Glossary of Terms
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, l2t
The measure of heat energy developed within a circuit during the
fuse’s clearing. It can be expressed as “Melting l2t”, “Arcing l2t” or
the sum of them as “Clearing l2t”. “l” stands for effective let-through
current (RMS), which is squared, and “t” stands for time of opening,
- Arcing Time
The amount of time from the instant the fuse link has melted until the
overcurrent is interrupted, or cleared
- Cartridge Fuse
A fuse consisting of a current responsive element inside a fuse tube
with terminals on both ends
- Class CC Fuses
600V, 200,000 ampere interrupting rating, branch circuit fuses with
overall dimensions .Their design incorporates a rejection
feature that allows them to be inserted into rejection fuse holders
and fuse blocks that reject all lower voltage, lower interrupting
- Class G Fuses
20A@600VAC, 25-604@480VAC, 100,000 ampere interrupting
rating branch circuit fuses that are size rejecting to eliminate over fusing.
These are available in ratings from 1 amp through 60 amps.
Class H Fuses
250V and 600V, 10,000 ampere interrupting rating branch circuit
fuses that may be renewable or non-renewable. These are available
in ampere ratings of 1 amp through 600 amps.
- Class J Fuses
These fuses are rated to interrupt a minimum of 200,000 amperes
AC. They are labelled as “Current-Limiting”, are rated for 600 volts
AC, and are not interchangeable with other classes.
- Class K Fuses
These are fuses listed as K-1, K-5, or K-9 fuses. Each subclass has
designated I2t and lp maximums. These are dimensionally the same
as Class H fuses, and they can have interrupting ratings of 50,000,
100,000, or 200,000 amps. These fuses are current-limiting.
However, they are not marked “current-limiting” on their label since
they do not have a rejection feature.
- Class L Fuses
These fuses are rated for 601 through 6000 amperes, and are rated
to interrupt a minimum of 200,000 amperes AC. They are labeled
“current-limiting” and are rated for 600 volts AC. They are intended to
be bolted into their mountings and are not normally used in clips. Some
Class L fuses have designed in time-delay features for all purpose use.
- Class R Fuses
These are high performance fuses rated 1/10 – 600 amps in 250 volt
and 600 volt ratings. All are marked “current-limiting” on their label
and all have a minimum of 200,000 amp interrupting rating. They
have identical outline dimensions with the Class H fuses but have a
rejection feature which prevents the user from mounting a fuse of
lesser capabilities (lower interrupting capacity) when used with
special Class R Clips. Class R fuses will fit into either rejection or
- Class T Fuses
An industry class of fuses in 300 volt and 600 volt ratings from 1
amp through 1200 amps. They are physically very small and can be
applied where space is at a premium. They are fast-acting and
time-lag fuses, with an interrupting rating of 200,000 amps RMS.
- Classes of Fuses
The industry has developed basic physical specifications and electrical
performance requirements for fuses with voltage ratings of
600 volts or less. These are known as standards. If a type of fuse
meets the requirements of a standard, it can fall into that class.
Typical classes are K, RK1, RK5, G, L, H, T, CC, and J
- Clearing Time
The total time between the beginning of the overcurrent and the
final opening of the circuit at rated voltage by an overcurrent protective
device. Clearing time is the total of the melting time and the
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-through 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.
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
- 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 5-6 times the fuse current
rating. The other element, the short-circuit section, operates on
short-circuits up to its interrupting rating.
- Electrical Load
That part of the electrical system which actually uses the energy or
does the work required.
- Fast Acting Fuse
A fuse which opens on overload and short circuits very quickly. This
type of fuse is not designed to withstand temporary overload
currents associated with some electrical loads, when sized near the
full load current of the circuit.
An overcurrent protective device with a fusible link that operates
and opens the circuit on an overcurrent condition.
- 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
- Inductive Load
An electrical load which pulls a large amount of current – an inrush
current – when first energized. After a few cycles or seconds the
current “settles down” to the full-load running current
- Interrupting Capacity
Actual test current an overcurrent device sees during the short circuit
- 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 standard test conditions
- Melting Time
The amount of time required to melt the fuse link during a specified
overcurrent. (See Arcing Time and Clearing Time.)
- “NEC” Dimensions
These are dimensions once referenced in the National Electrical
Code. They are common to Class H and K fuses and provide interchangeability
between manufacturers for fuses and fusible equipment
of given ampere and voltage ratings
- Ohm’s Law
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
- One Time Fuses
Generic term used to describe a Class H nonrenewable cartridge
fuse, with a single element
A condition which exists on an electrical circuit when the normal
load current is exceeded. Overcurrents take on two separate characteristics
– overloads and short-circuits
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 path of the
circuit – that is, it flows from the source, through the conductors,
through the load, back through the conductors, to the
- Peak Let-Through Current, lp
The instantaneous value of peak current let-through by a currentlimiting
fuse, when it operates in its current-limiting range
- Renewable Fuse (600V & below)
A fuse in which the element, typically a zinc link, may be replaced
after the fuse has opened, and then reused. Renewable fuses are
made to Class H standards.
- 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
- RMS Current
The RMS (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.
- Semiconductor Fuses
Fuses used to protect solid-state devices
Can be classified as an overcurrent which exceeds the normal full
load current of a circuit by a factor many times (tens, hundreds or
thousands greater). Also characteristic of this type of overcurrent is
that it leaves the normal current carrying path of the circuit – it takes
a “short cut” around the load and back to the source.
- Short-Circuit Current Rating
The maximum short-circuit current an electrical component can
sustain without the occurrence of excessive damage when protected
with an overcurrent protective device.
That condition which occurs when one phase of a three-phase system
opens, either in a low voltage (secondary) or high voltage (primary)
distribution system. Primary or secondary single-phasing can
be caused by any number of events. This condition results in unbalanced
currents in poly phase motors and unless protective measures
are taken, causes overheating and failure.
- Threshold Current
The symmetrical RMS available current at the threshold of the
current-limiting range, where the fuse becomes current-limiting
when tested to the industry standard. This value can be read off of
a peak let-through chart where the fuse curve intersects the A-B
line. A threshold ratio is the relationship of the threshold current to
the fuse’s continuous current rating.
- Time-Delay Fuse
A fuse with a built-in delay that allows temporary and harmless
inrush currents to pass without opening, but is so designed to open
on sustained overloads and short-circuits.
- 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.
- Withstand Rating
The maximum current that an unprotected electrical component
can sustain for a specified period of time without the occurrence of