Fuses

fuse is the weak link in a circuit, which will break when too

much current flows, thus protecting the circuit conductors

from damage. It must be remembered that the priority of

the fuse is to protect the circuit conductors, not the

appliance or the user. Calculation of cable size therefore

automatically involves the correct selection of protective

devices.

Fuses

Fuses

There are many different types and sizes of fuse, all designed to

perform a certain function There are three types of

fuses:

1 .The rewirable or semi-enclosed fuse

2 .The cartridge fuse and fuse link

3 .The high-rupturing-capacity (HRC) fuse.

rewirable fuseA rewirable fuse consists of a fuse, a holder, a fuse element and a fuse carrier, the holder and carrier being made of porcelain or Bakelite

disadvantages

1. The fact that it is repairable enables the wrong size of fuse

wire (element) to be used.

2. The elements become weak after long usage and may break

under normal conditions.

3. Normal starting-current surges (e.g. when

motors, etc. are switched on) are ‘ seen ’ by

the fuse as an overload and will therefore break

the circuit.

4. The fuse holder and carrier can become

damaged as a result of arcing in the event of

a heavy overload.

cartridge fuse

A cartridge fuse consists of a porcelain tube with metal and

caps to which the element is attached. The tube is filled with

silica These fuses are found generally in modern plug tops

used with 13 A socket outlets. They have some advantages

over the rewirable fuse they: do not deteriorate; maintain

accuracy in breaking at rated values; and do not arc when

interrupting faults . Cartridge fuses are, however, expensive to

replace.

HRC fuse

The HRC fuse is a sophisticated variation of the cartridge fuse

and is normally found protecting motor circuits and industrial

installations. It consists of a porcelain body filled with silica

with a silver element and lug type and caps. Another feature of

HRC fuse is the indicating element, which shows when the fuse

has blown. It is very fast-acting and can discriminate between a

starting surge and an overload

fusing factor

In order to classify these devices, it is important to have some

means of knowing their ‘fusing ’ performance. This is achieved

for fuses by the use of a fusing factor:

where the fusing current is the minimum current causing

the fuse to blow, and the current rating is the maximum

current, which the fuse can sustain without blowing.

fusing factor

For example, a 5 A fuse, which blows only when 9 A flows,

will have a fusing factor of 9/5 = 1.8.

Rewirable fuses have a fusing factor of about 1.8.

Cartridge fuses have a fusing factor of between 1.25 and 1.75.

HRC fuses have a fusing factor of up to 1.25 (maximum).

discrimination

Where more than one fuse protects a circuit it is clearly

sensible that the correct fuse should blow under fault

conditions. A fault on the appliance should cause fuse C to

blow. If fuse B blew, although it would break the circuit to the

faulty appliance, it would unnecessarily render the whole

radial circuit dead. If fuse A blew, instead of B or C, all circuits

from the distribution would be pointlessly disconnected. The

arrangement of fuses to protect the correct part of a circuit is

called discrimination .

discrimination