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Overcurrent Protection Glossary


Ambient Temperature

The ambient temperature is the measured temperature of the air immediately surrounding the fuse or circuit breaker.  Typically, the fuse or circuit breakers’ ampere rating is determined at “room temperature” or 23° C.  Fuses and circuit breakers must be re-rated where temperatures vary from 23° C.


An ampere is an international unit of measure based on the rate of flow of electrons in an electric circuit.  One ampere is equal to the amount of current that will flow through one ohm of resistance under one volt of potential.  (1A = 1V / 1Ω)

Ampere Rating

The ampere rating is the current carrying capacity of a fuse or circuit breaker (also collectively known as “over-current protective devices”).  When a fuse or circuit breaker 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 ampere-squared-seconds is a unit measure based on the energy or heat allowed to pass through the fuse or circuit breaker during a short-circuit condition.  The unit is derived by squaring the effective RMS current and multiplying it by the total time it takes the fuse or circuit breaker to open and extinguish the electrical arcing.  The total clearing ampere-squared-seconds (I²tc) is the sum of the “opening” ampere-squared-seconds (I²to) plus “arcing” ampere-squared-seconds (I²ta).  (I²tc = I²to + I²ta).

Anti-Surge Fuse – (See Time-Delay Fuse)

Arcing Time

The arcing time is the period of time between that when the fuse or circuit breaker begins to open and the time takes extinguish the fault current.

Breaking Capacity (See Interrupting Rating)

Clearing Time

The clearing time is the total time it takes a fuse or circuit breaker to open and clear an overcurrent.  The clearing time is the sum of the opening time and the arcing time.

Current Limitation

Current limitation refers to a fuse or circuit breaker’s ability to substantially reduce the amount of current experienced by the circuit if such a device was not present in the circuit.  In order for a fuse of circuit breaker to be labeled as a “current limiting” device, it must open and clear a short-circuit current before the instantaneous peak current is reached (typically within the first ¼ cycle).

De-Rating – (see Re-Rating)

Fast Acting Fuse

A fast-acting fuse is designed to open very quickly when the applied current exceeds that of the fuse rating.  This type of fuse is not designed to withstand temporary overload (surge) currents associated with some electrical loads such as motors and charging capacitors.

A fast-acting fuse designed to meet UL standards will typically open within 5 seconds when 200-250% of the fuse rating is applied.

A fast-acting fuse designed to meet European IEC 60127 standards will open within 0.001 to .01 seconds when subjected to 10x the fuse’s rated current and be marked with an “F”

A very fast-acting fuse designed to meet European IEC 60127 standards will open in less than 0.001 seconds when subjected to 10x the fuse’s rated current and be will be marked with an “FF”

Fast Blow Fuse (See Fast Acting Fuse)


A fuse is an overcurrent protective device with a fusible link that operates and opens the circuit in an overcurrent condition.

In-Rush Current (See Overload)

Interrupting Rating (Breaking Capacity)

The interrupting rating is also known as the breaking capacity or short circuit rating of any overcurrent protective device.  The interrupting rating is the maximum approved current which the fuse of circuit breaker can safely interrupt and clear a short-circuit at the rated voltage.

Melting Time

The melting time is the period of time before the fuse element begins to melt during an overcurrent condition.


An Ohm is the unit of measure for electric resistance. One Ohm is the amount of resistance that will allow one Ampere to flow under a pressure of one Volt.

Ohm’s Law

Ohm’s Law is the relationship between voltage, current, and resistance as theorized by Georg Ohm in 1827.  Ohm’s Law states that the voltage in a circuit is equal to the current multiplied by the resistance expressed by V = I x R


An overcurrent is a condition which exists in an electrical circuit when the normal load current is exceeded.  The two basic forms of an overcurrent are overloads and a short-circuits.  


An overload is an overcurrent condition where the current exceeds the normal full load-capacity of the circuit but where no fault condition (short-circuit) is present.  A momentary overload condition (also known as “in-rush” currents) may also occur when a circuit is first initialized due to capacitor charging and/or motor-startup.

Peak Let-Thru Current, Ip

The peak let-thru current of a current-limiting fuse or circuit breaker is the maximum instantaneous current allowed through the current-limiting device before the device opens and clears a short-circuit fault condition.

R.M.S. Current

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

Short Circuit

A short-circuit is an overcurrent condition where an abnormal, low-resistance, circuit path is introduced into the circuit.  This low-resistance path bypasses the normal load and can create extremely high currents (up to 1000x the normal current under some conditions). 

Slo-Blo Fuse (also see Time-Delay Fuse)

Slo-Blo fuse is a registered trade name of Littelfuse Corp. to reference time-delay fuses (See Time-Delay Fuse)

Time-Delay Fuse

A time-delay fuse incorporates a variety of techniques to delay the fuse’s opening time during temporary harmless over-load conditions such as circuit in-rush currents.  However, time delay fuses are also designed to open very quickly during dangerous short-circuit conditions or during sustained overload conditions.

Voltage Rating

The voltage rating is the maximum open circuit voltage in which a fuse or circuit breakers can be used, yet safely interrupt an overcurrent.  Exceeding the voltage rating of a fuse may cause the fuse to open but may not allow it to safely clear an overcurrent condition.



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