The Physics of... Fuses

The earlier subject was "APPS: Fuses for 24 Vdc Circuits." This presentation is to eliminate some myths, misconceptions, misunderstandings, and plain poppycock, as to how fuses function and what role Vac or Vdc plays. Furthermore, discussion is limited to "low" voltage systems, i.e., 600 Vac/Vdc or less. First. if voltage level were ignored there would be no need to certify their voltage rating. And secondly, if ac or dc didn't matter then they wouldn't have to be so marked. There are three basic types. They all "operate" (fuse, melt, clear, or trip) at a pre-determined current-level to interrupt a circuit. Also all three contain conductive element(s) calibrated so that the total series resistance, R, is as small as possible so as not to add any significant impedance in the protected circuit. Furthermore, R, is chosen so that power developed in the fuse element, I^R, provides a time-current characteristic curve that provides some repeatable degree of coordination with upstream and/or downstream protective devices. For simplicity I will refer to the three types as A, B, and C: A) The first type is called a general purpose, single-element type. It usually provides a fast-action operating characteristic on circuits having low to moderate electrical fault-current levels. B) The second type of fuse is referred to as Dual-element, Time-delay fuse. It is usually used on moderate to high fault-current levels. There are two internal elements. a) The first element operates for low to moderate overcurrents, such as overload. The operating principle is based on a spring-loaded, alloy-soldered unit (similar to thermal-alloy elements used as motor overload devices) connected to a larger component. The alloy element provides the time-delay feature, usually for loads (eg. motors and transformers) that have relatively high, long duration inrush currents. b) The second element, connected in series with the unit above, operates for much higher currents as would be expected from severe overcurrents, eg, short circuit. C) The third type of fuse uses a principle called current-limiting. This type limits the let-through energy to reduce the effects of electrical failure. It is very-fast acting and is usually used on systems exposed to extremely-high fault-current levels. While the above explains what fuses are, and how they work, it does not explain the impact of voltage level. Repeating what was stated earlier, the fuse element must interrupt the current flow. Furthermore, the "gap" produced must increase rapidly to overcome the effect of the arc and arc products. Extinguishing of the arc is insured in ac circuits because of the current passing through "zero." But, this is only true if the interrupted fault-current is within the fuse's capability. DC circuits impose another duty. Since there is no "passing through zero" the arc, hence, current, could be maintained due to the presence of the arc products. Furthermore, the thermal effects of the arc could cause the fuse enclosure to rupture. This wil cause collateral (thnx Ken) damage to the surrounding circuitry. An aside, have you ever wondered why passenger elevators having open-circuit faults in dc control circuits carried over trailing cables, always stops at a floor level, and not in between. The explaination... even though a conductor open-circuits, the arc maintains current until the elevator comes to a controlled stop because of action of the floor limit switch! Regards, Phil Corso, PE (EPSICON INC)