Resistor as fuse and resistor failure modes - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: September 23, 2020, 06:11:52 pm
content: Hi,with reference to the attached schematic (I previously posted here), I am thinking of removing the fuse because of the two 15K series resistors. But I am thinking that there might be a possibility of those failing in short circuit mode.1) First of all, can resistors fail in short circuit mode?2) If yes, then are there any resistors which are designed to ensure they would never fail in short circuit mode?3) With no fuse, if one failed (in short circuit mode) the maximum current would be 26mA peak. That would be considered adoublefailure AND also both such failures to be of the same type (short). Is that something that is considered so unlikely that is accepted as impossible and safe as far as the legislation requirements are concerned?Thank you

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 23, 2020, 07:34:14 pm
content: I can't say what regulations require, but most types of resistors can fail in a short circuit (or at least very low resistance) mode under severe electrical overstress (think inductive surge or lightning). Under these kinds of conditions the failure of the two resistors would not be independent.

author: TimNJ
author_rank: Super Contributor
timestamp: September 23, 2020, 07:38:52 pm
content: Is there a reason you want to remove the fuse? If you are worried about international requirements, it's not a good idea. There's a only a very limited class of electrical equipment that may be operated without a fuse, as far as I know, and I don't think yours qualifies. Resistors can certainly fail short circuit. Mains can be unpredictable with transients and surges caused by lightning and nearby equipment, to name just two. So, why take the risk? If you want to forgo a dedicated fuse, you at least need a UL listed fusible resistor. Something like this:https://www.vishay.com/docs/28915/pr02fs.pdfThe load is an incandescent bulb/array...right? Bulbs almost always fail open, and are allowed to go un-fused...but a concern is that you have a user-facing terminal block, to which a user could hypothetically attach any type load they want. I do not know the exact legal requirements, but to mitigate risk, I believe the load connection really ought to be fused. If the load fails short, Q1 and Q2 are probably going to fail short too, and then you have an uncontrolled short across the mains.That said, UK plugs have integrated fuse. I'm not sure the requirements again. Maybe the cord must be permanently attached to the equipment to make use of this fuse.I'd recommend fusing your equipment as attached.

author: bdunham7
author_rank: Super Contributor
timestamp: September 23, 2020, 07:43:15 pm
content: Look at these, I think they would fit the bill.https://www.mouser.com/ProductDetail/Yageo/FMP100JT-52-15K?qs=gt1LBUVyoHnRhtGPgJMc%252BA%3D%3DEDIT: I didn't look at approvals. The ones TimNJ listed might be better.

author: ricko_uk
author_rank: Super Contributor
timestamp: September 23, 2020, 10:00:49 pm
content: Thank you Tim, Catalina and Bdunham7Tim,so is a fusible resistor guaranteed to never fail into a short?Yes you are correct ref the fuse position in the circuit. Where it was it provided very limited protection.Thank you:)

author: KT88
author_rank: Frequent Contributor
timestamp: September 23, 2020, 10:27:13 pm
content: With AC things are a bit better to solve. DC can give you permanent arcing - until the house burns down though...You can get fuses that have all approvals that are required and you are off the hook if you haven't make any mistakes by violating other regulations...In addition to your fail safe challenge I would suggest to watch Dave's video from February:https://www.eevblog.com/2020/02/12/eevblog-1284-how-bad-product-design-kills-the-environment/.CheersAndreas

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2020, 02:42:48 am
content: Quote from: ricko_uk on September 23, 2020, 06:11:52 pmAbsolutely, some resistor types are likely to fail as a short. This includes common film resistors.QuoteYes, some resistor types are specifically designed to fail open and have high overload capability. Traditionally carbon composition resistors fulfilled this need but modern ceramic composition resistors and "fusible" film resistors are suitable. Metal oxide resistors might be suitable.

author: coppercone2
author_rank: Super Contributor
timestamp: September 24, 2020, 02:49:13 am
content: I have a feeling you are not going to beat a fuse with hacked resistors

author: james_s
author_rank: Super Contributor
timestamp: September 24, 2020, 02:58:41 am
content: Quote from: CatalinaWOW on September 23, 2020, 07:34:14 pmPerhaps possible but I can't say I've ever seen this happen. I suppose if the overstress was substantial enough an arc could form which is pretty close to a short circuit until it extinguishes. If in doubt, a fuse is unlikely to harm anything.

author: tkamiya
author_rank: Super Contributor
timestamp: September 24, 2020, 03:04:51 am
content: I have seen resistors completely charred and split apart. That would mean the initial failure mode was a short. Then it continued to heat up until insulator itself couldn't handle anymore and split open. PC board was completely charred through. This was a glass epoxy type PCB. If it was a cheaper phenolic type, it would have been worse. If there was something ignitable near by, it could be a lot worse.If max current was 26mA, then my recommendation will be a lot smaller fuse, but I wouldn't omit it. I could, for example, take it apart live, and drop a metal tool onto it. (I've done something like that in real life) Wires can chaff and short.As to regulation, I have no idea. I've seen many consumer equipment without one. Many USB power supplies don't have one. But that doesn't mean it's a good design, or welcomed practice. If it's my design, I wouldn't be caught without one.

author: bdunham7
author_rank: Super Contributor
timestamp: September 24, 2020, 03:46:47 am
content: 'Failing short" is a bit vague, especially for a 15K resistor. Ultimately, given enough current nothing fails short--except tantalum dips, which apparently can take millions of amps once they've blown. I've actually replaced 4 fusible or flameproof resistors this week that were destroyed by overloads and the 1K ones that were good and truly incinerated by a high voltage overload were indeed open, or close to it. However, a 30.9K 'flameproof' (they stop calling them fusible at high resistances) one was burnt and bubbling, but its resistance had dropped to 27K. So while I doubt a 15K flameproof would actually short in response to a gross overload, it might end up at something less than 15K.I agree with the previous comments regarding retaining some sort of fuse and moving it to include the whole circuit.

author: TimNJ
author_rank: Super Contributor
timestamp: September 24, 2020, 04:20:04 am
content: The discussion still seems to be about fusible resistors...but if you fuse the entire circuit (inclusive of load), then clearly a fusible resistor doesn't make any sense. As others have noted, when the regulations are not clear (or you do not have the resources to find out), it's best to err on the side of caution. I would argue that your "user accessible" load circuit poses a much greater risk of field failure than your small DC bias supply. Obviously(?) this means you need to size a (physically) larger fuse. If it's a purely resistive load, you don't need a crazy I2T rating, but you need a breaking capacity that is greater than the expected fault current...which can be quite high.A conservative choice would be something like Littelfuse 216:https://www.littelfuse.com/~/media/electronics/datasheets/fuses/littelfuse_fuse_216_datasheet.pdf.pdfIn reality, smaller fuse types like 2AG (5x15mm) or subminature style can probably break the current safely, even though their interrupt ratings can be significantly lower. I think the interrupt ratings shown on the datasheet correlate more to an IEC standard definition and less with the actual performance. For example, even though fuse A may only be 25% larger than fuse B, it may have an interrupt rating over 4000%(!) greater. Point is: Some of the smaller fuse varieties probably have higher breaking capacity than advertised, but if you don't want to do any validation testing, use the higher rated part.Finally, for IEC60950/IEC62368, if a low breaking capacity fuse is used in place of high breaking capacity, the test lab may repeatedly force a fault condition which causes the fuses to activate, in check if the fuses can handle all possible fault conditions. I believe each fault condition is tested 10 or 20 times. If the fuse(s) open safely (without bursting into flames or shooting off the board), then it's considered okay. If you're not getting 3rd party testing, again, I would rather stick to a conservative design.

author: ricko_uk
author_rank: Super Contributor
timestamp: September 27, 2020, 09:01:29 pm
content: Thank you all, much appreciated all the detailed info and knowledge shared!!I will remove other functionality to make space for the fuse in one configuration. And in another I will use a backplane.

author: Neilm
author_rank: Super Contributor
timestamp: September 28, 2020, 06:47:50 pm
content: I know that IEC61010 (test and measurement) allows resistors in series. I don't recall the exact requirements, but I think the resistor has to be rated for the maximum voltage (including transients) and can only be used at half the rated power. With 2 in series that meet that (assuming the other is short) this is acceptable.Again, that is one standard and I am going from memory. If you are trying to meet a specific standard get a copy and see what it says.