Cause for Alarm

The destructiveness of some electrical fires may ultimately be due to challenges in the detection and location. They can start in concealed spaces, behind walls, which gives them time to spread before triggering a smoke alarm. They also originate from something that isn’t actively being used. For example, if a fire originates from a stove, it tends to be because someone is cooking. But these occur in the hidden infrastructure of a building and they can take some time to grow and spread, starting with the accumulation of heat and then consuming surrounding combustibles. In that sense, they can be quite insidious.

One common source of electrical fires is household appliances. However, other causes of electrical fires that we often encounter can be traced back to the actual electrical work done on a house or building. When you hear that a fire was started by the electrical infrastructure in a building, it can be tempting to assume that fault must lie with the electrician. But in actual fact, damage can be inflicted by any number of contractors, and faulty installation can also be committed by anyone from the original builder, to the homeowner, landlord or tenant.

Hitting the Nail on the Head

When securing cables to a framing member, if a staple is driven crooked, too hard, or if the staple selection is wrong, it can damage the cable insulation. If the staple then comes in contact with one of the conductors (wires inside the cable) the electricity gets a path to ground through the actual framing member. However, because the wood provides a very poor connection, you get what is called a high-resistance failure, and over time, heating at the base of the staple where it meets the wood. It could take days, weeks, even months. But eventually, if it gets hot enough it can lead to a fire.

This can also happen with a nail used in the framing. If it isn’t driven straight and the cable is pulled against it, it can penetrate the wire insulation and touch one of the conductors. I was actually fortunate to have discovered this once before it caused an electrical fire. I could see the wood starting to turn black at the base of the nail. Unfortunately, this was at a scene where a fire had already occurred. In this case, we traced the greatest concentration of damage back to an area a few inches across, right where a staple would have been. And though it had fallen out, we were able to verify that staples had been misdriven in numerous locations, ultimately being the cause of the loss.

What is a High-Resistance Failure?

Some materials allow electricity to run through them better than others. Imagine you are an electron and have to get from point A to point B. Running on an asphalt track would be like travelling through a conductive material. Travelling through a resistant material would be like running through a swamp. Think of how much harder you have to work and how much extra energy you would expend. When electricity passes through a resistive material, that expended energy is converted into heat. The higher the resistance, the more heat, and the greater the chance a fire could occur.

Tying Up Loose Ends

Another common place electrical fires originate is in junction boxes where there are a number of wire nut connections. If one of the connections is loose, the wires can form an oxide which is a poor conductor. What can also happen is the space the current has to pass through is effectively made smaller, creating more resistance — imagine a four-lane highway converging into a single-lane. Both of these phenomena result in more heat being produced and create an environment where a fire could start.

Usually, it’s the plastic components inside the junction box that catch fire. Occasionally, if the connection is right on the wall of the junction box, combustible materials adjacent to the junction box can ignite as well, such as wooden building elements and insulation.

The frightening aspect of this failure is when you consider that these connections exist in virtually every fitting in your house. They are behind outlets, in light fixtures, light switches, ceiling fans — everywhere.

Wear and Tear

When wires are being installed, there are all sorts of opportunities for them to be damaged. They get pulled around corners, they get yanked on and dragging against rough wood and sharp metallic components. This can damage the wire insulation enough to compromise it. And because of the way it’s being pulled around, the damage usually occurs on the underside of the cable. So, it’s not immediately obvious to the installer.

This is less likely to be a high-resistance failure, and more likely that the two conductors come in contact with one another, or that the hot conductor and the ground could come in contact with each other. When the system is energized, or if the system vibrates and then settles over time you could get arching and sparking. Particularly if it is near a metal building element. Similarly, when the installer neglects to put a metal bushing around a wire that goes through a metal box like a junction box, as the house settles and there are vibrations, over time the wire’s insulation can wear down and cause a fire.

Electrical fires may not be as common as those caused by cooking or heating equipment, but the destruction caused can be extensive, and therefore, costly. These can be challenging determinations to prove for someone without the proper training. For example, because a staple or nail is right at the center of a fire, generally, as the wood is consumed, the staple or nail will fall out. It takes an experienced professional to connect the dots and not only strengthen the hypothesis but also eliminate all other ignition sources and causes. And in order to do this, the evidence must be documented as soon as possible.

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Martin Coles, P.Eng., CFEI

Martin is a licensed professional engineer specializing in electrical engineering and fire investigation. With more than 15 years of experience, he has performed over 130 fire investigations as well as electrical failure analyses, site reviews and code consultations. Martin is qualified as an expert witness in the Supreme Court of British Columbia.