I have had some interesting correspondence since I first posted on the subject of Fuses. One correspondent used to be a designer of HRC (High Rupture Capacity) fuses for switchboards. He wrote to me (in the face of the wonderings that I expressed in the last post) that the use of the fine silica sand is a constant, and that the various fuse characteristics are obtained by the design of the metal element. Interestingly, another correspondent used to be a designer of the sort of fuse that you see on a power pole. In that environment, it seems that special care it taken to design a fuse that will behave satisfactorily when there is a sufficient current to make the fuse very hot, but not (in the short term) enough to open it. A red hot fuse would be the sort of thing that would start a bushfire. Apparently in that environment an interesting innovation is to use boric acid rather than silica. Of boric acid, Wikipedia says:
Boric acid, also called hydrogen borate, boracic acid, orthoboric acid and acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, neutron absorber, or precursor to other chemical compounds. It has the chemical formula H3BO3 (sometimes written B(OH)3), and exists in the form of colorless crystals or a white powder that dissolves in water. When occurring as a mineral, it is called sassolite.
Obviously, even in the simple case with a silica filled fuse, the silica has a role to cool the element before it opens and to cool the plasma once that forms. These effects will obviously vary with temperature. In the world of heavy current fuses, I am informed of the following: “It is a common misconception that the fuse will eventually blow at the rated current. If you look closely at most fuse characteristics the minimum melt time asymptotes toward infinity at about twice the rated current. The reason for this is that the rated current is usually the maximum current the fuse can carry without being damaged. So if you have a 50A rated fuse, it will happily carry 50A from now till eternity. Given rating tolerances, which tend to be minus nothing/plus something, it will probably carry more than 50A with no ill effect.” Is this the case with small fuses of the type and style that we might install on a circuit board? Here is a set of graphs from a LittelFuse catologue.
The 1 Amp curve does seem to be at about 2 amp at the 100 second line. This is somewhat quicker than “till eternity”. It looks as if we need to look carefully at these details for each special case. The extremes are easy. The “Fuse Survives” and “Fuse opens promptly” regions seem pretty clear, but there be dragons in the middle there. Often in electronic circuits, a fuse may be desired, yet unless special measures are made, there might not be sufficient fault clearing current to open the circuit promptly. The use of a dirty great SCR as a crowbar used to be popular. The SCR can be triggered by either excess current, excess voltage, or indeed, any other fault condition for which we would like to open the circuit. In cases where we do not want to add the expense of a crow bar, we might nevertheless want to minimize the problems caused by dithering around in the badly defined region. Some circuits can make this situation worse. Here I will tell a story against myself. I had a design in gestation, and the client wanted a fuse added. The addition of a fuse was not to protect against any particular perceived fault condition: it was just that the existence of the fuse was going to add to the client’s sense of well-being. Here is the power supply part of the circuit that I drew up at the time:
(Hey you young blokes, don’t knock my drafting! This was drawn 2 years before the Protel (Altium) company came into existence!)
All three of the three terminal regulators have output current limit, and over temperature shut-down. Thus they rather protect the fuses from short circuits or over current faults down stream. It seems a bit silly now to spend time trying to justify details of a circuit that dates from 1983, but I suppose that if one of the regulators has a melt-down, we want to protect the power transformer from being cooked.
A big problem here is that the fuse is carrying the bridge current, and the regulator is drawing DC current from the res cap. As is well known, the RMS current at the rectifier is higher than the DC output current with such a circuit. I used to go by a rule of thumb that the RMS current was three times the DC current. Of course it all depends on the impedances in the curcuit, and how “spikey” the bridge current waveform is. Without worrying too much about the assumptions, I did a quick circuit model. Here is the circuit.
I have made all the resistances VERY low, and not included any leakage inductance in the power transformer at all. Refinement of this model could be for another day. The load current in the circuit ramps up to 10 amps. Here are the results:
In this low impedance, 10 amp output circuit, the results are:
With res cap value = 10mF, RMS current is 1.65 times the DC output current
With res cap value = 100mF, RMS current is 2.6 times the DC output current
With res cap value = 1F, the RMS current is 2.9 times the DC output current.
The capacitor values here are huge. These sorts of numbers would also be obtained with much smaller capacitors with a lower current circuit.
Thus if we have a design where the DC current to a regulator was 1 amp. we would need a 3.5 amp fuse at least. The problem is that with a regulator failure, the first thing that will happen is that the res cap will empty all its energy into the regulator without any damage to the fuse. The current in the regulator might then be (say) 10 amps. This is ten times the normal current, but the current in the fuse has only increased by a factor of about 3. The short circuit is a lower impedance than the res cap over the frequency range of interest, and the circuit is no longer acting as a peak rectifier. If a fuse from a family such as that represented by the curves above is used, it will open in about 20 seconds. Is that as fast as you would have thought for a ten fold increase in current to the regulator?
One of the experts told me that a problem with fuses of 10 A rating or less, is that they open very quickly. I made a quick attempt to model a fuse at the front of a switched mode circuit to see what overvoltages I might see with such a high di/dt in the leakage inductances. In the time I could spare, I did not get the model to work. Mental note – watch out for this.
I designed a circuit similar to the one shown above, but the power supply had to supply a dot matrix printer hammer driver. I do not have a record of the value, but this circuit had a very large res cap to minimize the volt drop on a printer hammer drive event. I was watching the prototype during software testing, and noticed that the wire in the 3AG glass fuse was running red hot. I increased the fuse rating. I had wanted to provide hardware to determine the timing of the hammer drive pulse, but the client insisted on doing this in software on cost grounds. During testing, the software left one (or more) of the hammers on for an extended time. The circuit board tracks failed before the fuse did. This was 20 years ago, and I have always taken care with my fault scenario analysis since then.