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The Basics: How Does a Fuse Work?
A fuse is a simple and highly effective way to protect a device from dangerous levels of current:
Current flowing through a conductor’s nonzero resistance leads to power dissipation.
Power is dissipated in the form of heat.
Heat raises the temperature of the conductor.
If the combination of current amplitude and duration is sufficient to raise the temperature above the fuse’s melting point, the fuse becomes an open circuit and current flow ceases.
Though the fundamental operation of an Axial Lead & Cartridge Fuse is not complicated, there are subtle points to keep in mind. The rest of this article will help you to understand some important details related to the behavior and use of fuses.
How a Fuse Is Tripped: Heat, Not Current
A fuse is not tripped directly by current; rather, the current creates heat, and heat trips the fuse. This is actually a rather important distinction because it means that the Automotive Fuse operation is influenced by ambient temperature and by the temporal characteristics of the current.
The specified current rating of a fuse is relevant only to a specific ambient temperature (usually, or maybe always, 25°C), and consequently you need to adjust your fuse selection if you’re designing a device that will operate outdoors in, say, Antarctica or Death Valley. The following plot shows how ambient temperature affects the actual current rating—relative to the nominal 25°C current rating—of three types of fuses.
Regarding the temporal characteristics of the current passing through the Power Fuse, we all know that the effect of heat accumulates over time (momentarily touching a hot skillet is nothing compared to picking it up and realizing that it’s hot when you’re halfway between the stove and the dining table). Consequently, the current rating of a fuse is a simplification of its real behavior. We can’t expect a fuse to respond to high-amplitude transients because the short duration of the higher power dissipation doesn’t increase the temperature enough to cause tripping.
The following plot shows the time-current characteristics for a group of surface-mount fuses made by Panasonic. The rated current is on top, and the curve represents the amount of time required to trip the fuse in relation to the amount of current flowing through the fuse.
Fuse Design Best Practices: Rated Current vs. Operating Current
It would be perfectly reasonable to assume that a fuse rated for 6 amps could be used in a circuit that might need 5 amps of steady-state current. It turns out, though, that this is not good design practice.
The current rating of a Resettable Fuse is not a high-precision specification, and furthermore (as discussed above) the actual tripping current is influenced by ambient temperature. Consequently, to avoid “nuisance tripping,” you should have a fairly generous gap between your expected steady-state current and your fuse’s rated current.
This document from Littelfuse suggests a “rerating” of 25% (for operation at room temperature); thus, a fuse with a rating of 10 amps would be used only if the circuit’s steady-state current will stay below 7.5 amps.