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.
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.
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 accurate in breaking at the rated value, and of not being subject to arcing.
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.
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.
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 performance. This is achieved by the use of a fusing factor:
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 • 1.75 = 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 x 1.25 = 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.