lamps can provide, this can kill a dimmer, especially if set low when first switched on.