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Ambient Temperature: The normal or typical temperature of the environment surrounding the device.

Automatic Reset: A thermostat that automatically resets to its pre-trip contact configuration at a factory preset temperature.

BimetalTwo metals with different rates of expansion molecularly bonded together to form a blade or disc for a thermostat.

Creep Action Thermostat: A thermostat with a bimetal blade that slowly makes the contacts or slowly breaks the contacts.

Current Derating: Term used to describe a predictable condition where increased current applied to a thermostat causes the device to open at a temperature below its set point. This condition is present only in Normally Closed bimetal thermostats that are both current and temperature sensitive.

Differential: (also known as Hysterisis) Refers to the difference between the opening and closing temperatures of a thermostat.

Functioning Temperature: (TF of Tf) In reference to a thermal cutoff, as defined by IEC Standards, the temperature at which a Thermal Cutoff opens a circuit under a no load condition. The IEC tolerance is +OC, -10C.

Holding Temperature: (TH or Th) In reference to a thermal cutoff, the maximum temperature that a Thermal Cutoff can maintain Rated Current for a period of 168 hours with functioning.

Manual Reset: A thermostat that requires an operator to perform a function before the device will reset. This is commonly achieved by use of a push button on a bimetal thermostat but other methods exist. Available only in Normally Closed contact configuration.

Manual Reset, Trip Free: A Manual Reset device that cannot be reset until the fault condition subsides.

Maximum Temperature:  (TM or Tm)  In reference to a thermal cutoff, the maximum temperature to which the device can be subjected for a period of ten minutes without resuming conductivity once it has opened.

Minimum Differential: (also known as deadband) The minimum number of degrees between the actual open and actual close temperatures.

Motor Protector: A thermal protector that is sensitive to both increases in current and increases in temperature.

Normally Closed: Contacts open on temperature rise.

Normally Open: Contacts close on temperature rise.

One Shot: A device that, once activated at its preset temperature, cannot be reset. Available only in Normally Closed contact configuration.

PTC (Positive Temperature Coefficient) Thermistor: A self-regulating heating element that maintains a constant surface temperature regardless or variations in voltage or ambient temperature.

Rated Current: The maximum allowable current that a device can carry.

Rated Voltage: The maximum voltage that should be applied to a device.

S.P.D.T. (Single Pole/Double Throw): A three terminal switch possessing both Normally Open and a Normally Closed contact configuration.

S.P.S.T. (Single Pole/Single Throw): A switch with a single set of terminals and a single set of contacts.

Self-Hold Protector: A device that, upon opening, will not reset until power is removed from its circuit regardless of ambient temperature. Available only in Normally Closed contact configuration.

Snap Action Thermostat: A thermostat with a Bimetal element that snaps to close the contacts when the activation condition is experienced. This construction permits the clean break of loads.

Thermal Cutoff:A thermal cutoff is an electrical safety device that interrupts electric current when heated to a specific temperature.

Thermal Cutoff Fuse: A device that melts its internal element to open a circuit in an over-temperature condition. Also known as a thermal fuse, thermal protector fuse or thermal switch.

Thermal Protector: A Thermal Protector is a protective device built into a motor that provides over-current or over-temperature protection that disconnects power to the ballast if internal temperatures rise above the trip point. This protects motors from overheating.  Types of switches include automatic rest, manual reset and resistance temperature detectors. 

Tolerance: The allowable range above and below a nominal temperature.