Purposeful Protection
Just because an electronic device is simple, you shouldn’t relegate it to an afterthought in your embedded system design. Such is the case with fuses. Robert explores the fundamentals of this seemingly simple device. In this article, he dives into the history, key specifications and technology of fuses. He also steps you through an experiment to analyze the performance of fuses and shares his results.
Welcome back to the “Darker Side.” As electronic system designers, we’ve become used to dealing with some fantastic and ultra-complex pieces of silicon in our projects—microcontrollers running at hundreds of megahertz, multi-core processors with billions of transistors, wireless transceivers with Gbps of throughput, miniature power converters with close to 100% efficiency and so on. Ok, of course some small discrete parts are still required around those key building blocks, but we’re inclined to disdain such components in the design phase. That’s because they represent a very small portion of the overall bill of materials and have low perceived value.
All that said, if you are a regular reader of this column, you already know that’s a bad choice. Some electronic components seem very simple—passives in particular. But such devices may be the source of incredible trouble if you don’t understand the intimate details of their behavior. If you have any doubt, go and re-read my articles on capacitors—for example, Circuit Cellar 283 “Dielectric Absorption;” Circuit Cellar 317 “Decoupling Capacitors and RLC Networks;” Circuit Cellar 321 “All Ceramic Capacitors Aren’t Equal.”
This month, I will talk about another very simple part that isn’t as simple as it seems: The fuse.
WHAT’S A FUSE?
Of course, you’ve all seen a fuse before. Fuses are as old as electricity. According to Wikipedia, their first documented use was in 1864 for telegraph installations [1]. The first patent on a fuse was registered by Thomas Edison (him again?) in 1890. Today, fuses are everywhere, and range from ultra-miniature, surface-mounted devices to massive units used in nuclear-powered generators. Let’s restrict the discussion to small fuses common in electronic devices, such as the ubiquitous 20 mm x 5 mm fuse cartridge, illustrated in Figure 1. The picture speaks for itself—a fuse is nothing more than a wire. It is designed to be a protection device, and open the circuit in case of overcurrent. The wire is designed to melt above a given current threshold and to open the circuit.
Let’s spend a few minutes on these words: “protection device.” What does this mean? What is protected by the fuse? The answer to this question is not as obvious as it seems, because a fuse serves two purposes. First, it helps to protect the components of the device itself—meaning the device after the fuse— from extensive damage in case of a fault. For example, a fuse at the input of a power supply could save sensitive parts from destruction if the power supply malfunctions. Second, a fuse isolates the device from the outer world when the device is faulty, and this helps to prevent greater damage to other equipment.
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“Protection device” also means that a fuse should not be, by itself, a potential source of hazard. When the wire in a fuse is melting, it will be hot and liquid, and could start a fire without adequate precautions. That’s why a fuse wire is always hermetically sealed, like the glass tube in Figure 1. That’s a requirement. Fuses are regulated by standards, mainly IEC 60269 [2] (for residential or large fuses) and IEC 60127 (for miniature fuses like my 20 mm × 5 mm example). Ok, Americans prefer UL248, which is a different standard—but the spirit is the same. In any case, these standards state that a fuse should not allow any external sign when a fault occurs. In other words, that means that everything should be contained within the fuse body. No smoke or other material is expelled. This is true as long as the fuse is used within its specifications. More on that in a minute.
The term “overcurrent” also needs some explanation. What is an overcurrent? Is it a current just above the nominal current? For how long? Or a short circuit with thousands of amperes? Let’s dig into more details …
FUSE SPECIFICATIONS
At this point, I encourage you to look for the datasheet of any standard fuse, and to read it carefully. Of course, you will find that a fuse is first specified by its package type and rated current. The rated current, written on the fuse, is simply the maximum current that it can continuously conduct without any problem.
The second key characteristic of a fuse is its speed. How fast will it blow in case of trouble? As you might expect, this depends on several parameters, and the first is the current. The greater the current passing through the fuse, the faster the wire will melt and cut the link. What are the tolerated limits? For miniature fuses, two speed grades are available and specified by EIC60127-2: Quick acting (“F” type, for “fast”) and time-lag (“T” type). Typical values, their respective minimum and maximum breaking times, depending on the effective current are given in Table 1. A caution here: Standards are evolving, so always consult the latest official version of the standards for any precise information. Now, look again at Table 1. You will see, for example, that a quick-acting miniature fuse, when a current 275% higher than its rating is applied, must cut the wire in less than 2 seconds, but not less than 10 ms. These durations become respectively 10 seconds and 600 ms for a time-lag version.
Two important points here. First, look again at the second line of Table 1. It might be surprising. When the current is 210% of the fuse rating (for example, 4.2 A for a 2 A fuse), the only requirement is that the fuse must blow in less than 30 minutes. And 30 minutes is an eternity, compared to the lifetime of a stressed transistor. That’s why fuses usually don’t protect semiconductors!
The second point is that this table comes from IEC60127-2, which, as noted, is the international standard. Unfortunately, it isn’t used by the US, Canada or Japan, which instead use UL/CSA248-14. Both standards are more or less similar, but are clearly not identical. That means that a 2 A, 20 mm × 5 mm “F” fuse according to the IEC standard is not the same part as its UL equivalent! The differences may seem minor, but they exist, and you should be aware. The specific reference standard should be defined in the bill of materials. You’ll find more information on this topic in the selection guide document “Fuseology” by Littelfuse [3].
AMBIENT TEMPERATURE
Another difficulty is that ambient temperature must be taken into account. Of course, metal melts more easily if the ambient temperature is high. This parameter—namely the temperature derating—is defined by the fuse manufacturer and should not be overlooked. A 10% current trip-off variation from minimum to maximum temperature is not unusual. Once again, this may or may not be an issue, but if some customers complain that their fuses blow when others don’t, you’d better check their ambient temperature. If the first lives in Hawaii while the second is in Iceland, then you might have a clue why.
Another important specification of a fuse is its so-called “breaking capacity.” This is the maximum instantaneous current at which a fuse will safely cut. Why such a limit? Because physics is, from time to time, insidious. Take, for example, a standard miniature fuse like the one shown in Figure 1, and apply a huge current, say 10,000 A. The wire will vaporize instantaneously, and the metal of the wire will probably be deposited on the inside of the glass tube.
If you are unlucky (and Murphy’s law states you will be), this deposit will make a conducting path between the two electrodes and the current won’t be cut. That’s why standard miniature fuses have a limited breaking capacity. IEC standard states it should be at minimum 35 A or 10 times the rated current, whichever is higher. If this could be an issue in your design, you’d better know that miniature fuses with high breaking capability do exist. These models are specifically designed to guarantee a 1,500 A breaking capacity, and are usually filled with sand. The sand limits the risk of metal deposition.
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Finally, fuses have a rated voltage, which is simply the maximum safe voltage that a fuse can cut. If there is a higher voltage between the two electrodes, then there is a risk of arcing. A plasma will be formed between the electrodes, and will conduct some current even if the wire is cut. This rated voltage must, of course, be higher than the circuit’s working voltage. A caveat here—you might think you’re clever and try to connect fuses in series to try to increase their rated voltage. This simply doesn’t work. One of the fuses might blow and may be arcing, while the second might still conduct a lower current. Just select and buy the appropriate fuse.
AN I2t EXPERIMENT
Let’s come back to the melting time. As explained earlier, it is linked to the current going through the fuse. But how? What is the relationship? How can you estimate this current breaking time for a given fuse? Let’s think about it. You might remember this version of Ohm’s law: P = R x I2. That means that the power dissipated in a fuse, which acts as a resistor, is proportional to the square of the current. You might also remember that energy is a power multiplied by a time. So, Energy = P × t, and therefore Energy = R × I2 × t. And in which situation will the fuse wire melt? When the energy dissipated in the wire is enough, meaning that Energy > threshold. This means that
R × I2 × t > threshold, or simply, assuming the resistance R is constant for a given fuse:
I2t > threshold
So, the melting time of a given fuse is, in fact, defined by a constant—namely the square of the current multiplied by the melting time, or I2t. If you double the current, then the melting time theoretically will be divided by 4 (the square of 2), and so forth. Simple rule, isn’t it? If you look at the datasheet of a given fuse you will find this so-called I2t value.
Why not experiment with this? With that in mind, I decided to destroy some 2 A, fast, 20 mm × 5 mm fuse cartridges for you. The required test setup is very simple, so you can easily reproduce it at home (Figure 2). You will need a lab power supply, a fuse holder, a low-value resistor and an oscilloscope connected to the resistor. And a box of fuses, of course. Put a fuse in place, set the power source to a given level, switch it on and measure both the current (proportional to the voltage measured on the scope) and the tripping duration (simply read on the scope’s horizontal axis).
My actual experiment is shown in Figure 3. I used a powerful TDK-Lambda GENH20-38 power source, a DC-DC compact unit able to supply up to 38 A and 20 V. I also used a 1 Ω, 10 W power resistor, enough to get up to 20 A for short durations (even if this creates 40 W of instantaneous current, as P= RI2). I measured the voltage on the 1 Ω resistor with a Keysight 34461A multimeter, while the current through the wire was measured with a Keysight DSO-X 3034A oscilloscope and an AIM-TTi I-prober 520 current probe. Yes, I could have just connected the oscilloscope voltage probe to the 1 Ω resistor, but using a current probe was, well … more fun.
THE RESULTS
What happened? Look at the oscilloscope plots reproduced in Figure 4, Figure 5 and Figure 6. On these three plots, the current was respectively 4.1 A, 6.51 A and 12.95 A. The durations required to blow the fuses were 1.38 seconds, 0.269 seconds and 0.06 seconds, respectively. I performed the test with five different currents, set by different voltages on the power supply. My results are given in Table 2. This spreadsheet also gives you the calculated I2t value—once again nothing more than the square of the current multiplied by the time required by the fuse to cut the circuit. And these results are impressively close to the theory. For all tests from 6.51 to 12.95 A, the measured I2t value is nearly constant, close to 10 A2-seconds.
However, for a lower current, here 4.1 A, the fuse time is far higher, more than double what was expected. Why? Probably because, as the fuse was heating slowly, this heat had enough time to dissipate through the fuse holder and ambient air, delaying the fuse melting. Interesting, isn’t it? Another interesting result of this experiment is, in particular, visible on the first scope plot (Figure 4). As you see, the current is starting to decrease a little just before the cutoff. What’s happening? My interpretation is that the wire was starting to become very hot or maybe to melt, and its resistance was increasing a little, reducing the current thanks to Ohm’s law. If you don’t agree with this interpretation, just contact me.
For your convenience, I’ve also graphed these current and tripping time measurements (Figure 7). As you will see, the measurement points—except the first one—are nearly perfectly aligned on the graph. As you will have noticed, I plotted this graph using logarithm scales. The fact that the measurement points on such a log/log scale are aligned is a direct consequence of the constant I2t property. (Note: the log of a product is the sum of the logs.)
WRAPPING UP
So, here we are. My only goal was to show you that things may look strange, but are easily explainable by simple laws of physics. Fuses are not magical, and shouldn’t be considered as more than what they are: simple wires that melt under proper conditions. By the way, if you have any doubt, just find a blown fuse and look at the wire under a microscope. You will easily see melted metal, and the melting depends on the current. I did the experiment for you, and one of my best pictures is shown in Figure 8 for your pleasure.
As usual, I’ve only scratched the surface of the subject. In particular, I didn’t talk about the different kinds of resetable fuses. That is another interesting but long topic. Maybe in another article? In the meantime, remember that the best way to learn is to practice. Fuses are inexpensive, so just do some tests by yourself! And have fun!
For detailed article references and additional resources go to:
www.circuitcellar.com/article-materials
References [1] through [3] as marked in the article can be found there.
RESOURCES
AIM-TTi | www.aimtti.com
Keysight | www.keysight.com
Littelfuse | www.littelfuse.com
TDK-Lambda | www.tdk-lambda.com
PUBLISHED IN CIRCUIT CELLAR MAGAZINE• August 2019 #349 – Get a PDF of the issue
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Robert Lacoste lives in France, between Paris and Versailles. He has more than 30 years of experience in RF systems, analog designs and high-speed electronics. Robert has won prizes in more than 15 international design contests. In 2003 he started a consulting company, ALCIOM, to share his passion for innovative mixed-signal designs. Robert is now an R&D consultant, mentor and trainer. Robert’s bimonthly Darker Side column has been published in Circuit Cellar since 2007. You can reach him at askrobert@lacoste.link.
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