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Bussmann®
www.bussmann.com e-mail: sales@bussmann.co.uk Sales Fax: +44 (0)1509 882788
Fuse Technology and Glossary of Terms
Basic Fuse Technology
Miniature fuses serve two main purposes:
a. To protect components, equipment
and people from risk of fire and
shock risk caused by overcurrents.
b. To isolate sub systems from the
main system once a fault has
occurred.
Overcurrents
Overcurrents exist when the normal load
for a circuit is exceeded. It can be either an
overload or short circuit. An overload is any
current flowing within the normal circuit
path that is higher than the normal circuit
full load current.
A short circuit is an overcurrent which
greatly exceeds the normal full load current
of the circuit. Components and equipment
can be damaged by both types of
overcurrents.
Selecting Overcurrent Protection
During normal load current conditions, the
fuse must carry the normal load current of
the circuit without nuisance opening.
However, when an overcurrent occurs the
fuse must interrupt the overcurrent and
withstand the voltage across the fuse after
arcing. To properly select a fuse the
following items must be considered:
•Voltage rating (ac or dc voltage)
•Full load currents (RMS Amperes)
•Available short circuit current
•In-rush characteristics
•Characteristics of equipment or
components to be protected
•Ambient conditions
•Standards requirements
In addition, consideration must be given to:
•Available board space
•Type of mounting
•Automatic or manual insertion
•Reliability
•Ease of field service.
Electronic circuits frequently exhibit
surges, caused by capacitors charging,
motors being momentarily stalled, or high
voltage components sparking over. It is
important that designers take account of
these temporary conditions during fuse
selection. The ability to resist surges is a
function of the fuse design relative to the
surge pulse, duration, frequency etc.
Comparison of a manufacturers I2t value
alone is not sufficient and Bussmann would
be pleased to advise on specific applica-
tions.
Voltage Ratings
The voltage rating of the fuse must be
greater than or equal to the circuit voltage.
Because the fuse has such low resistance,
the voltage rating becomes critical only
when the fuse is trying to open. The fuse
must be able to open quickly, extinguish
the arc after the fuse element has melted
and prevent the system open-circuit
voltage from re-striking across the open
fuse element.
Current Ratings
Each fuse is marked with a nominal current
rating.
Several factors can affect the ability of the
fuse to carry this rated current. First the
base material of the clip in which the fuse
is mounted may greatly affect the perform-
ance of the fuse.
Another important factor is the conductor
size used to connect the fuse to other
circuit components. If the conductor is too
small, it will generate heat. That extra heat
will be seen by the fuse, causing the fuse
to open before it should. It is also impor-
tant that the fuse be installed with clean
and tight connections. If the connections
are dirty or loose, they will cause increased
resistance, generating extra heat. That heat
will lead to a shortened fuse life.
Breaking Capacity
A fuse must be able to open the circuit
under a short circuit without endangering
its surroundings. The breaking capacity of
a protective device is the maximum
available current, at the rated voltage, that
the device can safely open without the
possibility of rupturing.
Fuse Resistance
In most applications, the voltage drop
across the fuse due to its internal and
contact resistances is negligible. There are,
however, certain critical applications where
the fuse resistance must be considered
and it is important that the circuit designer
understands the fuse characteristics in
order to select the proper fuse.
Physical Sizes
There are numerous physical sizes of
electronic fuses, including subminiature
fuses. The most common cylindrical
designs are 5x20mm and 6.3x32mm.
Subminiature fuses are often used when
board space is limited. For applications of
this type there are both through-hole and
surface mount devices available. Standard
package sizes for surface mount fuses are
1608, 3216 & 6125. These sizes are
standard throughout the electronic
industry. Through-hole axial and radial
leaded products allow fuses to be PCB
mounted. Standard 5x15mm & 5x20mm
fuses fitted with leads can also be mounted
in this way.
Our surface mount range will continue to
be expanded allowing them to be designed
into an even wider range of applications.
Glossary of Terms
Ampere squared seconds I2t
A measure of heat energy supplied to a
circuit while the fuse is clearing a fault. It
can be expressed as melting I2t, arcing I2t
or the sum of them clearing I2t. This is
usually used as a measure of short circuit
performance.
Arcing time
The amount of time from the instant the
fuse link has melted until the overcurrent is
interrupted, or cleared.
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 arcing time.
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 with some electrical loads.
Fuse
An overcurrent protective device with a
fusible link that operates and opens the
circuit on an overcurrent condition.
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 circuits.
Overload
Can be classified as an overcurrent which
exceeds the normal full load current of a
circuit.
Resistive load
An electrical load which is characterised by
not having any significant inrush current.
When a resistive load is energised, the
current rises instantly to its steady state
value, without first rising to a higher value.
RMS 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
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).
Time delay fuse
A fuse with a built-in time delay that allows
temporary and harmless inrush current to
pass without operating, but is so designed
to open on sustained overloads and short
circuits.
Time lag (surge proof)
A fuse which can withstand short high
current pulses and is particularly suitable
for television sets, amplifiers etc.
Voltage rating
A maximum open circuit voltage in which a
fuse can be used, yet safely interrupt an
over-current. Exceeding the voltage rating
of a fuse impairs its ability to clear an
overload or short circuit safely.