The fusing element must not hinder the normal circuit path, but it must also overheat and melt in response to excessive current (overcurrent). In simplified terms, the melting function depends upon the power expression of P = V * I, where P is the power in Watts. By algebraic substitution with Ohm's Law, the expression becomes P = I
2 * R, showing that a linear change in current produces an exponential change in power dissipation, which is useful in a fuse. But how quickly should the fuse respond? Fast? Slow? How fast or slow? The capabilities can be summarised with a TCC (Time/Current Curve). A logarithmic scale is typical, with current shown on the horizontal (x) axis and time on the vertical (y) axis. The TCC in
Figure 1 is illustrative of a very fast 1A fuse. First, we observe that no fuse of this type can fail from overcurrent until at least 1.25A is flowing, and none is guaranteed to fail until 1.5A is achieved, or 150% of nominal current. A factor of 1.5 is rather optimistic, and many real-world fuses are more in the range of two, representing the
safety factor that must be allowed when a fuse is specified.
Figure 1 - Possible TCC for a 1A fast-acting fuse Second, note that the TCC is a region representing unavoidable variations, since a thermal melting mode cannot be precise with real-world materials. Here, one fuse might pick up at 1.25A and fail, i.e.
clear the fault, in about ten seconds, and another fuse may not clear until 1.5A flows for ninety seconds. The gap between the nominal current rating and the minimum trip is also unavoidable, as a minimum trip too close to the nominal rating would subject the fuse element to large, thermally-induced mechanical stresses during normal operation. The curve characteristic is maintained in a family of fuses. A 10A fuse of the same family would yield a similar curve, but shifted right, with an initial pickup from around 12.5-15A. For any fuse, three data points are particularly interesting to a hobby user:
- Voltage Rating: When a fusing element separates under load, an arc can strike in the gap. Current flow is sustained by the arc, and the element may weld back together for a second break, or the arc may persist and continue melting the element outward. The voltage rating specifies the maximum voltage for which clearing can be guaranteed. The voltage rating must account for all possible failure conditions including the limits of the dielectric medium (type and degree of physical separation between ends), vaporised metal products formed in the chamber, and any support structures that may allow the arc to "track". AC ratings are usually higher than DC ratings since the zero-crossings accelerate the arc collapse. Fuses intended for 12V DC automotive electrical systems usually have a 32V DC rating. The AC rating is often 250V, but this cannot be assumed if data are not available to prove it.
- Current Rating: The nominal rating is the maximum current the fuse can carry continuously. A correctly specified fuse should have a long service life, but an overrated fuse may not protect the circuit if the required safety factor current exceeds what the circuit can source in a typical failure mode.
- Timing Characteristic: Smaller fuse styles will specify "fast acting" (FA) or "slow blow" (SB, or Time Delay) and not provide a TCC. Fast fuses are intended for applications where any failure condition must be quickly interrupted. Time delay fuses are useful when a momentary overload may legitimately exist for a short time (motor or transformer startup) and cause nuisance tripping of a fast fuse.
A nuisance (fatigue) failure occurs when the fuse has been aged by numerous operation cycles, or the fuse is operating slightly above its nominal current rating, or the fuse is subject to abuse such as inrush spikes or even mechanical vibration. Metal fatigue from aging is normal, but the latter cases can indicate a system design problem or an insufficient fuse rating. A fuse that is being run slightly above its current rating, or stressed by an inrush, can often be observed to move inside the cartridge. It may survive a number of cycles but it will eventually fail.