If I understand Lou's intent for many standards he is technically correct.... If you read UL/IEC60065 sub-clause 4.3, the standard states that if a circuit is less then 35Vp a.c. or d.c., not generating voltages above that value, and is not supplied by a circuit which can source 0.2A into any load for more then 2 minutes then fault conditions are not required. Whether you want to do faults in any case is certainly up to you and may be desirable as 7 watts can still heat up a low power component in a short time..................... From: richwo...@tycoint.com [mailto:richwo...@tycoint.com] Sent: Thursday, October 23, 2003 3:39 PM To: emc-p...@ieee.org Subject: RE: Fault condition test UL 60950
Lou, I have to take exception to your last paragraph. We often have to perform fault testing in low voltage circuits to verify conformity to SELV requirements at the output ports. We also perform fault testing to determine comformity to the limits for limited power circuits. Richard Woods Sensormatic Electronics Tyco International From: aiken [mailto:ai...@gulftel.com] Sent: Thursday, October 23, 2003 2:04 PM To: Van Compernolle, Eric; emc-p...@ieee.org Subject: Re: Fault condition test UL 60950 Eric. There are places in a circuit where film resistors are required by UL, such as where they bridge insulation. Ordinary carbon resistors are unacceptable in these cases - at least in the categories I usually work in. So film resistors are not considered to fault in a shorted state. As for caps and diodes you must make that decision based upon what the result of the fault will be. If an unlikely fault (short or open) will be something that can result in a hazard - within the meaning of the applicable standard - then it should be considered and provision made in the design to prevent the unacceptable increased risk. Sorry this is not such a clear cut answer but diodes and capacitors can fail in both shorted and open states, so a certain amount of circuit analysis is necessary Approved X, Y, Antenna coupling caps, & Antenna bypass caps are not shorted during fault testing - at least in the categories I ordinarily work with. Components in low voltage circuits are not investigated (for safety) when the available power is below certain limits. Those limits will be given in the standard applicable to the product. Regards, Lou Aiken, LaMer LLC 27109 Palmetto Drive Orange Beach, AL 36561 USA tel ++ 1 251 981 6786 fax ++ 1 251 981 3054 Cell ++ 1 251 979 4648
