I would like to thank all of you who replied my message. I'll read carefully all of them.
Now I would like to give my oppinion. It is my belief that, in most of applications, a bimetal cannot be considered as a protection device. As I said in my previous message, a bimetal is usually used to maintain the temperature within certain limits under NORMAL operation conditions. I mean, if everything works fine, the bimetal will not expose the user to a dangerous situation. However, if a FAULT occurs, the bimetal itself will not eliminate the HAZARDOUS CONDITION. It may open and close its contacts undefinetly until it fails (melts, weld its contacts, etc.). A proper protection against an ABNORMAL condition would be a thermal fuse or another device that would open its contacts and not close it again or will melt (like a fuse), requiring an intervention (maintenance, resetting, etc.) to put the product in order again. Now, let's say that a timer controls the operation of a heater. The duration of the operation of the heater is defined by both the timer and the bimetal that controls the temperature achieved in a compartment. Either can controls how long the heater stays on. This means that if the bimetal fails (welds its contacts), the timer will open the circuit after a certain time (already defined by previous tests). The timer can be either electronic or electromechanical. In this situation, would you require a second temperature actuated device (like a thermal fuse)? Consider that you are talking about a Class I product (grounded through the plug grounding prong) and the heater case is grounded!! Thanks again for your insights!! Luiz
