Electric Ranges and Cooktops
© 2005-2010 Dr. Ray Franco, PhD., PE r.a.franco@ieee.org
According to data from the U.S. Fire Administration's National Fire Incident Reporting System (2000-2004) and a survey by the National Fire Protection Association (NFPA), NFPA estimated that 32 percent of all home structure fires involved cooking equipment.
Range Infinite Heat Switches
Surface burner switches control heat from low to high and everywhere in between; hence, they are often referred to as "infinite" switches. The position of the switch can usually be determined from the position of the notch or detent in the plastic cam. The plastic housing and cam are made of a thermoplastic that does not melt. The switch stems and die casting on the back of the steel (2600°F) faceplate are made of plot metal, which melts at approximately 800°F.
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Infinite Switch Knobs turn Counter Clockwise.
The Outer Knobs control the Front Burners.
Typical Range Infinite Heat Switch.
Infinite Switch - Bimetal Arm and Terminals.
Infintie Switch Schematic.
Inner and Outer Cam Followers.
Indentions in Inner and Outer Cams Depicting the "OFF" Position.
When in the "OFF" Position, the Indention in the Inner Cam is aligned with the Metallic Notch of the Inner Cam Follower.
When in the "OFF" Position, the Indention in the Outer Cam is aligned with the Metallic Notch of the Outer Cam Follower.
When in the "ON" Position, the Inner Cam presses down on the Arm of Inner Cam Follower, Which closes the Pilot and Heater "1" Contacts.
When "ON", the Outer Cam presses down on the Arm of the Outer Cam Follower closing the Contacts. The Bimetal Arm opens the Contacts.
In the Max Heat Position, a High Spot on the Outer Cam insures that Heater "2" Contacts are Always Closed.
There is also a Seperate Cam Follower for the Max Heat Position and an Indention on the other Side of the Plastic Cam.
When the Switch is "OFF", the Notch on the Metallic Stem fits into the Indention on the Back of the Metallic Face Plate.
To Move the Switch from the "OFF" Position, the Metallic Stem must be pushed into the Plastic Cam.
The Knob has to be pressed "In" to be moved from the "OFF" Position.
Position of Metallic Notch on the Stem, when the Switch is in the "ON" Position.
Case History F09-067
A new Tappan range was purchased the weekend before Thanksgiving 2009. A kitchen fire occurs three days after the range was installed. The homeowner's daughter (age 15) and son (age 10) were out of school the week of Thanksgiving. Both parents were at work. All of the surface burners except the right rear had covers over them. A cast iron skillet was found on the range. When I examined the scene, all of the debris had been removed from the top of the range, and all that was visible of the infinite switches were the metallic cam follower arms, which were still attached to the wires. The two right infinite switches were found inside the back of the range. The green arrow points to the inner detent of the switch that was "OFF", the red arrow points to the inner detent of the switch that was "ON". If the green arrow is at 12 o'clock (OFF), the red arrow is at 8 o'clock (between Medium and High). The left two infinite switches were found on the floor. One of the two plastic cams was found, but it was not attached to the switch faceplate. This fire was investigated by the local fire department. It is presumed that they discarded the left two switches when they cleared the debris off of the top of the range. None of the plastic switch housings were found.
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New Tappan Electric Range (F09-67).
Position of Right Infinite Switches (F09-67).
The pot metal stems of both switches have started to melt.
Case History F05-015
A fire occurred at this home two months after a new Black & Decker coffeemaker was purchased. Guest stayed overnignt. They got up the following morning and used the griddle to cook bacon and eggs. Both the knob stem and the cam detent showed that the right front burner under the griddle was "ON".
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Electric Range (F05-15).
Vent Hood with Shaded Pole Fan Motor (F05-15).
Underside of Range Cooktop (F05-15).
Bottom of Range Cooktop (F05-15).
Bottom of Electric Griddle (F05-15).
Metalic Knob Stems - The Arrows Point to the Position of the Slot in the Stems (F05-15).
Range Control Panel (F05-15).
Surface Burner Control Switch - Infinite Switch (F05-15).
The Arrows Depict the Position of the Cam Indention, Which Requires the Knob to be Pressed "In" to be Moved from the "OFF" Position (F05-15).
Cabinet Space for Range (F05-15).
