Touch operated lamp

[Tall Paul]

I became involved recently with a small fire in a bedroom involving a touch operated lamp.  The fire was localised to the lamp, the wooden drawers that was sat on and a set of wicker drawers in a metal fram alongside.  The only ignition source in the area was the lamp.  The lamp was 'off' prior to the fire being discovered.  The damage within the lamp was extensive, with fire damage to the top surface of the wooden drawers, and the top drawer of the wicker set.

There was no evidence of candles, T-lights, smoking materials in use.

The lamp was overhanging the timber surface by a couple of inches and was in contact with the metal fram of the wicker drawer set.

This raises the question of a small earth current operating the lamp or setting up a repeated on-off contact...  But it is just a question.  The local trading standards office are going to run some tests on this basis.

Has anyone experienced a similar occurance?

[Wiz]

I became involved recently with a small fire in a bedroom involving a touch operated lamp.  The fire was localised to the lamp, the wooden drawers that was sat on and a set of wicker drawers in a metal fram alongside.  The only ignition source in the area was the lamp.  The lamp was 'off' prior to the fire being discovered.  The damage within the lamp was extensive, with fire damage to the top surface of the wooden drawers, and the top drawer of the wicker set.

There was no evidence of candles, T-lights, smoking materials in use.

The lamp was overhanging the timber surface by a couple of inches and was in contact with the metal fram of the wicker drawer set.

This raises the question of a small earth current operating the lamp or setting up a repeated on-off contact...  But it is just a question.  The local trading standards office are going to run some tests on this basis.

Has anyone experienced a similar occurance?

Obviously there is electronically operated circuitry in the touch switch which could fail and other components overheat.

Also watch out for operating voltage. These types of lamps became very popular in the U.S. some twenty years ago. These were manufactured to work at 110V ac. If it is a model converted for use in Europe then a voltage adapter might be in use (i.e 240V down to 110V). Could this have failed and introduced a higher voltage that allowed the touch switch circuitry components to overheat?

[John Webb]

Tall Paul,
A few questions:
(a) what material was the body of the lamp made from?
(b) what was the lampshade (if any) made from?
(c) what type of bulb? (ie wattage, voltage, halogen, long-life....)
(d) could something have been draped over the light (eg handkerchief) and overheated?
(e) any earth connection or was it double insulated?

[Mr. P]

I had one of those things.  By touching anywhere on the metal base the lamp comes on and each touch will increase brightness then finally off again.  The base never got hot but the glass shade did shatter.  The unit would make a buzzing noise too, sort of resonance humm.

[Tall Paul]

John,

The base was metal, the lampshade a stiff thin fabric, the bulb a standard 60W bayonet (not halogen), nothing draped over (confirm verbally and by the minute amount of ash surrounding lamp - just sufficient for shade), The lamp was earth connected.

Interested to hear about voltage adaptor Wiz, I'll throw that into the testing team.

By way of further info, the lamp was returned to manufacturer who, having run a series of their own tests, confirmed that the lamp had not malfunctioned.

[John Webb]

Only thing that occurs to me is that the lampshade caught fire for some reason and dropped burning chunks onto the top of the drawers etc. which damaged said surfaces. Not very much to go on if lamp was earthed and still working.

[Mike Buckley]

I assume that the lamp was rated for a 60W bulb.

Could there possibly have been a fault in the current regulating mechanism? If I remember my physics correctly (it was a long time ago) you can vary the current to the bulb by varying the resistance in the circuit. As the current passes through the resistor it creates heat.

As a second thought was the cable to the lamp trapped between the wooden drawers and the wicker set. I investigated a fire some time ago where an office had trapped the power cables to a computer between a wall and a stack of photocopy paper, although the fire was minor you could see the track of the cable burnt onto the surface of the wall.

[John Webb]

Mike,
The touch-lamps use electronics to drive triacs to 'chop' the current to reduce the voltage to the bulb; these only have a few watts to dissipate normally, but if there is a fault in the electronics there could be overheating from that.

[Tall Paul]

The lampshade was of such thin material that had it ignited and fallen from the frame it would not remain alight for long, nor would it transmit much in the way of heat energy once at desk level.  The cable showed only external fire damage and no signs of pinching.

I am happy with the slow burning of the upper section of the wicker drawers. I am happy with the degrees of pyrolisis to a nearby magazine, the level of charing to the desktop, the smoke patterns and the heat differential indicaters on the body of the lamp.

I am not yet able to put to bed the hypothysis that the metal frame and the lamp had some part to play.  I am not an electrical engineer, but it would seem logical to me that a very small current transfer or voltage differential would take place, presumably to earth, when a human hand touches the lamp base in order to turn it on.  My real question is, could the same effect be generated by an inadvertant contact with a metal frame?  Until I can put this one to bed, it remains one of the options in the bag.  The other options I can approach under the normal process.

[kurnal]

John- does the triac actually hold the lamp off and is the touch the trigger to the bias on the triac to switch it on?
I have never looked at how these things actually work but if we are relying on the bias on a triac to switch its state I would suggest theres great  potential there for a fault condition to arise- very much the tv in standby mode scenario.

[John Webb]

The triac is a bi-directional switch which requires a few mA/Volts to switch it on. The electronics to do this senses the start of each half-cycle of mains voltage and delays the switch-on pulse so that part of each half-cycle does not reach the lamp - thus the average voltage reaching the lamp is reduced and the lamp is dimmed. (Often refered to as "phase control".) The electronics adjusts the delay in accordance with the controls. Ordinary dimmer switches have a rotary control, the touch ones sense the length of touch and control the light according to the way they've been programmed - it does vary a bit with each maker.
 There is usually a small ferrite inductor in series with the lamp to reduce radio-frequency interference. Both this and the lamp will tend to emit noise when the current starts to flow in each half-cycle, giving rise to the 100Hz 'hum' often audible when the lamp is dimmed.
I'm not certain how the control electronics senses the touch, but I suspect that it is not any leakage current as such, since a body in bed is going to be quite well insulated. But the change in capacitance between a small metal body on its own and when it is touched by a much larger conductive human body may allow the means to detect the touch. So if the metal frame and the lamp were touching, then this may have altered the capacitance enough to make the controls think the lamp was being 'touched'. The result would presumeablely have been the lamp running continuously through its control programme. This might have led to overheating of the electronics; almost certainly the bulb would have on a lot.
Hope this may help.

[Tall Paul]

Thanks John.  I'd like to play with this a bit more:
When a normal light switch is being turned on slowly, there comes a point at which the light flickers between on and off because the contact has not been fully made.  Could such a state exist in this scenario, increasing the activity within the control programme beyond anticipated limits?

[John Webb]

Rather unlikely as there is is no mechanical switch as such, as I understand from your description. It's all 'solid-state' electronic switching in this case, I think.
Wall-mounted switch dimmers with knobs do have a mechanical switch, but this usually operates only when the bulb is out, or almost out, and the current flow is very low. The ones I've used have never had mechanical problems of intermittant contact.

[kurnal]

What is the probability of a fault with the electronics of the lamp causing the fire?
We are looking at a solid state device (triac) switching full mains potential. Theres no mechanical switch to cause arcing but theres a number of electronic components presumably on a printed circuit board. Not only may the triac have developed a fault but dry joints on the board can cause arcing especially where theres coils or capacitors or mains potential involved.
Was the body of the lamp metal? And if so was it earthed or double insulated? Could there have been a fault on the neutral side of the lamps wiring to the socket?
Could the metal frame of the wicker furniture have been earthed?

Dont want to start any hounds running  cos I am a total  amateur  who just dabbles in terms of solid state electronics   I could imagine that if the power control circuit, in the event of an open circuit neutral connection instead was faced with mains potential across its trigger circuit this could cause some pretty intense stress across components not designed to deal with this in normal circumstances.

Hey if this is complete piffle I dont mind being told so!

[John Webb]

As I understand it the electronic components are chosen to accept the voltage that is expected to be applied to them. If a fault develops then one or more components may then be subject to voltages above their expectations, which could cause overheating/fire etc.
But in this case we are told that the lamp had 'not malfunctioned' - and in any case the base is described as being metal. It seems to be an external fire to the lamp so even if the electronics had failed how could the fire be transmitted?
The probability of the isolated steel frame to the wicker-work being in contact with another earth and the light being faulty seem to be very, very low.
I've got a feeling that there must be another factor involved somewhere along the line which we have missed recognising in this case, but not the foggiest idea what it is!