Page 102 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Fuses
A fuse is a safety device connected in series to the circuit which protect the electrical apparatus from possible
damage when abnormal current flows through it. When normal current flows , the serves as a conductor but when
the current flows above the predetermined value through a circuit due to short circuit , heat s produced to melt the
fuse wire thus breaking the circuit to protect equipment from damage due to excessive value .
Classification of fuses

Fuses may be classified into two groups
1 Those designed to protect the circuit from short circuit only
2 Those designed to protect over load and short
Parts of fuse

1 Fuse wire 2 Fuse carrier 3 Fuse carrier contact 4 Fuse base 5 Fixed contact
Fuse wire
Fuse wire are bare wires made of easily melting material having high specific resistance usually standard alloy
63%tin and 37% lead it is used low current say up to 5 A and tinned copper for large value of current .
Fuse carrier
The part to which the fuse wire is fitted is called fuse carrier and is made of porcelain.
Fuse carrier contact
These are the contact strip which engage or disengage the fixed contacts of the fuse base and have a fuse wire
attached to them.
Fuse base
It is the fixed part of the fuse and is made of porcelain .
Fixed contact

These contact are provided in the porcelain base of the fuse and engage with the fuse carrier contacts
Minimum fusing current
Minimum fusing current is the least value of current at which the fuse wire is melt .
Current rating of fuse element

Fuse element current which it can carry without melting . It is the value is always less than the minimum value of
fuse current.
Fusing factor
The ratio of minimum fusing current and rating current of the fuse element is called fusing factor. Fusing factor
always greater than unity
Types of fuses
Two types of fuses are used

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CITS : Power - Electrician & Wireman - Lesson 20-25 CITS : Power - Electrician & Wireman - Lesson 20-25