Bimetal switches have been used for decades and proven generally to provide an adequate or better safety protection function. Not all bimetal switches are autoreset types; some require manual resetting. I've also experience bimetallic thermal protectors in motors in an open failure mode, more often than closed mode (but that only counts in my femtocosm of the world).
I have to disagree with your opinion, since bimetallic thermal protectors have long been in use and largely successful in their applications in wound products. This is not to say they're good in all applications (sometimes, a one-shot device is preferable) or that all bimetallic switches are created equal. Please note that several safety agencies evaluate bimetal switches as protective devices, both in and outside of products, lending to the credibility of their usefulness and suitability for their intended applications. Regards, Peter L. Tarver, PE ptar...@nortelnetworks.com -----Original Message----- From: Luiz Claudio [mailto:luizboni...@ig.com.br] Sent: Friday, September 22, 2000 3:53 AM To: 'emc-pstc' Subject: Protection or Control? 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
