Over Temperature Protection Options

Over-Temperature Options:

In layman’s terms there are basically 4 styles of Over-Temperature Protection:

• Digital Temperature Controller – for heaters with a Digital Temperature Controller a 2^nd Temperature Sensor and Controller is dedicated to ensuring the Process Sensor and Controller are performing as desired.
• Mechanical Over-Temperature Devices
  • Thermal Fuse – this is a “once and done” device that if tripped is blown and interrupts the circuit. The thermal fuse may be able to be factory replaced or the heater is rendered useless
  • Manual Re-Set – there are two types which if tripped require an operators intervention
    • Push Button – This limiter trips and pops a button which disconnects/open the circuit. This is easily re-set by an operator who simply pushes the button
    • Remove from Power – This limiter when tripped interrupts the heating by opening and then carrying the electrical load. The heater must be removed from the supply power, allowed to cool substantially, and then re-energized         
  • Auto-Reset – this essentially is a mechanical thermostat which will turn On/Off at the calibrated temperature. If this limiter gets tripped it will continue to manage the process at the calibrated temperature.  Danger here is an inattentive operator will NOT know the primary Process Control is not working as desired and the Over-Temperature Limiter is now running the process.

Other points to consider:

• Calibration Temperature
  • These over-temperature protection devices are there to avoid a disaster. That may be to protect the materials being heated, not to damage the vessel being heated or to protect the heater itself from being damaged.
  • The calibration temperature should be as high as possible while still performing its task and so it does NOT interfere with the normal operation of the heaters.
    • For example, we have seen customers request a Target Temperature of 60C with an Over-Temperature Protection calibrated for 70C. Because the over-temp device is placed in the hottest part of the heater it can get inadvertently tripped during normal operation. If nothing will be damaged or dangerous until say 100C is reached, an 80C or 90C calibration is a better choice.
  • Operator Role

If these Over Temperature devices are tripped/engaged that means something is wrong!  The heater should NOT simply be re-set and turned back on. The operator should determine the cause of the over-heating and not re-energize until the cause has been identified and rectified.