Fuses:  Why aren't they obsolete? - Page 1
Forum section: electronics / beginners

author: JoeN
author_rank: Frequent Contributor
timestamp: March 20, 2017, 08:00:43 pm
content: Can't you build a fast enough current sensor and semiconductor switch that could 100% of the time protect both the switch and the attached equipment from overcurrent conditions? Is tit not practical to do this or is it just too expensive? Worst case, you could always also have a fuse in-line, but it would never blow on you. Wouldn't this be ideal for DMMs, but also a huge range of other equipment? Thanks.

author: james_s
author_rank: Super Contributor
timestamp: March 20, 2017, 08:05:23 pm
content: Possibly, but in most cases it would not be cost effective. Fuses in most equipment never blow, if they do blow then it is because of a fault in the device and if one is inclined to repair the fault, the fuse is cheap compared to the effort and other parts. In something like a multimeter, I don't think you'd find something that is as reliable at interrupting large fault currents as a fuse. Semiconductor devices tend to fail shorted, mechanical relay contacts can weld. A fuse will blow and guarantee an open circuit.So fuses are still around because they're reliable and economical.

author: JRosario
author_rank: Contributor
timestamp: March 20, 2017, 08:12:11 pm
content: Quote from: JoeN on March 20, 2017, 08:00:43 pmI cannot.

author: JoeN
author_rank: Frequent Contributor
timestamp: March 20, 2017, 08:15:03 pm
content: Quote from: JRosario on March 20, 2017, 08:12:11 pmNice to know. I don't know how fast fuses blow, but it has to be on the millisecond timescale. For the conductor to burn away, it takes some time. Surely there is a circuit that can sense an over current condition and fully close a semiconductor switch in an even shorter time.

author: dmills
author_rank: Super Contributor
timestamp: March 20, 2017, 08:17:36 pm
content: A 10A fuse does not have to break a 10A current, it has to break a current of many thousands of amps in the event of a short from a stiff supply, and once blown stand off large voltages.Trying to do this with semiconductors in a way that does not destroy the switching device is a big ask (But IGBTs can sort of do it kind of, for low frequencies as long as the downstream stuff is over built and the PSC is not too high), doing it in such a way that it still appears resistive down to mV levels and can still cope with AC both large and small is a huge ask.$10 or so buys me a fuse that will protect me from short circuiting a 1,000V supply with 0.2 ohms of resistance, and yet will impose a negligible voltage drop in normal use, not an easy thing to do with semis.If you blow a multimeter fuse more then once in a blue moon, you are doing it wrong.73 Dan.

author: alm
author_rank: Super Contributor
timestamp: March 20, 2017, 08:19:46 pm
content: There are mechanical breakers and polyfuses. Neither can completely replace fuses due to size and performance. A polyfuse's response time degrades every time it is triggered.The fact that fuse fail catastrophically means that they are very likely to get replaced after being overloaded. What if said semiconductor device that 'protect 100% of the time' degrades after having to interrupt a high current a few dozen times? A shorted breaker could make for a very bad day. A fuse only has to be designed to interrupt a severe overload once.Probably the only example of a well-designed instrument that somewhat regularly needs a fuse replaced (depending on the skill and focus of the user), and DMM fuses are designed for some extreme overloads (e.g. up to 1 kV DC, 30 kA). A mechanical switch able to interrupt this would likely be huge. A semiconductor device would be very expensive and also quite huge.

author: TNb
author_rank: Regular Contributor
timestamp: March 20, 2017, 08:26:01 pm
content: There is such thing asResettable fuse(aka polyfuse, aka polyswitch).I haven't done much with it, don't have experience, but as far as I know it is used quite often, though not in all situations. Google about it if you haven't alreadyOr I am sure somebody has experience here and can tell something interesting.

author: Nerull
author_rank: Frequent Contributor
timestamp: March 20, 2017, 08:29:33 pm
content: This system would depend on several working systems within the device in order to trigger, which is a terrible assumption to make in situation where you need to blow a fuse.A fuse doesn't need a working power supply to blow.

author: AndyC_772
author_rank: Super Contributor
timestamp: March 20, 2017, 08:30:00 pm
content: It's very common to include electronic fault protection in a device. For example, suppose you have an active backplane which supports hot-swappable cards, and one of those cards develops a fault.You want to be able to remove the defective card and swap it out, but the backplane needs to remain powered up, so you can't turn it off to change a fuse. But, a simple microcontroller can detect that a faulty card has been removed, and can re-enable power to a slot which has been powered off because an electronic monitoring circuit detected a problem.A fuse has some desirable characteristics anyway. For example, a power supply with bulk capacitance draws a substantial surge current at start up, and an overly sensitive current limiting device will turn it off prematurely, before it's even had chance to start up. A fuse will pass that start-up current OK.Also, many semiconductor devices fail short circuit, which is the very last thing you want a fuse to do. Draw a modest current through an electronic device, and it might detect it and switch off as it's designed to do. Draw a massive surge through that same device, and it could fail short and allow the next weakest link in the chain to catch fire.

author: Gregg
author_rank: Super Contributor
timestamp: March 20, 2017, 09:44:43 pm
content: The one word answer is reliability.Quality fuses have a proven track record. They can be made to quench an arc form an overload that is exponentially higher than the fuse rating for normal use. They cleanly break the fault with no detection circuitry that could possibly compromise the ability to break the circuit.A solid state switch for breaking a fault would be relatively expensive to build and still couldn't produce the reliability of a fuse for a major overload.A mechanical breaker with contacts is vulnerable to a lot of possible failure modes and when it opens the contact points may be compromised; not to mention that some idiot may just reset it without thinking about why it may have tripped.Here is a picture of a toasted 100 amp breaker, no arc quenching and not a lot of contact area.

author: james_s
author_rank: Super Contributor
timestamp: March 20, 2017, 10:10:48 pm
content: Wow that's brutal.Ideally a fuse should be the secondary protection, something that blows if the primary protection like a mechanical breaker or PTC fails to respond in time. Often a fuse is a last resort that exists to prevent an equipment failure from turning into a fire. If the power supply in your computer fails and blows its fuse, the power supply is likely not economical to repair anyway, but the fuse safely disconnects the power before the fault can start a fire. There's no reason to use anything fancier than a fuse here because if the fuse blows the item is already going to be written off.

author: oldway
author_rank: Super Contributor
timestamp: March 20, 2017, 10:19:16 pm
content: There is a lot of reason why the fuse is a must-have component1) Its cost is very low2) it is a very simple device and therefore not susceptible to breakdowns3) it is able to interrupt very high currents in a small volume4) its behavior is totally predictable and is very well suited to the protection of equipment requiring a start-up current (charging of capacitor, magnetizing current, motor and transformer inrush current, ...)5) it does not need auxiliary power supply.

author: Gregg
author_rank: Super Contributor
timestamp: March 20, 2017, 10:30:01 pm
content: Quote from: james_s on March 20, 2017, 10:10:48 pmIf you think that's brutal, here is a picture of a 480V surge suppressor that blew up when someone pushed the re-settable breaker on the cover after it had tripped and the cover put a gash in a co-worker's forehead just above his eye. If it had been fused, they may have taken the time to consider the possibilities of re-powering a MOV that was basically a dead short. The main breaker ahead of this was rated at 800 amps and it tripped. If you look carefully at the cover picture the severed button of the breaker is at the top left and was rated at 20 amps.

author: Monkeh
author_rank: Super Contributor
timestamp: March 20, 2017, 10:35:30 pm
content: Quote from: Gregg on March 20, 2017, 09:44:43 pmWell that's pretty clearly a faulty design right off the bat. Same with that suppressor - they should never have resettable breakers like that.

author: james_s
author_rank: Super Contributor
timestamp: March 20, 2017, 11:08:40 pm
content: Yikes!I love seeing pictures like this of real world failures, it's baffling that they put a resettable breaker on something like that, especially with a cabinet unable to contain the blast should a fault like that occur.

author: ludzinc
author_rank: Unknown
timestamp: March 20, 2017, 11:09:41 pm
content: Quote from: dmills on March 20, 2017, 08:17:36 pm*than

author: rstofer
author_rank: Super Contributor
timestamp: March 20, 2017, 11:22:19 pm
content: Quote from: james_s on March 20, 2017, 11:08:40 pmIt's not a great breaker design but I don't see anything to suggest there was a case failure. Given enough fault current, it is reasonable to assume the breaker is toast but it would still be unlikely that the case failed. There is a UL test for that...Fuses are precision components. Look at the attached Fuse Curve and work through the examples. They will be enlightening. Remember that there are different curves for different fuse types. Time delay, standard, fast, semiconductor, etc.http://ep-us.mersen.com/fileadmin/catalog/Literature/Application-Guidelines/ADV-P-Application-Information-How-To-Read-A-Time-Current-Curve.pdf

author: retrolefty
author_rank: Super Contributor
timestamp: March 21, 2017, 12:08:31 am
content: QuoteWhile I agree in principle, there is an old saying many olders might recall. "The $10 output transistor blew open to save the 10 cent fuse". It usually gains smiles and memories of the times (however few) where that indeed happened. Reliability is really a system wide parameter, a single fuse is rarely a fix all.

author: james_s
author_rank: Super Contributor
timestamp: March 21, 2017, 12:14:19 am
content: Quote from: rstofer on March 20, 2017, 11:22:19 pmFrom the post with the picture "the cover put a gash in a co-worker's forehead just above his eye"Certainly sounds like the enclosure failed to contain the blast.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 21, 2017, 12:24:57 am
content: Electronic switches have a nasty tendency to fail short, so at overload currents that don't cause disintegration, they won't be very helpful.The other issue is that although you can probably emulate the function of a fuse in mist cases, for safety approvals you need to be able to prove that it has the desired function in all circumstances.Polyfuses and electronic fuses ( e.g. protected MOSFETs) have replaced fuses in some applications, ( or at least prevented one-time fuses blowing) but when it comes to safety, it's hard to beat a good old piece of wire.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2017, 12:43:36 am
content: One word: transients.Inrush current, and surge voltage, to be specific.Traditional fuses burn in the range of 100us (for ultrafast "semiconductor" and "current limiting" types, at rated peak current), up to 10s or 100s of ms (for a fault current about ten times continuous rated current), or 1 to 10s of sec for slow-blow types, breakers, polyfuses and so on. (Polyfuses span quite a range of size, so that an 0805 size part will open in less than 100ms, but a large leaded part may take tens of seconds.)It is adesirableproperty that a fuse can withstand large surge currents without opening. There are several reasons:1. Nuisance trips. You don't want a surge (from mains fluctuations, distant lightning, etc.) blowing the fuse every time.2. Inrush. Many loads gulp huge currents, for durations of a half cycle (e.g., transformers, most SMPSs) to hundreds of cycles (motors). Indeed, a type of resettable time-delay fuse is called amotor starter. (They're resettable because the current flows through a resistive strip, heating a small solder pot; a gear is secured in the pot. The breaker lever is sprung against the gear teeth. When the solder melts, the gear is able to move, releasing the lever. The mechanism is wide open to see, because the elements often need replacing...)3. Safety. Nothing else can endure the absolutely tremendous energy delivered by hot and angry pixies charged to 480VAC or more. Even 240V is on the angry side (and, I would suppose, this is reflected by the greater safety measures taken in European mains plugs, compared to the rather lax US style 120V plugs). The peak current ratings of fuses range from 1kA (for LV and residential circuits) to 200kA (for industrial circuits, and maybe more for medium voltage* parts).*Medium voltage is over 600VAC. 480V is still "low voltage". The practical side is, low voltage doesn't jump out and kill you, you have to touch it first. Medium voltage is on the margin, and includes voltages that jump.So. Can it be done with semiconductors? Unequivocally, yes! Can it be done as cheaply? Fuck no.I've designed and built fuse circuits before. At very low voltage, this is trivial: you can buy commercial devices for 5V, 500mA application (USB 2.0) that are tiny surface mount parts. They offer active current limiting and thermal resetting, with a thermal "trip" (on time) in the 2-10ms range, and off-time (recovery) in the 0.1-1s range.Protected switches are available up to automotive size, e.g., 60V and 10A (suitable for switching 24V loads at as many amps. The current limit is usually inaccurate, landing in the 20-60A range. During a current limit, the thermal trip time is in the 100us range, and recovery in the ~1s range.Making a power limit much higher than this (i.e., 24V * 60A is almost 1.5kW peak), for any length of time (i.e., 10s of ms, sufficient to start capacitors and motors), quickly becomes impractical. You need sheer volume of silicon now, and the price and size spirals out of control at the same time.You can employ alternative topologies: instead of limiting current in the transistor, design the circuit to dump faulting power into a dumb load resistor, or something like that. Now you need to switch at fault current levels, but this is still easier than dissipating it.Suppose you wanted to replace a 15A, 240VAC breaker.To simplify a little, suppose you make a unidirectional circuit: i.e., it works on DC. This is placed across the DC terminals of a FWB, and the FWB's AC terminals are used to replace the breaker contacts. (Thus, when the circuit is "on", a small voltage is dropped across the FWB, in either direction. When "off", full line voltage is dropped and no current flows.)Suppose you will design it for 4x current limiting, i.e., 60A. This should be good enough to start most capacitive and motor loads, without also having to handle full short-circuit current (which is around 2kA for a residential circuit, but a breaker employed for this service must safely interrupt 10kA). The nominal peak voltage is about 340V, but we should rate it for at least 600V, to account for lesser transients, which should be clamped with a MOV, which will drop about that much voltage in the process. This is 600V * 60A = 36kVA of switching area, still no small task, but a couple beefy transistors, wired in parallel, will handle that okay. We need a series inductor and a catch diode, to make a boost converter, and a dummy load to burn the dropped power, perhaps a resistor. (If this "fuse" can have a neutral wire routed to it as well, the excess power can be returned to the source, basically making a current limited AC buck converter.)But perhaps you've already noticed some problems. With clamping at 600V, this device can't be fully off: it will let transients through. Spikes of 2.5kV are largely let through to the load, even when off. That's not very safe!We could stack a bunch in series, but now the on-resistance goes way up. Or we could use big enough transistors, but 3.6kV IGBTs aren't exactly cheap, they switch slowly (low microseconds), and still have a relatively high voltage drop (about 3V, though that's really not much at all compared to the 3.6kV rating!). (On the upside, the power dissipation rating will be high, too, so we don't have to worry as much about switching efficiency.)So, it's hard.Tim

author: Gregg
author_rank: Super Contributor
timestamp: March 21, 2017, 01:10:22 am
content: The surge suppressor that blew up is no longer made with resettable push button breakers. I believe that the newer models have internal fuses that are not replaceable. Even with internal fuses, it is recommended that they be powered off of a breaker in the panel to facilitate replacement of the unit and for additional protection.The underlying cause of failure was two technicians without proper training connected a 750KVA 480/277V 3 phase generator to a curbside generator port that consisted of bus bars connected to an 800 amp 3 pole breaker which in turn was connected to a manual transfer switch located within the building. There was a smaller bus to connect the neutral wires clearly taped white tape [to indicate neutral in the US, a reduced neutral is common in industrial installations that mostly use 3 phase power]. The neutral bus is mounted on the back of the red fiberglass channel in the picture, not ideal. An even smaller connection to ground that was only connected to the port cabinet and the conduits but it was clearly taped green [which met code in the early 1980’s when it was installed].Because of lack of understanding of 3 phase power, the techs thought the small neutral bus was ground and connected it to the chassis of the generator, leaving the neutral floating free.At the time of the generator port installation, the only unbalanced loads [non 3 phase] were for lighting [277V, quite common in the US], but an upgrade to some of the HVAC installed two units that used 277V cooling the same space so that normally only one was on line at a time.The techs turned on the generator and threw the transfer switch and everything seemed OK for about 5 seconds until some of the fluorescent lights went way dim and some blew their ballasts. [One of the HVAC units turned on and pulled the neutral way off center making one phase very low voltage and the other two phases very high voltage; the second may have tried to run after the first wouldn’t run, but nobody was taking notes; if so one leg of the 3 phase was almost at full 480V.]After a couple of fast WTFs the two techs saw that one of the indicator lamps on the surge suppressor was off and the less cautious of the two reached across the shoulder of his co-worker and pushed the reset, resulting in the cover blowing off the surge suppressor resulting in a nasty gash on the forehead of the poor guy in front and probably some hearing loss. They quickly threw the manual transfer switch back to utility power and restored some semblance of order to their situation.What was really lucky is that nobody was touching the chassis of the portable generator that was ungrounded.I was called in after the incident and heard the story from both participants. Especially from the one that decided the neutral was really the ground and the same one that pushed the reset who claimed it wasn’t his fault because the installation wasn’t to code. He was partly correct in it not being his fault, but mostly because he didn’t get any training to understand the implications of playing with high power electricity.Since this incident, the transfer switch was replaced with a smart transfer switch that looks for proper power before it will transfer; basically an automatic transfer switch with a push button to start the transfer sequence. The neutral conductors have been repurposed as ground conductors and 480V delta to 480/277V Wye transformers installed to generate proper  neutrals. The generator port has been upgraded as well.I don’t know why someone put black, red, orange colors on the original generator port; those colors are generally used on 240V delta with a center tap on one phase for 120V.

author: Wytnucls
author_rank: Super Contributor
timestamp: March 21, 2017, 04:09:21 am
content: Quote from: JoeN on March 20, 2017, 08:00:43 pmThe Gossen MetraHit 30M, as a precision instrument, only measures low currents (100mA max).The original design had a soldered surface mount fuse, which blew so often due to careless handling, that the manufacturer eventually replaced it with a resettable polyswitch fuse on a later model revision.

author: rstofer
author_rank: Super Contributor
timestamp: March 21, 2017, 05:19:32 am
content: Quote from: james_s on March 21, 2017, 12:14:19 amIi must have missed that bit, my bad!The breaker was a) poorly designed or b) misapplied. One of the things most people overlooked is the maximum AIC (Ampere Interrupting Capacity). A breaker designed for 10,000 AIC shouldn't be applies to a system with 50,000 amps of available short circuit capability. That particular breaker doesn't look like any I have ever used.

author: Neilm
author_rank: Super Contributor
timestamp: March 21, 2017, 06:34:15 pm
content: Quote from: JoeN on March 20, 2017, 08:00:43 pmIf I remember correctly, IEC61010 does not allow semi-conductors being the only protection againts fire / explosion.Quote from: Wytnucls on March 21, 2017, 04:09:21 amI am surprised they could fit a polyswitch on a CAT rated meter. I can't say I have ever seen a polyswitch that would be good for CAT rated applications above 300V.

author: SeanB
author_rank: Super Contributor
timestamp: March 21, 2017, 06:46:41 pm
content: So thankful I work where you only have a single supply system, triplicated for 3 phase, but still the same thing. Not the US mix of however many they have, single phase, split phase, assorted 3 phase varieties, high leg delta and all with similar looking voltages and scarily enough various floating voltages on the conductors.Here 230VAC to neutral all round, 400VAC between phases and current is going to be the only differentiator for the breakers. The incoming ones are a lot bigger, better rated and have to live being fed by a great big 200kVA transformer via some 200A fuses, but downstream there is a lower requirement from the reduced cable size providing increased source impedance. Biggest breaker is the lift, 100A and cables to match, but the draw is 20A or so per phase, but the supply has to be capable of providing the all important overrun test motor current without tripping before the motor room dashpot breaker.You can have whatever mechanical, electronic or semiconductor overcurrent devices you want, but always upstream of them there will be a fuse of some kind, there as the in case all else fails device, and generally if it goes for anything other than old age something downstream is going to be really badly borked.

author: krish2487
author_rank: Frequent Contributor
timestamp: March 21, 2017, 06:56:41 pm
content: This!Fuses have been around for so long. They are well characterized and have specified response times and capacities. Fast blow, slow blow, delayed trigger, HRC so on. Each has a specific application and characteristics. A single semiconductor alternative will not suit all the use cases.Second, variance in semiconductors. Granted, todays manufacturing has the tolerance to a predictable limit. Then factor in the accumulated tolerances of other semiconductors and passives. Now the net tolerance of the system in terms of response time, capacity, MTBF might or might not be with in the required spec of the application.All these factors make a semiconductor replacement unviable for a repeatable, predictable behavior. Atleast for the technology available today.Quote from: oldway onYesterdayat 09:19:16 AMe

author: Wytnucls
author_rank: Super Contributor
timestamp: March 21, 2017, 08:12:14 pm
content: Quote from: Neilm on March 21, 2017, 06:34:15 pmI'm not sure which PPTC they used, but some of them are rated for 600V/1A fault voltage and current. The impedance of the mA shunt circuit is 14 Ohms on the 100mA range. There is no Amp range.

author: james_s
author_rank: Super Contributor
timestamp: March 21, 2017, 08:27:16 pm
content: How about a polyswitch in series with a slightly higher rated fuse? A big overload will blow the fuse, but the polyfuse will take care of the usual careless mistakes.

author: Wytnucls
author_rank: Super Contributor
timestamp: March 21, 2017, 08:35:42 pm
content: That's a possibility, but is not mentioned by Gossen. I'll see if I can trace the protection components when I have some time.

author: james_s
author_rank: Super Contributor
timestamp: March 21, 2017, 09:45:16 pm
content: Oh it doesn't really matter to me how it's done in that particular instrument, that's just how I would be inclined to design one.

author: Vtile
author_rank: Super Contributor
timestamp: March 22, 2017, 03:34:16 pm
content: It is the most robust solution.Install and forget.PS. I forgot, fuses aren't for direct equipment or human safety, their main purpose is to prevent fire and further collateral damage from the failed system. This in electrical perspective, then low energy electronics might look at them more of an equipment perspective.