Electrical circuit protection:Fuses and Rewireable fuse

Every electrical installation must be protected from current overload. This is achieved by locating a protective device at the commencement of each circuit in the form of a fuse or circuit breaker. Their function is to protect the circuit conductors (cables) and not the appliance or user.

Fuses

Three types of fuse are used (Figure 100); the rewireable or semi-enclosed fuse; the cartridge fuse or fuse link; and the high rupturing capacity (HRC) fuse.

Rewireable fuse

This comprises a fuse holder, an element and a fuse carrier. The holder and carrier can be made of porcelain or bakelite. The fuse holder is colour coded as follows:

45 A green

30 A red

20 A yellow

15 A blue

5 A white

This type of fuse is popular for domestic appliances and small commercial units because of cheapness and ease of replacement. It is not recommended for commercial refrigeration duty because of these disadvantages:

1 The fuse carrier can be loaded with the wrong size fuse wire.

2 The fuse element tends to weaken after long usage owing to oxidation of the wire by heating in air. This causes it to fail under normal conditions,

i.e. normal starting current surges are sensed by the fuse as an overload.

3 The fuse holder and carrier can be damaged as a result of arcing in the event of an overload.

Cartridge fuse

This consists of a porcelain tube with metal end caps to which the fuse element is attached. The tube is filled with silica.

These fuses may be used in plug tops with 13A socket outlets and in distribution boards. They are recommended for refrigeration duty. They have the advantages over the rewireable types of not deteriorating, of being more accu- rate in breaking at the rated value, and of not being subject to arcing.

HRC fuses

This is a sophisticated version of the cartridge fuse. It is normally used for the protection of motor circuits in commercial and industrial installations. It consists of a porcelain body filled with silica and with a silver element; the body terminates in lug-type end caps.

These fuses are fast acting, and can discriminate between a starting surge and an overload. An indicating element shows when the fuse is ruptured.

Rating

The selection of fuse ratings depends on the full load current, the locked rotor current and the cable size. The current ratings for tinned copper wire are shown in Table 6.

Fusing factor

Different types of fuse provide different levels of protection. Rewireable fuses are slower to operate and are less accurate than cartridge types.

To classify the protection devices it is important to know the fusing perfor- mance. This is achieved by the use of a fusing factor:

fusing current fusing factor D current rating

Here the fusing current is the minimum current causing the fuse to rupture, and the current rating is the maximum current which the fuse can sustain without rupturing. For example:

1 A 5 A fuse ruptures only when 9 A flows; it has a fusing factor of 9/5 or 1.8.

2 The current rating of a cartridge fuse is 30 A and the fusing factor is 1.75; the fuse will rupture at 30 ð 1.75 D 52.5 A.

3 The current rating of an HRC fuse is 20 A and the fusing factor is 1.25; the fuse will rupture at 20 ð 1.25 D 25 A.

It must therefore be realized that a fuse is rated at the amount of current it can carry, and not the amount at which it will rupture. Rewireable fuses have fusing factors of approximately 1.8; cartridge fuses of between 1.25 and 1.75; HRC fuses of up to 1.25 maximum; and motor cartridge fuses of 1.75.

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