HRC Fuse is known as totally enclosed type fuses, In this type of fuses the fusing element is completely enclosed in a tube of glass or porcelain or fiber etc. AS the fusing element does not come in contact with the atmosphere, so the fusing current remains controlled and constant.
Fig. 4.4. HRC fuses.
Cartridge type fuses. In a simple construction, as shown in Fig. 4.4 the name is kept after its construction similar to the construction of a cartridge say bullet. The container is made of insulating material generally glass, fiber or porcelain. The container is filled with quartz or sand, so that the heat dissipation is maintained uniformly from the fusing element, thus maintaining the fusing current constant. The sand or quartz also helps in quenching the spark if any. Each end Of the tube is covered with the metallic cap. The caps are made of copper and then a sufficient provision is made to tight or insert, in the fuse carriers to enable the continuity in operation. The fusing element generally of silver, is fixed between these two caps. The element is made of a number of chambers, to enable the heat to dissipate. There is a white spot on the cylindrical construction like a peep hole, which in normal condition is clear and becomes black in case of fuse blown out, so by seeing it can be judged about the condition of the fuse. The HRC fuses (high rupturing capacity) are designed with a time lag arrangement, i.e. for a short time, the fusing element will carry the overload current and will dissipate the heat produced. But if overload remains certainly it will blow out and can be seen from the spot already mentioned.
Operation. In case of normal working, the current flows through the fuse as it is connected in series, the heat developed is quite bearable and fuse will not blow. But, if the current exceeds the normal working limits, then the heat developed will be much more than the heat dissipated and the condition Of the fuse whether blown or safe will be indicated by the spot.
These fuses are available from 1 A to 300 A. These are designed for accurate currents and are used in light and power circuits or in switch gears of high current capacity.
What are the points to be kept in mind while selecting the HRC fuses? Also state the advantages and disadvantages of these fuses.
There are some important points to be kept in mind before selecting the HRC fuse:
(i) The normal current Of the circuit,
(ii) The time required for the overloading to the circuit.
(iii) The circuit and fuse voltage.
(iv) The rupturing capacity of the fuse should not be less than the current to be broken.
Advantages:
There are the following advantages:
(i) The fuse is very sensitive.
(ii) The fuse does not deteriorate with high speed (as the circuit cut off before the first peak value).
(iii) These are quite reliable.
(iv) These are cheap.
(v) NO maintenance is required.
(vi) Consistent is performance.
Disadvantages:
There are the following disadvantages:
(i) It requires replacement after every blowout, so they are costly in operation.
(ii) Inter-locking is not possible.
(iii) Cannot be repaired for same current capacity.
(iv) Produces over heating of the adjacent contacts.
Can the HRC fuse with tripping device be possible, if so give an idea with neat sketch?
The H.R.C. fuse with tripping devices are possible and the brief description is as under. ‘The body of this type Of fuses is ceramie, provided with metallic caps at the ends. The cap at one end is provided with a plunger facility, which hits the tripping contact of the circuit breaker. The plunger is electrically connected through fusible link, chemical charge and tungsten wire on the other end of the cap as shown in Fig. 4.5.”
Fig. 4.5. HRC fuse with tripping device.
There are a number of fusing elements made of silver, depending upon the current capacity Of the fuse, These elements are connected to both caps as shown in Fig. 4.5. Whenever the fault occurs, the fuse elements are first to blow off, With the flowing of this current, the weak link in series, tungsten wire heats up and blow the chemical charge and deteriorate, thus the plung is pushed up as shown in Fig. 4.7 with a force and trip the circuit breaker.
Then with one fuse blowing, only the circuit breaker is off, as a result the single phasing is eliminated and motor etc. will not run on two phases.
Fig. 4.6. Normal Conduction
Fig. 4.7. Tripping operation.