UL single fault conditions - Page 1
Forum section: electronics / projects

author: bestboy4
author_rank: Newbie
timestamp: October 30, 2017, 07:06:15 pm
content: So I have a board going through UL. UL claims that since it is over 15W that it needs to go through "special" testing. It is based on the 60730-1 standard (annex H.27). Apparently this gives them the license to short or open random components on the board (one at a time) to induce a fault. I have been an engineer for over 10 years and have never heard of such a test for industrial controls.In my particular case, they removed a resistor in my 5V switcher circuit that sets the output voltage. When this occurs the output goes negative and causes a nice flame out of the my CAN transceiver.Has anyone here have any experience with this UL requirement?

author: AndyC_772
author_rank: Super Contributor
timestamp: October 30, 2017, 07:23:23 pm
content: Not that standard specifically, but testing for electrical safety under a single fault condition is pretty normal.The basic idea is to simulate any one fault which could be reasonably expected to occur. Any component going open circuit is fair game, given that's exactly what happens if a solder joint cracks.Short circuit faults might take a bit more justification; I know, for example, according to some standards used in ATEX testing, a high value resistor is deemed 'infallible', ie. it might fail open circuit, but absolutely cannot fail short. Other standards just refer to worst-case faults, so opens and shorts are both considered, however unlikely they might be.It sounds as though UL have, in this case, correctly identified a potential safety issue with your design; namely, a failed solder joint on that resistor would cause excessive heat dissipation in your CAN transceiver. That may actually be OK provided the energy involved is limited, and nothing can actually start a fire. If it can, and you literally do mean flames came out of the chip, then UL have done their job by failing a potentially dangerous design, which you now need to fix.Maybe you could use two resistors in parallel, so the effect of one of them going o/c is less severe. Or, maybe you could put a suitably large diode across the output of the supply, so it shorts out and causes the PSU to shut down if the output tries to go negative. Maybe you could change your CAN transceiver for one less likely to fail in such a dramatic way, or put a PTC on the output of the PSU to limit the current which can ever flow out of, or into, it.There are probably a dozen other relatively minor modifications you could make to the design.My experience with UL is they're mainly interested in fire. Your equipment can break any way you like, provided nothing initiates or supports combustion.

author: bestboy4
author_rank: Newbie
timestamp: October 30, 2017, 08:46:41 pm
content: Thank you for the reply Andy,I agree that there are some minor adjustments that can be made to mitigate the specific fault condition I mentioned. Where I get nervous is that I have no idea what components they are going to test next or how they are going to test. It's like Pandora's box, who knows when they'll stop. I can literally see this dragging on for weeks. I could fix one thing, resubmit, then oops here is this other thing you need to fix. I haven't designed fully redundant systems since my aerospace days. These are cheap refrigerator controls, and I'm not sure if we've got the profit margin to institute all the changes that might crop up.Hopefully you are right about UL's interest in flammability only. This has been my experience as well. Apparently they use cheesecloth as they're litmus to determine charring.

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 30, 2017, 10:16:33 pm
content: It doesn't need to be redundant, it just needs to fail gracefully. Basically design it so if X failing could cause catastrophic meltdown, then sacrificial components Y or Z will fail rapidly cutting power to the dead circuit. Having a few sacrificial components cut power if any number of faults occur is the way to go. Basically cover high current draw, overheating, and sudden voltage spikes.

author: jbb
author_rank: Super Contributor
timestamp: October 31, 2017, 12:37:25 am
content: This kind of testing can get really painful, especially if you go back again and again.I suggest you ask them whether they can do a ‘debug’ evaluation of the design for you. This wouldn’t give you a certificate at the end, but they will try to test - or at least flag - as many issues as possible for you.Also, it might be possible to speak to one of the assessor directly. (S)he might be able to quickly point out areas of concern that you can then check out yourself. Ask nicely, because they’re probably quite busy.