Question:
I need some information about the current approach to safety of new equipment with regard to fire hazards created by a fault. I appreciate there are generic standards covering the ejection of molten metal and so on, but I am wondering about the application of the "single component failure" concept in situations where a failure could overload a semiconductor with the possible, though unlikely, result that it ignites or ignites an adjacent part. I'm not asking about *techniques* to avoid hazard, I'm asking about what is legally required. Trick question - I'm not asking for legal advice, just information concerning best current practice. I'm posting from the UK but I suspect the regs will be substantially the same in all of Europe and the US. My particular concern is a small transistor driving an external alarm. The power supply has a fuse but as it feeds several circuits, it doesn't provide much protection for the external alarm circuit. I am considering an active current limit in the supply but I still have some reservations as to whether this meets the letter of the law. For example, one scenario involves two events as follows: 1 The current limit fails spontaneously, but as this is not monitored, the defect remains undetected, waiting for the second event to happen... 2 Someone fiddles with the external wiring and causes a short. The result is that the driver overheats, catches fire and there is hell to pay. Now, it is perfectly true that this involves two independent "failures" so at first sight would meet the "single component failure" criterion. However, I suspect that a fault that is never detected (and just lies there waiting for a chance to create a hazard) may not count. Likewise, a fault that could be caused by Uncle Fred with his screwdriver is hardly a spontaneous component failure. So overall, would such a system meet the "due care" criterion? I have severe doubts as to whether much equipment is designed with this degree of concern but it would be goot to be ahead of the field - without incurring too much cost. Also, if this isn't the best website could someone point me in the right direction? Most electronics groups seem to be full of people selling stuff.

Answer: Generally equipment design for general use is only concerned with a single failure criteria. In my experience adding more circuitry increases the complexity to the point the failure and problems are more likely. The problem with all redundant circuits is the need to test them to insure that all of the redundant circuits are working. Statistical methods may be used to determine how often the redundant circuits must be checked to achieve a certain confidance level the system will operate properly when required. In your case I would see no need for extra circuitry Execeptions are things like Safety Shutdown systems, Intrinsically Safe Equipment to insures that sparks or hot spots don't trigger an explosion in explosive atmospheres and control systems in nuclear power plants.