Fuses
Normally, the ampacity rating of a conductor
is a circuit design limit never to be intentionally
exceeded, but there is an application where ampacity
exceedence is expected: in the case of fuses.
A fuse is nothing more than a short length
of wire designed to melt and separate in the event of
excessive current. Fuses are always connected in series with
the component(s) to be protected from overcurrent, so that
when the fuse blows (opens) it will open the entire
circuit and stop current through the component(s). A fuse
connected in one branch of a parallel circuit, of course,
would not affect current through any of the other branches.
Normally, the thin piece of fuse wire is
contained within a safety sheath to minimize hazards of arc
blast if the wire burns open with violent force, as can
happen in the case of severe overcurrents. In the case of
small automotive fuses, the sheath is transparent so that
the fusible element can be visually inspected. Residential
wiring used to commonly employ screw-in fuses with glass
bodies and a thin, narrow metal foil strip in the middle. A
photograph showing both types of fuses is shown here:
Cartridge type fuses are popular in
automotive applications, and in industrial applications when
constructed with sheath materials other than glass. Because
fuses are designed to "fail" open when their current rating
is exceeded, they are typically designed to be replaced
easily in a circuit. This means they will be inserted into
some type of holder rather than being directly soldered or
bolted to the circuit conductors. The following is a
photograph showing a couple of glass cartridge fuses in a
multi-fuse holder:
The fuses are held by spring metal clips,
the clips themselves being permanently connected to the
circuit conductors. The base material of the fuse holder (or
fuse block as they are sometimes called) is chosen to
be a good insulator.
Another type of fuse holder for
cartridge-type fuses is commonly used for installation in
equipment control panels, where it is desirable to conceal
all electrical contact points from human contact. Unlike the
fuse block just shown, where all the metal clips are openly
exposed, this type of fuse holder completely encloses the
fuse in an insulating housing:
The most common device in use for
overcurrent protection in high-current circuits today is the
circuit breaker. Circuit breakers are specially
designed switches that automatically open to stop current in
the event of an overcurrent condition. Small circuit
breakers, such as those used in residential, commercial and
light industrial service are thermally operated. They
contain a bimetallic strip (a thin strip of two
metals bonded back-to-back) carrying circuit current, which
bends when heated. When enough force is generated by the
bimetallic strip (due to overcurrent heating of the strip),
the trip mechanism is actuated and the breaker will open.
Larger circuit breakers are automatically actuated by the
strength of the magnetic field produced by current-carrying
conductors within the breaker, or can be triggered to trip
by external devices monitoring the circuit current (those
devices being called protective relays).
Because circuit breakers don't fail when
subjected to overcurrent conditions -- rather, they merely
open and can be re-closed by moving a lever -- they are more
likely to be found connected to a circuit in a more
permanent manner than fuses. A photograph of a small circuit
breaker is shown here:
From outside appearances, it looks like
nothing more than a switch. Indeed, it could be used as
such. However, its true function is to operate as an
overcurrent protection device.
It should be noted that some automobiles use
inexpensive devices known as fusible links for
overcurrent protection in the battery charging circuit, due
to the expense of a properly-rated fuse and holder. A
fusible link is a primitive fuse, being nothing more than a
short piece of rubber-insulated wire designed to melt open
in the event of overcurrent, with no hard sheathing of any
kind. Such crude and potentially dangerous devices are never
used in industry or even residential power use, mainly due
to the greater voltage and current levels encountered. As
far as this author is concerned, their application even in
automotive circuits is questionable.
The electrical schematic drawing symbol for
a fuse is an S-shaped curve:
Fuses are primarily rated, as one might
expect, in the unit for current: amps. Although their
operation depends on the self-generation of heat under
conditions of excessive current by means of the fuse's own
electrical resistance, they are engineered to contribute a
negligible amount of extra resistance to the circuits they
protect. This is largely accomplished by making the fuse
wire as short as is practically possible. Just as a normal
wire's ampacity is not related to its length (10-gauge solid
copper wire will handle 40 amps of current in free air,
regardless of how long or short of a piece it is), a fuse
wire of certain material and gauge will blow at a certain
current no matter how long it is. Since length is not a
factor in current rating, the shorter it can be made, the
less resistance it will have end-to-end.
However, the fuse designer also has to
consider what happens after a fuse blows: the melted ends of
the once-continuous wire will be separated by an air gap,
with full supply voltage between the ends. If the fuse isn't
made long enough on a high-voltage circuit, a spark may be
able to jump from one of the melted wire ends to the other,
completing the circuit again:
Consequently, fuses are rated in terms of
their voltage capacity as well as the current level at which
they will blow.
Some large industrial fuses have replaceable
wire elements, to reduce the expense. The body of the fuse
is an opaque, reusable cartridge, shielding the fuse wire
from exposure and shielding surrounding objects from the
fuse wire.
There's more to the current rating of a fuse
than a single number. If a current of 35 amps is sent
through a 30 amp fuse, it may blow suddenly or delay before
blowing, depending on other aspects of its design. Some
fuses are intended to blow very fast, while others are
designed for more modest "opening" times, or even for a
delayed action depending on the application. The latter
fuses are sometimes called slow-blow fuses due to
their intentional time-delay characteristics.
A classic example of a slow-blow fuse
application is in electric motor protection, where inrush
currents of up to ten times normal operating current are
commonly experienced every time the motor is started from a
dead stop. If fast-blowing fuses were to be used in an
application like this, the motor could never get started
because the normal inrush current levels would blow the
fuse(s) immediately! The design of a slow-blow fuse is such
that the fuse element has more mass (but no more ampacity)
than an equivalent fast-blow fuse, meaning that it will heat
up slower (but to the same ultimate temperature) for any
given amount of current.
On the other end of the fuse action
spectrum, there are so-called semiconductor fuses
designed to open very quickly in the event of an overcurrent
condition. Semiconductor devices such as transistors tend to
be especially intolerant of overcurrent conditions, and as
such require fast-acting protection against overcurrents in
high-power applications.
Fuses are always supposed to be placed on
the "hot" side of the load in systems that are grounded. The
intent of this is for the load to be completely de-energized
in all respects after the fuse opens. To see the difference
between fusing the "hot" side versus the "neutral" side of a
load, compare these two circuits:
In either case, the fuse successfully
interrupted current to the load, but the lower circuit fails
to interrupt potentially dangerous voltage from either side
of the load to ground, where a person might be standing. The
first circuit design is much safer.
As it was said before, fuses are not the
only type of overcurrent protection device in use.
Switch-like devices called circuit breakers are often (and
more commonly) used to open circuits with excessive current,
their popularity due to the fact that they don't destroy
themselves in the process of breaking the circuit as fuses
do. In any case, though, placement of the overcurrent
protection device in a circuit will follow the same general
guidelines listed above: namely, to "fuse" the side of the
power supply not connected to ground.
Although overcurrent protection placement in
a circuit may determine the relative shock hazard of that
circuit under various conditions, it must be understood that
such devices were never intended to guard against electric
shock. Neither fuses nor circuit breakers were not designed
to open in the event of a person getting shocked; rather,
they are intended to open only under conditions of potential
conductor overheating. Overcurrent devices primarily protect
the conductors of a circuit from overtemperature damage (and
the fire hazards associated with overly hot conductors), and
secondarily protect specific pieces of equipment such as
loads and generators (some fast-acting fuses are designed to
protect electronic devices particularly susceptible to
current surges). Since the current levels necessary for
electric shock or electrocution are much lower than the
normal current levels of common power loads, a condition of
overcurrent is not indicative of shock occurring. There are
other devices designed to detect certain chock conditions
(ground-fault detectors being the most popular), but these
devices strictly serve that one purpose and are uninvolved
with protection of the conductors against overheating.
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REVIEW:
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A fuse is a small, thin conductor
designed to melt and separate into two pieces for the
purpose of breaking a circuit in the event of excessive
current.
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A circuit breaker is a specially
designed switch that automatically opens to interrupt
circuit current in the event of an overcurrent condition.
They can be "tripped" (opened) thermally, by magnetic
fields, or by external devices called "protective relays,"
depending on the design of breaker, its size, and the
application.
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Fuses are primarily rated in terms of
maximum current, but are also rated in terms of how much
voltage drop they will safely withstand after interrupting
a circuit.
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Fuses can be designed to blow fast, slow,
or anywhere in between for the same maximum level of
current.
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The best place to install a fuse in a
grounded power system is on the ungrounded conductor path
to the load. That way, when the fuse blows there will only
be the grounded (safe) conductor still connected to the
load, making it safer for people to be around.
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