The U.S. Consumer Product Safety Commission (CPSC) estimates that in 1998, clothes dryers were associated with 15,600 fires, which resulted in 20 deaths and 370 injuries. Fires can occur when lint builds up in the dryer or in the exhaust duct. Lint can block the flow of air, cause excessive heat build-up, and result in a fire in some dryers.
According to data gathered between 1999 and 2002 by the National Fire Protection Association (NFPA), 4 % of all reported structural fires involved a clothes dryer .
|Dryer Fires : 1999 - 2002 |
 NFPA: U.S. Home Product Report, Appliances & Equipment, November 2005.
UL Standard 2158 for Electric Clothes Dryers is a harmonized standard with the Canadan Standards Assocation, CSA C22.2 No. 112. This standard required that electric clothes dryers manufactured after March 29, 2013 pass four aggressive fire containment tests. This is as close as your are going to get to the dryer manufacturers admitting that they can not design electric clothes dryer to prevent fires; therefore, they have to contain them.
The predominant cause of dryer fires is lint build-up in the dryer or the exhaust duct. Lint build-up restricts air flow, which results in the clothes taking longer to dry. Lint build-up often occurs because the flexible exhaust duct gets pinched or crushed. Flexible dryer ducts are known to collapse. In addition to prohibiting the use of plastic and vinyl flexible ducts that support combustion, dryer manufactures, do not allow using metal foil ducts. Except for the fact that they will not burn, metal foil ducts have the same problems as plastic and vinyl ducts; air flow can be restricted, and they can collapse. Sears, Lowe's, Home Depot, and Best Buy have had class action lawsuits against them for failing to follow the manufacturer's instructions and installing foil metal exhaust ducts. Although a foil metal duct is a flexible metal duct, it is not "the" flexible metal duct that manufacturers refer to in their installation instructions. This is a heavy metal flexible duct that can not collapse. Again, dryer manufacturers do not allow the use of foil metal ducts. See Manufacturer's instructions for installing dryers below:
Most dryers have at least an operational thermostat, a high limit thermostat, and a thermal fuse. The operational thermostat measures the temperature of the air flowing through the dryer. That is, the air that is drying the clothes. Depending on its location, the operational thermostat is usually between 120-160°F. The high limit thermostat and thermal fuse are usually located on the heater box. The value of the high limit thermostat depends on the design of the heater box, but 250°F is a typical value. The value of the thermal fuse is usually 100°F higher than the thermal fuse, but again its value depends on its location. The thermal fuse is a one shot device; when it melts, it has to be replaced.
When the dryer vent is pinched or clogged with lint, air does not circulate through the dryer. Hence, the operational thermostat is not measuring the temperature of the air flowing across it because there is no air flowing across it or the amount of air has been severely limited. Air circulation is also what cools the heater coils in the heater box. Without air flowing, the heat in the heater box will continue to rise until it activates (opens) the high limit thermostat. Now the dryer is cycling according to the temperature of the heater box; the operational thermostat is not working because there is no air flowing across it. Usually high limit thermostats are not designed for the millions of openings and closings that operational thermostat sees; it is a safety device. If the high limit thermostat sticks or the contacts become welded together, the temperature continues to increase until the thermal fuse melts. At this point, the dryer stops heating. The thermal fuse is a last resort device. No one wants to go in the interior of their dryer and replace a thermal fuse because they forgot to clean the lint filter
When the thermal fuse melts, it almost always means that some other device in the dryer has failed. For example, when the thermal fuse in my dryer failed, I found out that I could not just order a thermal fuse. I had to order at kit that contained both the high limit thermostat and the thermal fuse. I tested the old high limit thermostat, and the contacts were electrical welded together. I have rigid metal ducts. My problem was not with the rigid metal ducts, but the PVC pipe that when into the wall and to the outside. I had not cleaned my exhaust system in 17 years. Most manufacturers state that the entire lint duct system should be cleaned at least every 18 months.
I have investigated a number of dryer fire caused by someone bypassing the thermal fuse. However, fires can occur even before the thermal fuse blows. When air is not properly circulating, the lint does not have anywhere to go but into the dryer. This can result in lint getting on the drum motor and into the heater box, which can ignite the lint.
The autoignition temperature of lint is 511°F (266°C).
Another problem with dryer is that they vibrate. This is especially true when the load in them become unbalanced. Dryer vibrations can cause electrical screw connections to become loose and wires to chaff against sharp objects.
To prevent wire chaffing, where the cable enters the frame of the dryer, there should be a metal cable clamp with smooth edges. However, they are often not installed or installed improperly. In 1996, the National Electrical Code (NEC) changed the requirement for dryer receptacles from a 3-slot to a 4-slot receptacle with a separate, safety, ground-wire. The change was for new homes built after 1996. This change affected the dryer power cord. If the dryer was being installed in a home built before 1996, it needed a 3-wire cable with a 3-prong plug, and if it was being installed in home built after 1996, it needed a 4-wire cable with a 4-prong plug. This stopped the dryer manufacturers from connecting or supplying a power cable with the dryer. Instead, they sold separate kits for the dryer power cable, and they allowed the retailer to charge for installing the dryer power cable. Alternatively, the dryer power cable could be purchased from Wal-Mart, Home Depot or Lowe's. To save money, the dryer purchaser often installs the cable himself. The dryer cable sometimes comes with the cable clamp around the cable. However, the clamp must be removed from the cable and installed in the frame of the dryer before the screw connections are made to dryer. This often does not occur because the installer is primarily concerned with which wire goes to which terminal. It's only after the connections have been made, that he realizes that he should have removed the clamp from the cable and installed it first. The inexperienced installer often does not properly connect or torque the connections to the dryer. I've actually seen power connection to a dryer resting on the screw without a nut to secure the connection. When I asked the owner of the dryer what happened to the nut that secured the connection, he said that he dropped it, and it fell inside the frame of the dryer.