The measurement of intensity of rate of flow 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 ampere rating, it will open the circuit after a predetermined period of time.

Ampere Squared Seconds (I²t)

The measurement of heat energy developed within a circuit during the fuses clearing. It can be expressed as melting - I²t, arcing - I²t, or the sum of them as clearing - I²t. "I" stands for effective let-through current (RMS), which is squared, and "t" stands for time of opening, in seconds.

Arcing Time

The amount of time from the instant the fuse link has melted until the over current is interrupted, or cleared.

Breaking Capacity

The rating which defines the fuses 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 over current protective device is intended to interrupt under standard test conditions."

Clearing Time

The total time during the beginning of the over current and the final opening of the circuit at rated voltage by an over current protective device. Clearing time is the total of the melting time and the arcing time.

Current Limitation

A fuse operation relating to short circuits only. When a fuse operates in its current limiting range, it will clear a short circuit in less than 1/2 cycle. Also, it will limit the instantaneous peak let-thru current to a value substantially less than that obtainable in the same circuit if that fuse were replaced with a solid conductor of equal impedance.

Disconnect Mounting

The disconnect mounting allows the fuse unit to be removed (off load) using an insulated hook stick. The hook-stick grabs a pull ring and disconnects the fuse unit, which may then be lifted out of its mounting.

End Fittings

End fittings are metal parts that attach to each end of a fuse unit’s ferrules (end caps). As previously mentioned, they are used solely with disconnect fuse applications or when converting a non-disconnect to a disconnect fuse configuration. When end fittings are ordered,  a fitting for each end of the fuse is included. Keep in mind that end fittings can become damaged in use and, therefore, are sold separately from the live parts when necessary. It is not necessary to purchase an entire set of live parts when only the end fittings are required.

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.

Live Parts

Live parts were discussed as part of the “Mounting” definition. Everything above the insulators on the mounting excluding the fuse unit, fuse holder, and the fuse end fittings (if required) are considered the live parts. Fuse end fittings are discussed next and are not required with non-disconnect
live parts, but are required and included with disconnect live parts. Live parts may be sold separately as replacement parts or for new OEM applications.

Melting Time

The amount of time required to melt the fuse. Link during a specified over current.


A mounting provides all the necessary parts to safely mount a fuse in its intended piece of equipment. The base is the metal support to which all other pieces attach. Insulators attach to the base and insulate the live fuse unit from the base and everything beyond the base. Live parts are the parts of the mounting that are energized once electricity is flowing. The live parts provide the means to hold the fuse unit in place, electrical contact, and a place to make line and load connections.

Non-Disconnect Mounting

A non-disconnect mounting does not provide a means for removing the fuse unit until the circuit is dead and the fuse unit can be removed manually. The fuse unit is held in place by friction through the use of fuse clips or by a cross bar.


The unit of measure for electric resistance. An ohm is the amount of resistance that will allow one ampere to flow under a pressure of one volt.


The relationship between voltage, current, and resistance, expressed by the equation E=IR, where E is the voltage in volts, I is the current in amperes, and R is the resistance in ohms.

Over Current

A condition which exists on an electrical circuit when the normal load current is exceeded. Over currents take on two separate characteristics--overloads and short circuits.


Can be classified as an over current which exceeds the normal full load current of a circuit. The current does not leave the normal current carrying path of the circuit--that is, it flows from the source, through the conductors,
through the load, back through the conductors, to the source again.

Peak Let-Thru Current, IP

The instantaneous value of peak current let-thru by a current limiting fuse, when it operates in its current limiting range.

Power vs. Distribution

The differentiation is intended to indicate the test conditions and where fuses are normally applied on an electrical system, based on specific requirements for generating sources, substations and distribution lines. Each class has its own unique set of voltage, current and construction requirements (see ANSI C37.42, .44, .46 and .47).

Replaceable Fuse Unit:

A replaceable fuse unit is a phrase used to describe a fuse that does not have a separate holder and refill assembly.  In a replaceable fuse unit, the fuse is its own holder and is completely replaced after interruption.

Resistive Load

An electrical 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.

R.M.S. Current

The R.M.S. (root-mean-square) value of any periodic current is equal to the value of the direct current which, flowing through a resistance, produces the same heating effect in the resistance as the periodic current does.

Short Circuit

An over current which exceeds the normal full load current of a circuit by a factor many times (tens, hundreds or thousands greater). The over current also leaves the normal current carrying path of the circuit--it takes a "shortcut" around the load and back to the source.

Voltage Rating

The maximum open circuit 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.