Page 80 - Physics_Form_2

Physics for Secondary Schools

A fuse is connected to the live cable to suitable casing and placed in series with
ensure that if it blows, the appliance’s the circuit to be protected.
body is cut off from the live electrical
connection, preventing potential danger Fuses are categorised into two types;
to users. The earth pin on the plug is rewirable fuses and cartridge fuses. In
generally longer than the other two pins, rewireable fuse, the fuse element is carried
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enabling it to open the safety shutter in a removable fuse link that is. made of
to the terminals of the socket. It also porcelain or other suitable insulating
material. This ensures no danger to an
ensures an appliance is earthed before operator when removing the fuse link.
being connected to a power source. When Figure 2.43 shows a rewirable fuse.
connecting a three-pin plug, the colour
code must be strictly adhered to avoid an
electric shock. The switch must be OFF
when pushing the plug into the socket.
The two-pin plug does not have a fuse
or an earth pin. Thus, an appliance using
a two-pin plug does not have its body
connected to the earth. Figure 2.42 shows
a two-pin plug. The connections for this
plug follow the same colour code as the Figure 2.43: Rewireable fuse
three-pin plug.
Cartridge fuses consist of a porcelain tube
with metal end caps attached to the fuse
element. Figure 2.44 shows a cartridge
fuse.

Figure 2.42: A two-pin plug

A fuse and a circuit breaker
When an electric current exceeds the Figure 2.44: Cartridge fuse
rated value of an electrical appliance, it
causes damage to the device. To protect Action of a fuse
an appliance against this damage, a Blowing (melting) of a fuse occurs
fuse or a circuit breaker is used. A fuse when a circuit is overloaded or a short
consists of an element, usually a piece of circuit occurs. A short-circuit fault is an
copper or tin-lead alloy wire, that melts unwanted connection allowing current
when current exceeds a predetermined to flow along a path not part of a circuit
value. The element is contained in a design. The new path might re-route

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Student’s Book Form One

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