The following points highlight the four different types of cut outs found in a fuse-wire. The types are: 1. Grip Fuse 2. Totally Enclosed Fuse 3. Cartridge Fuse 4. High Rupturing Capacity Fuse or H.R.C. Fuse.
Type # 1. Grip Fuse:
It is a semi-enclosed type of use. The fuse-wire in this type of fuse is neither totally enclosed nor kept open in air. It is mostly used for household installations. Fig. 84 shows a particular type of grip fuse. It consists of two parts, namely fuse carrier and fuse base. These two parts are usually made of porcelain or Bakelite, but for small fuses plastic is also used now-a-days.
The fuse carrier is placed on the top of the base. The base has two copper or brass terminals with two contact clips at two ends. With one terminal the incoming live line of the circuit or sub-circuit is connected. From the other terminal the outgoing live line goes to the load circuit.
Fuse carrier has also two copper or brass terminals at the two ends. These terminals are fixed with the carrier by means of two bolts. Each bolt is provided with a fly-nut or a screw. In fig. 84 the screw of the terminal on the right-hand side is seen. A similar screw is provided with the other terminal on the other side of the carrier. The two ends of a fuse wire are tightened up with these screws.
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The carrier is then pressed upon the base so that its terminals are pressed into the contact clips of the base. The main advantage of this fuse is that, while replacing the fuse wire, one does not come in contact with the live line. The carrier is taken out of the base and the melted fuse-wire is replaced by a new one.
Type # 2. Totally Enclosed Fuse:
The fuse-wire remains totally enclosed in this type of fuse. Hence, it provides more safety to the operator and has larger field of application than grip fuse.
Totally enclosed fuse is similar in construction to a grip fuse having a carrier and a base. Such a fuse in shown in fig. 85(a) & 85(b). The main advantage of this fuse is that, even if the fuse-wire melts as soon as the carrier is pressed upon the base, the operator will not have any burn injury in his finger. But such an injury is possible with a grip fuse.
Type # 3. Cartridge Fuse:
Cartridge fuse is also a totally enclosed fuse. Fig. 86 shows the different parts of such a fuse. The main part containing the fuse-wire is the part B. It appears to be a cartridge of a fire-arm and hence it is called cartridge fuse.
Part B consists of a cylindrical tube made of porcelain with two ends open. The bottom end is covered by a metal rod and the top end by a metal disc. The fuse wire remains tightened in between the rod and the disc within the cylinder. In order that the hot gas produced due to melting of the fuse-wire does not come out, the space around the fuse-wire is filled up with a powder like French chalk.
The cartridge is placed within a hollow porcelain base (part D shown in fig 86) and the top of the base is covered by a cap (part A) which is screwed into the base. The cap is hollow and covered with a piece of glass at the top. So long the fuse-wire remains intact, the glass at the top of the cap remains clean. But as soon as the fuse – wire melts, the inner surface of the glass is stained due to fume. This indicates the melting of fuse-wire. The collar shown in part C prevents any bigger size of cartridge to be placed within the base.
The cartridge fuse mentioned above is not common now-a-days. Instead, an alternative shown in fig. 87 has large field of application.
Type # 4. High Rupturing Capacity Fuse or H.R.C. Fuse:
High rupturing capacity fuse is a special type of cartridge fuse. Outwards it appears to be the same as the cartridge fuse shown in fig. 87. Its body is made of heat resisting ceramic materials having low temperature co-efficient of thermal expansion. The body can also withstand high internal pressure. A silver wire is used as its fuse element. Surrounding the fuse-wire the space inside the cover is filled with a quartz powder which acts as arc-extinguishing agent.
Due to short-circuit etc. when a high current flows, the fuse-wire melts and an electric arc is formed between two contacts. The melting of silver wire produces a gas inside the cover. This gas and the powder together form a high pressure inside and offer high resistance to the flow of current through the arc. As a result the arc extinguishes.
H.R.C. fuse is much costlier than any other similar type of fuse. Once fused it should be replaced by a new one. But its use has some special advantages, and therefore it has a wide field of application.
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Use of H.R.R. fuse has the following advantages:
(i) H.R.C. fuse costs less than any other protective device of same capacity.
(ii) If a short-circuit occurs in a line or in a machine or in an apparatus, the performance of this fuse is excellent.
(iii) H.R.C. fuse can give the required discrimination with complete reliability.
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(iv) There is remote chance of deterioration of H.R.C. fuses during service period.
(v) The arcing time of this fuse is inversely proportional to current flowing through it, i.e. under more severe condition the time taken for clearance of fault is comparatively, less.
(vi) The fuse has consistency of operation, i.e. it works within very close limit of desired performance.