Fuses are certainly the best known and, in concept, the simplest of the defensive tools in the Engineering armoury. They are accepted generally as being extremely unreliable but few know why and but seldom blame the real culprit. As a general rule it is best to be careful when things seem simple and this rule certainly applies to fuses. They are designed as the "weak link in the chain"; they are meant to interrupt an electrical circuit when the current increases to a dangerous value and they provide the obvious place to look when seeking the break. Any "failure" of modern fuses to do their assigned job has more to do with what happens after a fuse has ruptured than with the actual fusing process itself.
As stated above fuses are used to protect circuits from the heating effect of excessive current; as such they are connected in series with the circuit they protect and so why does voltage enter the argument? The answer is that a fuse is a thermal switch; when the switch opens the emf of the Source appears across its terminals and it matters very much whether that emf is 5 volts, 50 volts or 5,000 volts. Fuse manufacturers are faced with the kind of compromise which is met so often in engineering. A fuse must have a small thermal capacity to ensure that it heats rapidly and ruptures before damage is caused in the remainder of the circuit; i.e. it needs to be a fine-gauge wire. However fine wires tend to have high resistance which causes a voltage drop; current flowing through the fuse-wire with voltage developed across it spells power-dissipation and so heat is generated thus encouraging a fuse to blow.
The real difficulty arises from the fact that the fuse-wire is required to melt; in turning to vapour it (a) fills the space between the fuse terminals with an ionised and conducting media which keeps the current flowing by means of an arc (b) when the arc is quenched and the vapour cools it deposits copper over the surrounding area and so tends to re-establish a conducting path. A fuse which interrupts a circuit driven by a 5-volt emf is not very likely to cause an arc but the very same fuse used to interrupt a high-voltage circuit can be guaranteed to do so. Then again it matters greatly whether the circuit that is interrupted is resistive in nature or reactive; an inductive circuit even at 5 volts can generate enough voltage to set up an arc while a capacitive circuit can supply a very-large current for a short time and so may sustain a high-current arc (if one becomes established) long enough to allow damage to ensue.
As stated above the most important factor in fuse rating is what happens once the fuse has melted. Most non-designer users will not be interested in the factors which determine the choice of fuse for a given job of protection but, when replacing a ruptured fuse, it is important to understand the markings on that fuse and to choose either an exact replacement or a suitable substitute.
The second most important factor in choosing a fuse is the speed with which it is required to operate. Digital circuits, for example, are extremely intolerant of overloads and any protective fuse must rupture very quickly if it is to be effective. At the other end of the scale there is need for a fuse to “wait and see”; before it interrupts a circuit it has to decide whether there is a genuine fault condition or an acceptable transient. Typical of this requirement is the so-called in-rush current that occurs when equipment is first switched-on. On the output side of a mains transformer there are large-value reservoir capacitors which virtually short-circuit the transformer until they acquire their normal operating charge; trouble comes in large doses if the Law of Maximum Perversity (Sod’s Law or, in the USA, Murphy’s Law) decrees that switch-on takes place just as the Mains reaches the peak of a voltage cycle. (240 volts ac has a peak value 336 volts).
In-rush current is a reality and, before modern fuses became available, it was a menace. I remember throwing the main switch in a television studio to be greeted by a heavy “crump” followed by the task of locating and replacing fifteen sets of mains fuses! Doubtless some of the fuse-failures during the following weeks had their origins in the same incident and in-rush current probably accounts for many of the so-called “tired-fuse failures”. Equipment likely to suffer in this manner is now fitted with time-delay or anti-surge fuses.
Fuses are manufactured and tested under set standards laid down by the Underwriters Laboratories Inc. (UL) in the USA, Canadian Standards Association (CSA) in Canada and the International Electromechanical Commission (lEG) in Europe and Asia (there are others). All fuses are batch tested and bear markings as follows:
These are specified for protecting semiconductor devices or other equipment where the “quick-acting” F-fuses are too slow.
These are intended for use in circuits which are not subject to surges or transients and which will not produce high short-circuit currents. Some F-type fuses are provided with a filler that assists in quenching arcs and these may provide low-cost protection for semiconductors. In American terminology these fuses are known as ‘Normal Blo’ fuses.
M—type fuses will withstand the small transients or surges encountered in normal operation.
These are intended for use in circuits subject to high in—rush currents. They are referred to as anti—surge or delay fuses and, in American terminology, as 'Slo—Blo' fuses. They are characterised by a built—in thermal delay.
These are similar to the T-fuses but they provide an even greater time delay before rupturing.
The current-value which appears on a fuse is the maximum current to which the fuse should be loaded. The exact meaning of this depends somewhat on the Standards to which the fuse was tested. Fuses tested to the IEC Standard should be selected so that the normal current equals the fuse-rating; fuses tested to the UL Standard should be rated 25% greater than the normal circuit current.
The rated voltage marked on a fuse refers to the maximum voltage at which that fuse can safely clear a short-circuit. A fuse can be used at any voltage which does not exceed its rated voltage; it should not be used in circuits where the emf is greater than the rated voltage.
Otherwise known as short-circuit or interrupting rating this refers to the maximum short-circuit current that the fuse can safely interrupt without risk of explosion.
The obvious has been stated already in that fuses are thermal devices but what perhaps is little realised is that their performance is affected by the temperature of their surroundings. The term ambient temperature usually refers to the atmospheric or room temperature but fuses usually live in a private world probably surrounded by a fuse-holder and close to heat-generating components; their surrounding temperature is likely to exceed the ambient temperature. The temperature assumed by any Body stabilises when the heat which it generates is exactly balanced by the heat which it loses; the higher its surrounding temperature the higher becomes that stabilised temperature. A fuse which is running hotter than the test-value must rupture more readily.
Hence in determining the rating of a fuse its operating temperature must be taken into account; if, when replacing a fuse, you are tempted into doubting a particular fuse-rating then take care. A fuse runs hotter also as the normal operating current approaches the rated-current of the fuse; this perhaps might explain why that rogue fuse persistently fails for no apparent reason? As an example: Fuses are tested for their current-carrying ability at a temperature between 20C and 25C but a fuse rated at say 1.6 amps would have to be replaced by a 2-amp fuse if it is to perform at 70C. Some fuses are manufactured with wire connections; in general it is best to crimp or clamp these into place but, if soldered, it is most important to use a heat-sink (sometimes mistakenly called a heat-shunt). The application of heat to any fuse is likely to re-flow the internal solder and so change the characteristics of the fuse.
END OF INFORMATION SHEET 4