Fire Bulbs - ECN Electrical Forums

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These are two bulbs that show the phenomenon of light bulbs that survive a fire. The thin glass envelope softens as the gas inside expands. Because they are so delicate this happens at the earliest stages of the fire and once the gas is expelled they harden pointing to where the fire started.

In 25 years and many fires I have been able to save only these two. Many firefighters have never seen one. As the one shows they do not even have to be installed or on for this to happen.

(Alan Nadon)

The "point on this one is near the base:

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This is the best. The glass is almost too thin to handle. It was in a ceiling lampholder, base up and pointed straight at the paper basket, (dust bin) where the fire started.

Everyone knows electrical equipment can start a fire, but not too many know it can solve where and how a fire started.

Alan Nadon

Cool pictures Alan.

I'm in the volunteer fire service and never seen a bulb like this because the fire had destroyed most of the evidence but great to see this and work it back to the origin of the heat source.

As an addition to the V pattern on burnt walls it is an other very good pointer for fire investigation afterwards.

"The gas inside expands" ?

I thought they were filled with... Vacuum?

They have a bit of some inert gas in there to keep the inside of the glass bulb from getting excessively blackened as the tungsten filament evaporates from the heat. It also enables manufacturers to use thinner glass.

How in the world would this point to where the fire "started"? It seems as though it would more likely point to wherever the hotter gas was collecting, irregardless of where the actual initial flames were.

It's probably a reliable an indicator as a lot of the other "science" used by fire investigators:

http://www.washingtonpost.com/wp-dyn/content/article/2006/12/29/AR2006122901920_pf.html

http://malabar.livejournal.com/263698.html

Wonder if anyone is doing jail time based on a melted lightbulb...

My delay in getting back to this item and the comments is because I have been searching for the science behind the light bulb pointing in the direction of the source of fire.
It is in NFPA 921 part 4-20.1 (1995 edition) This NFPA document was the result of the lack of science that the news articles reference.
Titled "Distorted Light Bulbs" it goes on to state. Incandescent light bulbs can sometimes show the direction of heat impingment. As the side of the bulb facing the source is heated and softens the gases inside a bulb of greater than 25 watts can begin to expand and bubble out the softened glass. This has been traditionally called a "pulled" light bulb, though the action is really a response to internal pressure rather than a pulling. The bulged or "pulled" portion of the bulb will be in the direction of the source of the heating.
I wasn't aware of the 25 watt limit which is based on 25 watt or less bulbs being vacuum rather than inert gas filled, and the glass will be dimpled instead of pulled.
The NFPA is more than just the Electrical Code.
Alan--