Electrical circuits need a protection device. The protective device may be preventing damage to the load, or damage to the supply and associated circuitry.
Fuses and circuit breakers are two different methods of protecting against large current flows. Fuses and circuit breakers are protection devices, or a type of automatic switch, which can interrupt the circuit to block against current over a safe limit.
Fuses and circuit breakers are rated by their maximum current value. For example, an 18 A fuse will allow currents up to 18 amps to flow, but will open ("blow") at current values greater than 18 amps.
A fuse is a one-time only device because when it "blows" due to an excessive amount of current flow, it must be replaced. Some common electrical symbols for a fuse are shown here.
Fuses come in many different shapes and sizes. They may fit into a fuse box, or may be connected "inline".
An operational fuse has a very low resistance (close to zero ohms). A blown fuse has infinite resistance. A fuse can easily be checked with an ohmmeter.
There are two basic types of fuses:
The fast acting fuse will open very quickly when its particular current rating is exceeded.
Slow blow fuses are designed to open only on a continued overload, such as a short circuit.
The slow blow fuse has a coiled construction inside the glass which prevents the fuse from blowing on a temporary current surge.
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