Fires at home and work don’t just “happen”. Often times with fires involving electrical components or heating appliances, there are ways to identify and prevent unsafe situations before the first puff of smoke appears. In many of these fires there is a commonality to the method in which heat being generated translates into the ignition of materials causing a fire.
I’m going to concentrate on the concept of “Pyrolysis” and how heat from an overheated electrical wire/fixture, or an improperly shielded heating appliance or chimney will cause a fire to occur. To illustrate this concept, I’ve included a video at the end of this article to watch how I take a simple piece of bread and a toaster…toast the bread repeatedly…which eventually causes the toaster to catch fire.
But before we get to the demonstration…what’s going on? Why will this toaster catch fire? How was I able to heat a piece of bread to over 5 times its normal temperature of 118°F when it pops up, in about 2 minutes using a normal kitchen toaster?
When organic materials, like wood or bread, are repeatedly reheated the moisture content evaporates. As this occurs along with the thermal degradation of the materials it lowers the auto-ignition temperature of the material to a point in which the simple application of a substantial heat source (not even an open flame) is enough to cause the organic material to catch fire. Pyrolysis occurs through thermal decomposition of organic materials in an oxygen deficient atmosphere. As this process continues the material eventually will reach its auto-ignition temperature, combined with a little oxygen, and a fire occurs.
Here’s a scenario: You’ve just bought a house where the former owner had a wood stove installed some years ago. After years of dutiful yearly inspections and cleanings and enjoying the warmness and ambience of your wood stove, you are surprised by your neighbor pounding on the door to warn you of flames coming from the eaves on your home. The fire investigation reveals that the chimney was improperly installed, allowing combustible materials too close to the pipe. Over the years the wood was repeatedly heated and cooked due to the close proximity of the metal chimney pipe. Eventually enough oxygen combined in the void space around the outside of the chimney to allow a fire to ignite.
This same scenario plays out every year around the country. The chimney was properly cleaned, a spark was not able to escape, yet the improper installation allowed combustible materials to be located too close to a chimney pipe.
The same occurrence of Pyrolysis can occur with faulty wiring or electrical fixtures. The repeated heating through electrical arcing, acting much like a heating element, will cause the thermal degradation of the surrounding combustible materials until the right mixture of heat and oxygen is present to allow combustion to occur. Just because electrical arcing is occurring, doesn’t mean that your circuit breaker will necessarily trip.
As this thermal degradation continues the organic material is less able to dissipate heat, so even if they heat source temperature remains constant it will gradually heat up to higher and higher temperatures. The material also becomes much easier to ignite. A good example of this is wood charcoal. Wood biomass is cooked under pressure to pyrolyze the material into charcoal briquettes for the BBQ grill. These briquettes are far easier to ignite than the original wood biomass.
So how do you prevent this from occurring?
- Ensure any continuous use, high draw electrical appliances are plugged into an outlet located on a 20amp circuit (or greater depending on the appliance manufacture’s specifications). Most kitchens, bathroom, and garage outlets are rated at 20amp with other parts of the home wired with 15amp circuits.
- Any evidence of flickering lights, circuits that work intermittently, or electrical anomalies should be investigated by a qualified and licensed electrician.
- Any solid fuel heating appliance or fireplace should be installed by a qualified professional, ensure that all clearance distances are followed appropriately.