A fuse is a thermal protective or safety device that opens a circuit when current passing through it becomes excessive. Most fuses have a thick metal strip through which current passes into a circuit. If too much current starts to flow the metal melts and cuts off the current.

• Plug & Type S - The metal strip is visible through a window that melts when there is an overload.
• Time Delay - These fuses have a spring-loaded strip that allows temporary overloads.
• Cartridge - These fuses show no sign of overload. They must be tested to reveal whether they’ve blown.

• Ampere: The measurement of intensity of rate of electrons in an electric circuit. An ampere is the amount of current that will flow through a resistance of one ohm under a pressure of one volt.
• Ampere Rating: The current carrying capacity of a fuse. When a fuse is subjected to a current above its rating it will open the circuit after a predetermined time.
• Arcing Time: The amount of time from the instant the fuse link has melted until the overcurrent is interrupted or cleared.
• Dual Element Fuse: Fuse with a special design that utilizes two individual elements in series inside the fuse tube. One element, the spring actuated trigger assembly, operates on overloads up to 5 times the fuse current rating. The other element, the short circuit section operates on short circuits up to their interrupting rating.
• High Speed Fuses: Fuses with no intentional time-delay in the overload range and designed to open as quickly as possible in the short circuit range. These fuses are often used to protect solid-state devices.
• Interrupting Rating: The rating, which defines a fuse’s ability to safely interrupt, and clear short circuits. This rating is much greater than the ampere rating of a fuse. The NEC defines “interrupting rating” as the “highest current at rated voltage that an overcurrent protective device is intended to interrupt under stated conditions”.
• Melting Time: The amount of time required to melt the fuse link during a specified overcurrent.
• Overcurrent: A condition which exists on an electrical circuit when the normal load current is exceeded. Overcurrents take on two separate characteristics-overloads and short circuit.
• Overload: Can be classified as an overcurrent which exceeds the normal full load current of a circuit. Also characteristic of this type of overcurrent is that it does not leave the normal current carrying the path of the circuit, meaning that it flows from the source, through the conductors, through the load, back through the conductors to the source again.
• Resistive Load: An electrical load which is characteristic of not having any significant inrush current. When a resistive load is energized, the current rises instantly to its steady state value without first rising to a higher value.
• Short Circuit: Can be classified as an overcurrent which exceeds the normal full load current of a circuit by a factor of many greater than designed. Also characteristic of this type of current is that it leaves the normal current carrying path of the circuit. It take a “short cut” around the load and back to the source.
• Time-Delay Fuse: A fuse with a built in delay that allows temporary and harmless inrush currents to pass without opening, but it is designed to open on sustained overloads and short circuits.
• Voltage Rating: The maximium value of system voltage in which a fuse can be used, yet safely interrupt an overcurrent. Exceeding the voltage rating of a fuse impairs its ability to clear an overload or short circuit safely.