What is a Thermal Cutoff Switch?

A thermal switch is an electromechanical device which opens and closes contacts to control the flow of electrical current in response to temperature change. The term Thermal Cutoff Switch generally refers to how the switch is used, ie. It cuts off the current to critical machinery when a temperature limit is exceeded preventing potential burn out or failure.  The applications of, and need for electromechanical thermal switch devices are broad and cover a huge diversity of industry applications. In their most basic form, they can be found on home appliances such as dryers for over temp protection.  Thermal cutoff switches are also used widely in sophisticated industrial equipment as well as commercial jetliners.

There are a number of different technologies used to implement a thermal cutoff switch that rely on expanding elements to provide the movement to open or close contacts, including vapor filled, rod and tube, bimetal, and  bimetallic disc to name a few.

CPI has been making electromechanical thermal switches since we made our first ones for the military in 1946. We offer a broad selection of off-the-shelf switches as well as completely customized implementations for OEM’s that leverage our deep expertise, and field proven technologies.

 

Thermal Cutoff Switches by CPI

For thermal cutoff switch applications, CPI draws from three standard platforms described as follows:

SnapStats (0°F – 300°F): Contact movement in the temp switch is achieved using a bimetallic snap disc. Snap action provides shock and vibration immunity and a broader differential. Set point range: 0°F-300°F (-17.8°C-148°C)

Thermal cutoff switch cutaway - SnapStat

PlugStats (0°F – 650°F): Contact movement is achieved via the different expansion rates of two metals fused together. This is a slow-make-and-break device, which provides very close tolerance temperature sensing, with a small differential. Set point range: 0°F-650F (-17.8°C-343°C)

Thermal cutoff switch cutaway - PlugStat

Rod & Tubes: (0°F – 1750°F): Contact movement in the switches is achieved via differences in coefficients of expansion of two materials, in this case the outer tube and the internal rod. This product offers very rapid response time and very high temperature sensing. Set point range: 0°F-1750°F (-18.3°C-954°C).

Thermal cutoff switch cutaway - Rod & Tube

 

Thermal Cutoff Switch Applications

Thermal switches are specified as cut-off switches because they represent a straight-forward approach to shutting down a system if a critical temperature is reached. The simplicity of an electromechanical thermal switch is what makes this approach so desirable to designers, as they are passive devices which require no power, and will reliably change state at the specified set point. Applications include:

  • Plastics extruder barrel overtemp detection.
  • Brake overtemp indication
  • Engine cooling fan control
  • Clutch overtemp in escalators
  • Bleed air overtemp indication on aircraft environmental control systems
  • Window defrost overtemp on military vehicles
  • Overtemp in refinery process
  • Avionics overtemp on aircraft avionics
  • Gas shut-off  flame detection on railroad switch de-icing
  • Flame detection in aircraft engine

<images: plastics extruder, large dozer, aircraft>