What Causes Electrical Fires in the Home
Electrical Fire Causes
Unfortunately, home fires are an all too common occurrence and in a great many cases the root cause is an electrical failure of some kind. The causes of electrical fires are many; sometimes it is mechanical failure of an electrical device and sometimes it is from improper usage or design of the electrical system in your home.
Nearly all electrical fires can be prevented, though, if we understand the causes and take appropriate steps to prevent them. A little added convenience, saving a couple of dollars - these things aren't worth risking the loss of your home or life from a fire.
The National Electrical Code (NEC) is widely accepted throughout the US as the definitive standard for electrical work and a good portion of that code consists of procedures to prevent fire. As a professional electrician for many years I am expected and required to understand and comply with the guidelines of the NEC at all times and that typically means an understanding of why those rules are in place.
The NEC's Role in Home Fire Prevention
The NEC, some 800 pages describing proper methods of doing nearly all electrical work, is just a small part of the NFPA - National Fire Protection Association. It is not designed to produce efficient electrical systems nor to make profits for the inspectors enforcing the rules. It is not intended to sell particular electrical devices and it will not describe how to wire a house cheaply or quickly. The only reason for the code is safety - all 800 pages describe how to install electrical systems that will operate safely and this includes fire prevention.
For example, table 310.16 in the 2008 code lists the minimum wire size for a given circuit breaker or fuse - in the table we can see that a 20 amp breaker requires a 12 gauge wire. This isn't because a smaller 14 gauge wire won't work, it is because a 12 gauge wire will carry at least 20 amps of current (with a considerable safety factor) without overheating and causing a fire. If your home is wired with 14 gauge wire (most homes are) but the breaker keeps tripping it is not only illegal but very risky to replace it with a 20 amp breaker. That 14 gauge wire can now overheat before the breaker trips and cause a home fire, and that's the first lesson for the homeowner - never make changes to the electrical system in an effort to bypass the safety devices installed.
GFCI outlets, circuit breakers, fuses, arc-fault breakers - all of these can be irritating at times but they are there for a very good reason - your safety and the safety of anyone in the home. They are designed to shut the circuit off before either a fire or shock can cause any harm and should never be removed, modified or defeated in some manner.
In years past homeowners with screw in fuses rather than modern circuit breakers used to remove the fuse, stick a penny in and screw the fuse back in. Any and all protection afforded by the fuse is completely lost as a result and overloaded circuits will simply heat up until they either melt the wire in two or (more likely) cause a fire. Such modifications all too often have literally deadly results.
Poor Electrical Connections Can Cause a Home Fire
Any time electricity flows it creates heat. Under normal conditions, this heat is minimal - your home wiring is designed and installed to prevent any large amount of heat build up in the electrical system. Sometimes, though, failing equipment or devices create far more heat than they are designed to. Some examples might be:
Loose or Worn Outlets
Loose and worn outlets. If cords can be plugged into an outlet very easily or if they simply fall out as time passes, the outlet needs replacement. If appliances plugged into an outlet operate only intermittently it can be caused by poor connections either in the outlet itself (outlets with wires plugged into the back of them instead of being secured by a screw will fail with time) or in wire nuts in the box behind the outlet. Either tighten the wire nut connections and/or again replace the outlet.
Like outlets, switches will wear out over time. They will usually last for many years, which makes it easy to think they will last forever, but they won't. Just like outlets, the wire connections in the back or side of the switch may be loose and, again like an outlet, the switch needs replacement. It is also possible that wire nuts are loose behind the switch.
Old and Corroded Fixtures
As electrical parts age the wiring degrades very slightly each year. A light fixture may be good for 25 or even 50 years, but eventually the wire insulation inside chars, the connections corrode and the light bulb sockets won't hold a bulb well. If lights flicker or won't stay lit and potential switch problems have been eliminated it may be time to replace old light fixtures.
Other Electrical Problems That Can Result In A Fire
The use of extension cords should be evaluated for necessity; they are a very common cause of home fires. When a cord is kinked or smashed (walking on it too many times, for instance) the electrical resistance of the wire inside rises, which creates heat. Eventually the insulation may well begin to melt but this is not obvious; it may just be a small portion between the copper wires inside. As this happens more and more current begins to flow between the wires and eventually causes a fire. If you absolutely must use an extension cord for long periods of time make sure it is completely out of the way and not subject to physical damage. Under a couch, perhaps, or tucked into the edge of carpeting - whatever it takes to make sure that feet, vacuum cleaners, pets and small children can't reach it. Better to simply add a new outlet to an existing one than use an extension cord for months or years.
Many older homes are really marginal in their ability to safely provide enough power to operate all the electrical things we use in modern life. There just isn't enough power available to the home or there aren't enough individual circuits. In years past only one circuit was designed into kitchens, and that isn't enough to properly operate a microwave, refrigerator, toaster, electric grill, mixer, toaster oven, etc. The result is that the circuit breaker or fuse is always tripping. The same is true in bathrooms; the bathroom circuit commonly supplies bedrooms as well, but add in a curling iron and a hair dryer along with the bedroom TV and electric blanket and that circuit is often overloaded as well.
Every time a circuit trips or a fuse blows it is an indication that the circuit is overloaded, but most people will simply reset the breaker and do it again. The inevitable result is that the circuit breaker is slowly damaged to the point it doesn't work properly anymore and the panel itself can be damaged as well. As an electrician, I've seen house panels that have the massive bus bars inside, carrying the entire power for the whole home, half melted from repeated overloads and this is a recipe for disaster - it most certainly is one way that home fires start.
If you are commonly tripping a breaker or having to replace blown fuses, have additional circuits installed as necessary. It can be expensive but it's better than burning the house down.
In addition to electrical overloads is overloading a light fixture with lamps that are too large. According to the NFPA, the largest source of electrical fires are lamps and light fixtures. Such fixtures are designed to withstand only the heat from the rated size light bulbs - do not overload the fixture with bulbs that are too large. While it is quite acceptable to use a 100 watt equivalent CFL bulb to replace a 40 watt incandescent bulb as an energy saving measure in your home, the key is that the CFL is only using a few watts of energy and isn't nearly as hot. It won't overload either the thermal insulation or the electrical wiring of the fixture. It isn't the same as screwing in a 100 watt bulb into a 40 watt light.
Occasionally equipment of various kinds can fail in such a way that it may not trip a breaker or blow a fuse, but is still drawing more current than it should. Circuits in general are allowed to carry only 80% of the current the breaker will allow as a safety factor but when that is removed the safety becomes marginal. While the current may not trip a breaker, it does cause heat and, over time, can harm the electrical components such as breakers or the wire itself. Eventually resistance rises to the point that enough heat is produced to cause a fire.
If a fan, for instance, turns too slowly, if the cord warms to the touch, if it makes strange noises - replace or repair it! Don't simply keep using it until the cord catches fire. Electrical heaters are equipped with a "tip over" switch that turns them off if they are tipped over - never, ever defeat such electrical protections. They can be irritating, but they also save lives.
Improper Wiring Methods
A qualified and conscientous electrician will always follow the requirements of the NEC to produce a safe and effective electrical system, but the same isn't true of homeowners who often don't know what is safe. Another example from my experience: I once purchased a used hot tub from a homeowner. Upon finding that I was an electrician, he asked me to unwire it from the house and I agreed to help him out there. He had run wire from the hot tub to a dryer outlet just inside, but what appeared to be adequately sized wire turned out to be 14 gauge wire wrapped with many layers of electrical tape. Now, that is at least 3 sizes too small for the hot tub and when I removed the large cover from the dryer outlet it simply crumbled in my hands - completely charred to nothing but tiny bits of plastic just waiting to be disturbed to fall apart. The homeowner looked at it, looked at me, and commented that "I guess that's what causes fires, huh?".
He was right - that's what burns homes down and it was completely unnecessary. If you don't know what is needed to complete a wiring job safely, either find out or hire someone that does know. Electricians have, in most states, completed years of study and work learning how to ply their craft and are usually quite knowledgeable. Hire one if necessary.
Holidays often bring home fires with them. Extension cords are strung everywhere, lights are added all over rooms and vegetation outdoors, and even candles are left burning without supervision. All of these are dangerous and can cause a home fire at the worst possible time. Make sure that your Christmas tree isn't drying out with lights or candles still on it. Put candles out when you're not right there. Protect extension cords, and particularly don't leave the ends of them in puddles or snow. Arrange cords so that children or pets can't play with them; a dog chewing a cord can not only electrocute themselves but can burn the house down as well. A little common sense can go a long way.
Questions & Answers
I recently had a garage fire. I hadn't been in the garage in over a week. Nothing was on and the door was locked. It has been ruled electrical. What could've caused the fire to start?
It is entirely possible for a piece of equipment, or even an outlet or light switch, to develop a very minor short in it. Just a trickle of power and nowhere enough to trip a breaker.
But it can be enough to heat the device or wires and, very gradually, deteriorate the insulation. Over time it gets worse and worse until there is enough heat being produced to start a fire.
Animals, perhaps mice or termites, even a bees nest, can cause damage to wires with the same result - a minor short that is enough to build heat over long periods to the point that a fire starts. I once worked an industrial site that had a large well with automatic switch gear in the well house to start and stop the big pump. A squirrel got inside and got across the main, 480V, terminals. It was electrocuted of course, but remained there, causing a short between the two terminals. The resulting fire was contained within the metal box, but the entire relay had to be replaced.
Can a device or circuit catch fire if the power supply is off?
If a power supply, internal to a device such as a power supply in a computer, is off the circuit can still catch fire. An extension cord may be in use, for instance, or a wiring fault inside the wall. It is even possible, however unlikely, that there is a fault in the on/off switch which could cause a fire.
But if the power is turned off at the fuse box (and there are no batteries in use), there can be no electrical fires on that circuit.
Can light bulbs cause an electrical fire?
If you use a bulb with a higher wattage than the fixture is designed for, it can absolutely cause a fire. Note, though, that fluorescent bulbs (the "twisty" kind used to replace an incandescent bulb) and led bulbs are usually listed as being an "equivalent" wattage to an incandescent but actually draw far less current and those bulbs can be used in a larger equivalency than the original incandescent bulb.
In addition, long term use of a fixture (meaning decades) will eventually dry out the wiring insulation and that can be a fire hazard. Not the fault of the bulb, just the result of years and years of heat.
The cord on my vacuum hasn't been working properly, I plugged into the socket and it caught fire, I unplugged it and it stopped. Is it the electrical or the vacuum cord?
Assuming you mean that either the cord or the vacuum caught fire, the problem is one of those. If you mean there was a fire in the electrical outlet, the problem is in the electrical system. Either the outlet or the breaker that didn't trip when it should have.
© 2012 Dan Harmon