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Fuse Basics - The Essential Concepts of Working With Fuses

This webinar, given by Dave Komm at Mersen, covers the essential concepts of working with fuses including their construction, basic design, operation, performance, and maintenance. This webinar helps you understand how a fuse operates during a short-circuit or overload situation, and also discusses cycle fatigue.

See the full transcript of the webinar below.

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Full Transcript of the Video

Dave Komm: Alright, we're going to get started here. I want to thank EasyPower for putting on this webinar. My name is Dave Komm, I'm a solutions engineer with Mersen. For those of you who are on the call that aren't familiar with Mersen. We were formally Ferraz Shawmut and about 7 years ago we changed our name Ferraz Shawmut or Gould Shawmut, whichever of those names you remember. So our primary focus is fuses and over current over voltage protection so we're gonna go through some basics on fuses today. So, getting into the presentation we're gonna talk about typical construction methods. Some of the components we use in our, in our fuses or in fuses in general, types of operations whether short circuit, overload, cycle fatigue. Go a little bit into UL (00:01:00) fuse classes and different types and why we have each type. If you guys have any questions or is there a question box, just type them in there and I'll try to answer questions at the end.

First type of fuse construction, it's going to be a ferrule design, typically up to 60 amps in the UL style fuses, and this is a traditional when you hear dual element time delay fuse this is that type of construction. So we call it a strict spring and plunger assembly. So we start with this little housing to house a spring and a plunger which is shown here. And then we add a heater tab and really all this is doing is helps to focus some heat which is the electrical current within the fuse. Then we add a short circuit element. We'll talk about how these parts interact with each other a little further on, Typically (00:02:00) short circuit elements are going to be copper or silver depending on the fuse, depending on cost, things like that, performance, would determine what types of metal we use in there. And you can see this element has kind of a U shape to it. That's just for manufacturability. that if we had it with a flat element it'd be a lot more flimsy. So adding those U bends to them is really just adding a little more rigidity to it.

So when we're assembling it in our factories it's easier for, for people to handle and work with. Then we add this little rubber grommet, and really all that's doing is helping keep any of the filler material from binding up and getting stuck in that plunger assembly when that tries to operate, so really just making sure there's no sand getting wedged in there. Then typically we'd slide a body on the fuse, put one end cap on. Once we've done this then we will fill it with sand. Typically were using a silica (00:03:00) sand. We know right down to the grain size of the sand we are using in there and different mixtures to get performance out of them. And we'll fill in the fuse and then we'll actually put it on a vibration table to try to compact that sand down as much as possible. We really want it to be a good compact fill in there. So we'll do that step a few times, maybe two or three times of filling it, vibrating it, topping off again by vibrating it.

And really making sure there's a good compaction of fill in there, fill media. Then we, we put the other end cap on and typically it's a blind solder where it's brazed after the end cap is put on and then everything is heated up and brazed together to make a good solid electrical connection in the fuse. So it's typically up to 60 amps, could be 100 amps, 100 amps is kind of a, could be a mixture of this and the next design we're gonna see depending on the fuse types (00:04:00) that's being used. So when we get over 100 amps typically a lot of manufactures start to do a blade block assembly. When we look at these it starts with a block and then we braze a blade to it. You can see in this, this drawing it looks like two different materials, typically they are. A block would be a brass material and then the blade being copper and really just to get, really just to get costs down. If we made that whole thing out of solid copper it's gonna have a higher cost on the fuse and really the performance benefit of doing that versus brass doesn't outweigh the cost benefit. So, traditionally it's two different types of material and then do plate then unusually.

Some fuses do use aluminum, like a solid forged aluminum end cap. Then from here we (00:05:00) start to add elements into the fuse. So, this fuse typically is gonna have more than one element in it. Again, elements will be typically copper or silver. We do use some other types of materials if they're real low current rated fuses. If you get into some different materials they are typically copper, aluminum, or gonn, or copper or silver, excuse me. Are gonna be the most common materials and you can see in here there's these little notches, little circle. So those are stamped out, smaller cross-sectional areas. and that's what's going to open on an over, over current short circuit type condition. Each set of those notches is roughly about 125-150 volts. Somewhere in that range depending on design. So, when we're talking fuses typically when you see a medium voltage fuse, (00:06:00) you know, 15 KV it's gonna be a lot longer than a 600-volt fuse and that's because we have more of those notches in there. Typically, those elements I don't show them in this, but typically wound around a core or 600 volts we can do it in a straight line. Then on this fuse because it is a higher current ratting we're gonna stack elements. So, if we said this was a 400 amp fuse we're gonna have 4, essentially, 100 amp elements.

And again, that's kind of a performance thing. It's easier, or we can melt four smaller elements quicker than we could if we had one element that was 400 amps versus 4 100 amp elements. When you get up into 1000 rated fuses, couple 1000 amp rated fuses there's probably 100 different elements in there and it's really to get the speed out of the fuse, (00:07:00) get it to be very current limiting rather than try melt one big solid element we split it out across many smaller elements. Then from here, so what we would do is solder on the other blade block assembly and put a body on it. Again, we would add filler at point, typically a silica sand, through one of the holes you can see there at the end of the block Again, vibrate it down and really make sure there is a good compaction of fill material inside and then once that's done the fuse gets plugged up.

So, I wanna try doing a poll question. I've never tried these before. So, I just opened a poll question. So, I want to get everyone's opinion. What's a harder voltage for a fuse to clear? Direct current or alternating current? And this is kind of going the lines of each notch section. So (00:08:00) 125-150 volts. So, it looks like a lot of people have answered. About 75%. So, we're at 41% saying alternating current, 59% saying direct current. So, direct current is harder for a fuse to open. I always like to ask that question because it's always split almost 50/50 on what people think. Generally, when don't see, when you see a voltage rating on a fuse and it doesn't say AC or DC it's assumed that it's AC current. DC is much harder for a fuse to clear because it's a steady state voltage versus an alternating where at some point it's going to be driving itself back down to zero current. The fuse has to drive everything back down to zero. And with that, voltages are a maximum rating. So, I've got another poll question. (00:09:00) What happens when we use a fuse over its maximum voltage rating? So, I just opened up a poll. So, nothing happens, the fuse acts like it should. Opens safely as soon as power is turned on. Opens catastrophically when is turned on. Or nothing until a fault occurs and then it opens catastrophically. So, we're about 70% response, 75% response. So, the winner is nothing happens until a fault occurs, then the fuse opens catastrophically. 60% of people responded to that. That is the correct answer.

So, if you install a 600-volt fuse on a 1000 volt system, nothing's gonna happen as long as there's no over current present. Once the fuse tries to open and it sees (00:10:00)is that excess voltage. What happens is it kind of runs out of notches here. So we only have so many notches to spread the voltage out across, and by the time it hits that voltage it's got nothing else to do and it starts burning away element, portions of the element where it's not designed to. It can lead to a catastrophic failure. We have a high powered test lab so we get to test different things within our facility. So, I always like to show some videos. This is, this was actually a DC test. It's a 600 DC fuse that we are trying to see if we can push 1000 volts through it. Just to try to see what the fuse would do, maybe we over designed it. As you can see the fuse really did not like trying to clear out 1000 volts where it was only 600 volt rated. We kind of let it sit here and burn, (00:11:00) burn for a little bit. And if I had the sound on you'd actually be able to hear our lab manager in a few seconds here saying, "I think that's enough." What happens is the fuse is essentially completely out there and now it's arcing across our rectifier terminals is what wound up happening there. Alright, I can show this one again and I do have a couple more videos I'll show during the presentation from our lab. So, moving on talking about fuse performance, over currents.

Just to make sure everyone's on the same page here we'll talk over currents. Anything that's in excess of the continuous current rating. (00:12:00) Two types of over currents. Short Circuit or faults, can be used interchangeably, those terms, and then an overload. So, what are we talking about here. Again, I would assume most people on the call are familiar with these terms I'd just kinda like to set a baseline. So, anything that's in excess insulation breakdown, falling metal objects, those are the types of things when we're talking short circuit or faults. Accidental phase to phase fault conditions, high current, hopefully short time period, and then what are we trying to prevent? Whether it's using a fuse, a circuit breaker, whatever the device is. So, this is another lab test we did. This one is in slow motion, this is about 750 MCM cable that we looped between the bus powers and we're gonna apply a short circuit to it. So again, slow motion, I know it's not the best quality video. (00:13:00) But you can see those electromagnetic forces begin to whip around the cable. There's a tremendous amount of heat there, we left the insulation on, that's what all the smoke is, insulation burning off, so that's what we're trying to prevent when we're talking short circuit fault conditions. Overloads, just the opposite of that. Anything that's gonna be a lower current and will cause damage overtime if allowed to persist. Typically fires, that would be the biggest cause, or biggest effect of it.

This is real time video now. 750 MCM cable, between 2 bus powers in our lab. We did strip the insulation off for this test. And then we're just gonna sit here and overload the cable. And we'll see right about (00:14:00) now it's gonna start sagging under its own weight as that heat starts to build up in the cable. And we let this go to the point of failure really to show, if there's nothing there to take out an overload they can be pretty hazardous. This point it's like a little, large light bulb, bright red. And then eventually the cable's going to get to the point it no longer can hold it, melts completely off. This incident in our lab actually did cause a fire, we had a little rubber mat. You can see all the black smoke that the cable landed on. See the flames kind of hitting the bottom of the screen and then the fire extinguisher. So, this overload we actually did cause a fire. (00:15:00) So fuse performance within EasyPower, you guys can pull up all these characteristics, all the curves. Time current curves, so typically the lower portion's gonna be the short circuit region, the 0.1 and lower and then above is gonna be considered more, the overload portion of the curve. How do we read them? Fairly easy. So, we find the available current on the horizontal axis, so in this case 400 amps or less are available. And then the fuse will take 30 seconds to open 400 amps, in this particular example.

Another data performance, kind of hand to hand operation is peak left of the current. So, on the horizontal axis we have available current in RMS. Peak current going up the vertical (00:16:00) axis. So, to read these again, we would have our available fault current on the bottom. In this case 20, 30, 40, 50,000 amps available. We follow it up till we hit the fuse curve, go over and we can read the peak let-thru which is 9000 amps. Now we gotta remember, in this case we're not comparing apples to apples, it's RMS current peak current, and I'm sure some of the people on the call have heard of the up over down method, if you want to get RMS let-thru. So, you can take that 50,000 amps, follow it until you hit the fuse curve, over till you hit this greyer line. What that grey line is, is the actual maximum amount of current the test current could produce if the fuse wasn't there. So, when we look at this curve we can also determine the current limiting point of the fuse where it splits off from that (00:17:00) grey line. That is where the fuse is going to start entering its current limiting range. Before that it's going to be thinking it's more of an overload and it's not going to be current limiting. It'll take a longer duration to open.

So, when we look at a fault here we have a full load current, in green. Our available fault in red. The fuse is going to open. Typically, if it's a short circuit, our current limiting fuse is going to open during that first quarter cycle. And limit the amount of peak current that's there. We zoom in on that peak current. It is just the peak value of that let-thru curve. And then when we're talk performance on short circuit, how a fuse operates. So the first part of this, the rise time, (00:18:00) is going to be a melting "i squared t", it's a measure of thermal energy, and what's happening at this point, the fuse is melting and all these notches up on the top, the 4 notches that we have in the short circuit portion, they going to be continuously heating and heating and heating, and when it hits that peak what happens is that there is an arc that form because the element has melted at those portions and the voltage is arcing across them.

And then the second half of this let-thru curve is the arcing time, or the arcing "i squared t", the total is a clearing "i squared t". So as that's arcing all that sand filler that we compacted in there fills in around the arc and quenches it. And the sand actually melts and kind of turns into a glass like material which is a great insulator and helps disconnect the circuit. (00:19:00) Why are these values important? If we're talking about selectivity or coordination between 2 fuses, we have to look at the time current curves, but we also have to consider the portion that's not represented on the time current curve which is below 0.01 seconds. And we have to look at the branch fuse of what the clearing "i square t" is. So that fuse versus the melting "i squared t" of an upstream fuse. So, in this example the downstream current has a clearing "i squared t" of 1000 amps^2 seconds. The main fuse has 1500 amps^2 seconds. That downstream element will fully clear before we melt the element in the upstream fuse. And that's a portion so there's another side we have to look at when we're trying to coordination fuses, not just looking at time current curves.

So, overload portion, (00:20:00) I'm gonna show the time delay dual element. So, what happens on the spring plunger assembly where the connection is there is a eutectic solder here, and that solder is designed and calibrated to melt at a specified temperature. Once that heats up and hits that temperature it melts, and this whole spring draws that plunger back and disconnects the circuit. You can see up in the top picture the fuse cut away. And that's how we're doing typical on a dual element time delayed fuse. How overload protection is achieved. When we're talking fuse operation, standard motor, motor characteristic, starting motor characteristic. Just gonna draw, represent that with lines across the top on time current curve. So, the importance of a time delay fuse is that we can size it (00:21:00) closer to what the full load current is, and achieve motor starts is really where it is. So, in this example we have the blue fuse which is a fast acting or a non-time delay fuse versus a time delay fuse in red. And you can see where the curves line up versus a motor starting characteristic, that the 100-amp time delay, we get a little more room in the middle of the curve and we're allowed to start motor.

So, this brings me to the last poll question I made. So out of these 2 fuses, which fuse is more current limiting? The non-time delay which is the A4J represented here or the time delay fuse, the AJT? About 3/4 of the people voted. So, the results from what everyone voted on. 51% the fast acting fuse is going to be more current limiting. 16%, the time delay is more current limiting, (00:22:00) and then 33% said they're equal. Not uncommon results when we ask the question. From this curve, we can't tell which one is more limiting. We have to look at a peak let-thru curve, and I do pick the 100 amp fuses in this example for a particular reason. When we look at this. Here's our time delay fuse. So, peak let-thru, if we remember where we split off, that's where the current limitation starts on this fuse. Now I draw our fast acting fuse over it and we can see now that the time delay fuse is going to be more current limiting. And I generally ask the question because people this fast acting and they think that means more current limitation and it does not necessarily mean that. It just means that it doesn't have time delay built in to it. In many cases time delay fuses are newer design (00:23:00) and they're going to be the better design of the 2 and have more current limitation built in to them just because they are newer designs and newer technology, things like that.

You can always use a time delay fuse, even when you don't need the time delay. Unless we're talking semiconductor applications, which is a whole different world. But in the general scheme of wire protection, things like that you could always use a time delay fuse and generally speaking they'll be more current limiting. Now when we talk cycle fatigue, particularly with motors. Obviously, that can be a big things with motors starting, stopping, pumping things. What happens in a fuse is, when you think about it, the fuse is heating up and then it cools down, and then it heats up and then it cools down, heats up, cools down. You keep going through those cycles and if you cut a fuse open that has opened and you suspect cycle fatigue you actually look at where (00:24:00) these notches are, and we get little S-bends, and what that is, is the heat expanding and contracting the metal, and kind of form little S's you can see in the smaller areas when you dissect the fuse. Over time that can only expand and contract so many times, you one set of them that just breaks. It's like folding a piece of copper back and forth a bunch of times. At some point, it's gonna break. And that's generally what happens on cycle fatigues. You can tell that by dissecting it, looking at these areas. Sometimes you can see it by the naked eye, a lot of times you need to use the microscope to look and see those bends. It kind of depends on what the current is there that we're cycling through. How drastic it is and how much it's expanding that metal and contracting it. But there is a lot you can tell by doing that. Now, general overview of UL, (00:25:00) UL 248 is the fuse standard for all UL low voltage fuses. So, these are the primary classes, class cell fuses go from 100 amps all the way up to 6000 amps. 600 volts only. Class J's are going to be 600 volts. 0 amps to 600 amps. Class T we offer them in either 600 volts or 300 volts, and those can go up to the 300 volt is up to like 1200 amps, 600 volt is up to 800 amps in these classes. Generally real small compact size no time delay in the class T's just because the body is so small we can't really get the performance we need for a time delay.

And then we have class K5, which is an older one time, if you hear someone say "one-time fuse" (00:26:00) that's generally what they are talking about. Not the greatest fuse but it's there. 600 volts, 250 volts, 0 to 600 amps. Then we jump to an RK5, which would be, again, 600 volts, 250 volts, 0 to 600 amps. RK1 same thing. It's 250 volts, 600 volts, and we can go up to 600 amps. CC's that's a control circuit fuse. 30 amps and lower. 600 volts is what those are offered in. And then midget fuses, again, typically 30 amps and lower, 600 volts. But they could be 250 volts. There's no real UL definition. It's whatever the manufacture picks for the fuse. And those are supplemental protection only. Different body breaks for anything below 600 (00:27:00) volts. 60 amps and bellow, like I said earlier it's going to that ferrule construction, ferrule design. And then when we get above 60 amps we get into bladed product for UL. So, I just wanted to show those body size breaks for the fuses. Then when we talk, when I had K5, RK5, and RK1 on the slide a few slides ago, they're all very similar and it's really like a history lesson here. In this one I added class H which hopefully no one out there sees or uses. They're the old renewable, screw on end caps. Not great fuses. There's no fill in them. Like I said the fill helps quench arcing so it actually help drive current limitation in a fuse.

So, we go from H which is non-current limiting, (00:20:00) only 10,000 amp interrupting rating. So fairly low by today's standards for as far as what it can handle for current. Then K5, we get into a fill but it's not a grade, not a silica fill. Typically, they're gonna be a lower cost fuse. Those are going to be 50,000-amp rated. And then into an RK5, which we start getting into silica sand. Typically, it's gonna use a copper element. 200,000-amp rated, and then an RK1 is going to use a silver element and again, 200,000-amp rated. So it's kind of a history lesson of how we've progressed from class H being the original design of the fuse going way back 100 years. And then getting to RK1 which would be the newest progression. Latest and greatest design of them. And what that means when we're looking at let-thru curves here. So, class H (00:29:00) and K5. K5 would, would be on here but it's not considered current limiting by UL so it won't limit to the first quarter cycle. Class H is not current and won't really limit even close to a quarter cycle either. When we look at RK5 and RK1 there's a drastic difference here. It's an 80% reduction in let-thru when we go from an RK5 to an RK1.So, very drastic there. And then class J is going to be a little closer to RK1, but a little better.

Typically speaking, class J fuses are used for new installations. RK1 would be used for retro fits in a plants. We have existing safety switches, things like that. You'd throw an RK1 or an RK5 in there. and then from an arc flash stand point (00:30:00) I like to show this video. So, first test, no fuses and an arc flash. This is gonna model a molded case breaker. This is real time. 18,000 amps, real time test. So, we can see the arc flash with 6 cycle breakers is what's actually disconnecting the circuit here. Now we're gonna install RK5 fuses. And that's we did was remove the breaker and install RK5 fuses. So, hard to tell. It is a little better as far heat that's available. This is just gonna replay that same test. Hard to tell with the naked when we're seeing it real time. It's hard to tell if there's really any difference between that and the first test with no fuses. (00:31:00) Now we're gonna go to the RK1 style. So, like I said, 80% more current limiting than an RK5. That's pretty drastic. We can see that one with the naked eye on what we're doing just by jumping to that newer, latest, greatest fuse. And essentially it's switching from copper elements to silver elements. The film may change a little bit but it is gonna help reduce arc flash safety. And that's kind of high level overview. (00:31:34)