It is perfectly acceptable to use an auto reset thermal switch to prevent a hazardous condition from occurring provided that it conforms to an appropriate standard and is used is a situation that it is designed for. You will find that something like a Thermik auto reset switch has been tested for 10,000 open/close cycles.
Consider the following: the most common failure of a budget/mid range audio amplifier is the mains transformer overheating when it is thrashed at a party (high end transformers are usually so overrated that they only get warm). The transformer insulation will be protected by a thermal switch embedded in the windings. If the switch is a "one-shot" the transformer needs to be replaced - believe me, I have replaced a few transformers for friends in my time. If the switch is an auto reset, all that happens is that the party becomes a little quiet until the transformer cools down. If you use an approved auto reset switch, you will find that safety test agencies will have no problem in accepting it (for IEC60065 anyway). You will only get problems if you use an unapproved switch, then you are letting yourself in for extended fault conditions testing at the very least. Regards Chris Colgan EMC & Safety TAG McLaren Audio Ltd mailto:[email protected] > -----Original Message----- > From: Luiz Claudio [SMTP:[email protected]] > Sent: 22 September 2000 03:53 > 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 > > ===================================================== Authorised on 09/22/00 at 11:00:54; code 37f48bf3D60F1A41. ************************************************************** Please visit us at www.tagmclarenaudio.com ************************************************************** The contents of this E-mail are confidential and for the exclusive use of the intended recipient. If you receive this E-mail in error, please delete it from your system immediately and notify us either by E-mail, telephone or fax. You should not copy, forward or otherwise disclose the content of the E-mail. TAG McLaren Audio Ltd The Summit, 11 Latham Road Huntingdon, Cambs, PE29 6ZU Telephone : 01480 415600 (+44 1480 415600) Facsimile : 01480 52159 (+44 1480 52159) ************************************************************** Please visit us at www.tagmclarenaudio.com ************************************************************** ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
