Electric Clothes Dryer
© 2006-2011, Dr. Ray Franco, PhD., PE r.a.franco@ieee.org
Statistical Data - 15,000 Dryer Fires per Year
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. CPSC Document # 5022
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 [1].
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[1] NFPA: U.S. Home Product Report, Appliances & Equipment, November 2005.
Lint Build-Up, Flexible Ducts, & Dryer Thermostats
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:
Whirlpool Dryer Installation Instructions
Frigidaire Dryer Installation Instructions
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 running. 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 made a 90° turn to get 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.
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.
Dryer Power Cords
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.
Lint Build-Up - My Dryer
On January 3, 2009, my wife said, "the dryer is turning, but it will not heat". Upon investigating, I found the thermal cutoff, which is a non-resettable part, had activated. I knew that there had to be a reason that it activated. The culvert was PCV pipe going to the outside vent. It was packed with tint. It had only been 17 years since I cleaned it. After cleaning the outside vent pipe. I told my wife, I had to order a part. She said, "She needed her clothes dry". I bypassed the thermal cutoff, and I told her to be very carefully, and not to leave the house with the dryer on. She thought I was a genius; the truth is I was stupid for bypassing the manufacturer's safety device. Furthermore, I know now that when the thermal fuse melts, the contacts on the high limit thermostat are usually welded together! I have seen more than one fire occur because someone bypassed the thermal fuse, and they never got around to replacing it. On some dryers, you have to take the drum out to get to it. Who wants to disassemble and reassemble a dryer - twice!
For Maximum Resolution, Click on the Body of the Picture.
My Kenmore Washer & Dryer Purchased in 1992.
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Lint Build Up - It happens in the Best of Homes.
An Electrical Schematic was on the Inside of the Bottom Cover.
My Dryer with Bottom Cover Removed.
My Dryer with Lint Shoot Removed.
My Dryer's Lint Shoot was full.
Lint removed from my dryer's Lint Shoot.
Both the Thermal Cutoff (non-resettable) and the High Limit Thermostat are located on the Dryer Heater Box.
Both have to be replaced. If the Thermal Cutoff activated, then the High Limit Thermostat Failed.
You can not purchase just the Thermal Cutoff - the High Limit Thermostat comes with the Thermal Cutoff.
When I tested my High Limit Thermostat (250°F), it didn't open at 400°F.
Both the Operational Thermostat and the Thermal Fuse are located on the Return Air. These contacts are always energized!
Lint Build-Up & Bypassed Thermal Cutoff (F09-021)
In December 2008, lint build-up and a bypassed thermal cutoff caused a dryer fire. Clothes and wood paneling in the laundry room caught fire. The operational thermostat for the Maytag dryer was located near the bottom of the dryer on the exhaust vent. With lint clogging the exhaust, there was no air circulation, and hence, no hot air for the operational thermostat to monitor. The only thing regulating the temperature was the high limit thermostat at the heater box. The high limit thermostat is an overload device; it is not designed for millions of operations. Eventually, the contacts on the high limit thermostat became welded together. The only other safety device was the thermal cutoff. Apparently, this was not the first time this dryer had been clogged with lint. In fact, the thermal cutoff had previously activated. To get the dryer to work, they bypassed the thermal cutoff. Because they failed to replay the thermal cutoff, a fire occurred. You can not blame fires like this on the manufacturer.
For Maximum Resolution, Click on the Body of the Picture.
Top, Front & Side of Maytag Dryer.
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Back of Dryer - Heat Patterns on Heater Box Vent Holes.
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Excessive Lint in the Lint Filter.
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Excessive Lint in the Flexible Vent Duct.
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No Damage to Wire Insulation in the Control Section of the Dryer.
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Excessive Heat at Heater Coils & Damaged High Limit Thermostat.
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High Limit Thermostat Contacts Welded Together.
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Heat Pattern inside the Drum due to Overheating of the Heater Coil.
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Heat Patterns on the Heater Box.
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Missing Thermal Cutoff & Taped Wires.
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Bypassed Thermal Cutoff.
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Parts of the Thermal Cutoff were found at the Bottom of the Dryer.
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Heater Coil Resistance - 11.7 ohms.
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The Operational Thermostat was located near the Exhaust Fan.
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Operational Thermostat opens at 146°F and closes at 122°F (146-24).
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Bottom of Dryer with Drum Removed.
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Drum Motor - 115 Volts, 5.5 Amps, 1/3 HP, Thermal Protected.
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Electrical Schematic Found in Control Panel of Dryer.
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Electrical Diagram Found in Control Panel of Dryer.
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Kenmore/Whirlpool Dryer Fire
The Kenmore/Whirlpool depicted below was purchased at a garage sale. Five of the wires in the bottom on the dryer were spliced. The female spade connectors were not properly crimped to the wires. This resulted in a loose connection, which caused resistive heating. After four years of use, the connection failed and ignited lint in the dryer.
For Maximum Resolution, Click on the Body of the Picture.
Very Localized Dryer Fire (F06-009).
The Fire is on the Opposite Side of the Heater Box (F06-009).
Wires Spliced with Wirenuts and Butt Splices (F06-009).
A Screwdriver and a Socket Were Found in the Lint Chute (F06-009).
Electrically Severed Wire with Copper Beads on Both Ends (F06-009).
Dryer Thermostats (F06-009).
Dryer Heater Coils (F06-009).
Burnt Lint on Heater Coils (F06-009).
Dryer Motor (F06-009).
Dryer Motor Tag - 1/3 HP; 1645 RPM's; 115 Volts; 5.6 Amps;
Thermally Protected 40° C (F06-009).
Dryer Motor Winding Resistance (F06-009).
Back of Dryer Control Panel (F06-009).
Frigidaire Dryer Fire - Coat Hanger
This dryer fire was caused by someone putting a steel coat hanger in the drum with a load of clothes. The dryer was manufactured in 2007 by Frigidaire, Model No.: FER3111FS, Serial No.: XD72207921. Electrical Schematic.
Top and Front of Dryer (F10-036).
Control Panel and Back of Dryer (F10-036).
Coat Hanger in Dryer Drum (F10-036).
Hook End of Coat Hanger Stuck in Drum Perforation Hole (F10-036).
Only 1/3 of Heater Coil is in Place. The Rest of the Heater Coil is wrapped around the Rear Drum Bearing, which has turned Blue (600°F).
Heater Coil Wrapped Around Rear Dryer Bearing (F10-036).
Dryer Power Cable - (F09-015)
This young couple purchased a new dryer and attempted to install the cable themselves. The wife purchased the dryer cable from Lowe's. When she got home, she made sure she had the right cable. She put the plug of the dryer cable close to, but not touching, the dryer receptacle. She also made sure that the lugs on the other end of the cable were separated and not touching. Supposedly, electricity jumped out of the receptacle and onto the plug. The force of the electric current was such that it went through the cable, and arced across the separated lugs on the other end of the cable. It partially melted the copper lugs (over 1980°F). The heat generated by the electrical current damaged the wire insulation and burned her hand.
There was damage to the cable insulation where her hand had been (F09-015).
Closer View of Damage to Dryer Cable Insulation (F09-015).
There was Damage to all three Terminal Ring Lugs (F09-015).
There was no Damage to the dryer Plug Blades or the Receptacle (F09-015).
The husband purchased a new dryer cord, and he installed it himself. Even if he could not read, you would think that he would at least look at the picture on the back of dryer. The power cord was not routed correctly, and the cable clamp (strain release) was still in the sealed plastic bag. Not one of three terminals was properly connected to back of the dryer. When I asked him where the missing terminal nut was, he said that it fell into the frame of the dryer.
Needless to say, I advised this couple to hire an electrician.
Photo on the Back of the Dryer depicting how to attach the Power Cable (F09-015).
The Cable was not routed correctly and No Cable Clamp was used.
The Cable Clamp was still in the Sealed Plastic Bag (F09-015).
There was a missing terminal nut, and one of the nuts was only Half On (F09-015).
On the other Hot Wire Terminal, the Ring Lug was against a Plastic Ridge meant to separate the Terminals (F09-015).
Maytag / Amana Dryer: Model #: NDE2330AYW Manufactured in June 2004.
This Amana dryer was being used when a fire was discovered in the carport. The Laundry room is adjacent to the carport. The electric service entrance, meterbase, and two electrical distribution panels were in the carport. The cause of this fire is believed to be a recalled cruise control deactivation switch on a Ford Expedition. Since the dryer was in use at the time of the fire and the cable that powered it passed through the attic of the carport, it was necessary to ascertain that the dryer did not overload the circuit.
For Maximum Resolution, Click on the Body of the Picture.
Amana Washer & Dryer (F06-013).
Back of 2004 Amana Dryer (F06-013).
2004 Amana Dryer Drum (F06-013).
Back of Amana Dryer Control Panel (F06-013).
Amana Dryer Exhaust/Lint Chute (F06-013).
Amana Dryer Thermostat Locations (F06-013).
Amana Dryer Cycling Thermostats (F06-013).
Amana Dryer Heater Box (F06-013).
Amana Dryer Heater Resistance (F06-013).
Amana Dryer Heating Element.
Amana Dryer with Drum Removed (F06-013).
Amana Dryer Motor (F06-013).
Amana Dryer Motor Winding Resistance (F06-013).
Amana Dryer Electrical Schematic (F06-013).
