1 Fuses

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Fuses

fuse is the weak link in a circuit, which will

break when too much current ows, 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.

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Fuses

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Fuses

There are many dierent 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 "#$C% fuse.

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rewirable fuse

& rewirable fuse consists of a fuse, a holder, afuse element and a fuse carrier, the holderand carrier being made of porcelain or'akelite

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disadvantages

(. The fact that it is repairable enables the

wrong size of fuse wire "element% to be used.

). The elements become weak after long

usage and may break under normal

conditions.

*. +ormal 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.

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cartridge fuse

& cartridge fuse consists of a porcelain tube

with metal and caps to which the element is

attached. The tube is -lled with silica These

fuses are found generally in modern plug

tops used with (* & 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, e0pensive to replace.

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#$C fuse

The #$C 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

-lled with silica with a silver element and lug

type and caps. &nother feature of #$C fuse is

the indicating element, which shows when thefuse has blown. It is very fast!acting and can

discriminate between a starting surge and an

overload

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fusing factor

In order to classify these devices, it is

important to have some

means of knowing their 1fusing 2performance. 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 ma0imum current,

which the fuse can sustain without blowing.

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fusing factor

For e0ample, a 3 & fuse, which blows only

when 4 & ows, will have a fusing factor of

453 6 (.7.

$ewirable fuses have a fusing factor of

about (.7.

Cartridge fuses have a fusing factor of

between (.)3 and (.83.

#$C fuses have a fusing factor of up to (.)3

"ma0imum%.

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discrimination

9here more than one fuse protects a circuit

it is clearly sensible that the correct fuse

should blow under fault conditions. & fault on

the appliance should cause fuse C to blow. If

fuse ' blew, although it would break the

circuit to the

faulty appliance, it would unnecessarily

render the whole radial circuit dead. If fuse &

blew, instead of ' or C, all circuits from the

distribution would be pointlessly

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discrimination

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#ow fuses are marked

The amp ratings of all fuses are printed on them, butthere are two systems of rating in use.

In the latest system, the rating marked on a fuse

indicates the continuous current it can carry, while also

withstanding brief surges of up to double that strength. It

is usually called the CR (continuous resistance) rating.

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Fuse replacement

& lower amp fuse can be used in an

emergency.......but never a higher amp

fuse...#igher can cause a -re or burn out

:ower may pop and need to be replaced, but

will not cause a -re. ;ust as a lower amp may

simply keep blowing if what is drawing upon

it needs a higher amp draw.

&lways best to

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1 Fuses