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Basic Electricity
Fuses
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What is a fuse?
• A fuse is a simple device used to protect circuits from
over-loading and overheating.
• Most fuses contain a strip of metal that has a higher
resistance than the conductors in the circuit.
• This strip also has a very low melting point.
• Since it has a higher resistance, it will heat up faster
than the conductors.
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• When the current exceeds the rating on the fuse, the
strip melts and opens the circuit.
• Fuses are one time devices and must be replaced when
the strip or element melts.
• The cause for the circuit overload must be identified
and corrected before replacing the fuse.
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Plug Fuses
• Plug fuses have either an Edisonbase or a type S base.
• Edison base fuses are used in
older installations and can be used
for replacement only.
• The type S fuse can be used only
in type S fuse holders. Otherwise
an adapter must be used.
• Plug fuses are rated up to 125 V
and 30 A.
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Dual Element Plug Fuses
• Many circuits have electric
motors as the load or part of the
load.
• Motors draw more current
when starting and can cause a
plain fuse to burn out or open
the circuit.
• Dual element fuses are
frequently used in this situation.
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• One element in the fuse will melt when there is a large
overload, such as a short circuit.
• The other, will melt and open the circuit when there is a
smaller current overload lasting more than a few
seconds.
• This allows for the larger starting current of an electric
motor.
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Cartridge fuses
• For 230 v to 600 v service up to 60 A,
the ferrule cartridge fuse is used.
• From 60 A to 600 A, knife-blade
cartridge fuses can be used.
• A cartridge fuse is sized according to
its amperage rating to prevent a fuse
with inadequate rating from being used.
• Many cartridge fuses have an arcquenching material around the element
to prevent damage from arcing in
severe short circuits.
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Knife- Blade cartridge fuse
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Circuit Breakers
• A circuit breaker can
function as a switch as well
as a means for opening a
circuit when an overload
occurs.
• Most modern houses and
commercial buildings use
circuit breakers.
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• Circuit breakers use two methods to protect the circuit.
• One is a bimetal strip that heats up with a current overload and trips
the breaker, opening the circuit.
• The other is a magnetic coil that causes the breaker to trip and open
the circuit when there is a short circuit or other excessive current
overload in a short time.
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Internals of a circuit breaker
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