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Design & Operating Principles

The heart of the 3MP self hold consists of the pre-set Klixon® snap action bimetallic disc welded into a metal housing with integrated terminal. The housing is covered by the plate on which a heater can be welded to increase the current sensitivity of the device. A PTC pill is mounted on top of the metal housing by a clip to meet the requirements for self hold. The combination of a variety of standard terminal configurations and carefully select-ed materials provides easy handling and mounting. Customized terminal configurations are available on request. Wires including connectors can be automatically attached to the standard crimp terminals. Sensata Technologies supplies a range of standard leads configurations; customized solutions are available on request. In construction where the 3MP self hold device is contacting conductive parts of the motor assembly, Sensata Technologies can deliver the self hold devices with a Mylar™ insulation sleeve. Customized coding and coloring is an option on request.

The operating principle of the 3MP self hold is both simple and effective. The protector is actuated by current passing through it and by the heat received from surrounding parts.

The electrical circuit is interrupted when the disc reaches its pre-set open temperature. Due to the heat supply from the PTC pill, the bimetal disc does not cool down below the close temperature. The contacts will remain open until the appliance is switched off by the user, assuring a higher level of safety. The self-hold condition immediately after the first trip cannot be guaranteed by the protector itself. This depends on the application in combination with the selected 3MP self-hold and must be checked during application verification tests (same as for winding temperatures). When switched off the device cools down to a safe temperature again and the contacts will automatically reset. The bimetal disc provides excellent thermal and current sensitivity in overload situation. Under locked rotor conditions the integrated heater in combination with the bimetal disc provide very accurate trip times for maximum protection.


The 3MP self hold is used in domestic and industrial electric motors for washers, vacuum cleaners, chain saws, trimmers, lawn mowers and pumps in the 120 and 250 VAC applications.


Standard operating temperature rangeFrom 80°C - 170°C (Increments 5K)
Tolerance on open temperature± 8K
Peak Temperature (5 Min)200°C
Ambient Temperature to Guarantee
Stable Self Hold function
Max. Ambient TemperatureT-Open +20°C
Time Check at T-Ambient 25°C4 to 10 seconds depending on current level
Contact Rating18 A @ cos 0.6 / 250 VAC / 300 cycles


Declarations to EN60730-2-2
Purpose of the Control Thermal Motorprotector
PTI of Insulation Materials PTI 250
Method of Mounting Off-winding, fixed position, no mounting limitation
Type of Action Type 3C
Reset Characteristic Automatic
Maintained by PTC Heater. Device resets by interrupting power supply.
Control Pollution Degree Degree 1