Luiz, I am coming from a risk assessment perspective only, not a standards or regulation one.
What is the worst case result of every device going bad and leaving the heat on? What is the probability of this occuring? Base your decision on these points. Where is your limit of risk? What if the bi-metalic strip failed at turn-on the heater would be on for the maximum time the timer would allow. Would there be a hazardous condition if this happened? Many standards call for protection from single faults but we all know that most mishaps occur after multiple faults. What is the failure mode of the timer or the circuit associated with the timer? Can it fail ON? Would you know if either of these devices or circuits failed? If there is no way to detect a protective circuit failure then it cannot be relied on to be a safety device. IMHO you should include a thermal fuse. Oscar luizbonilla%ig.com...@interlock.lexmark.com on 09/21/2000 10:53:27 PM Please respond to luizbonilla%ig.com...@interlock.lexmark.com To: emc-pstc%majordomo.ieee....@interlock.lexmark.com cc: (bcc: Oscar Overton/Lex/Lexmark) 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
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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
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