Low voltage fuses have a low normal current rating and rupturing capacity. They cannot be, therefore, employed successfully on modern high voltage circuits. The main problems that start at high voltage is that of the corona. Therefore, high voltage fuses have got special designs. These are of two types viz., cartridge type and liquid type.
Cartridge Type HV HRC Fuse
The construction of HRC fuse is similar to low voltage fuse except that some special feature is integrated. In some designs, the fuse element is wound in the ring shape so as to avoid the corona effect at a higher voltage.
In some designs two fuse element are placed in parallel – one of low resistance carries the normal current, and the other is of high resistance made up of tungsten wire and, after the low resistance has blown off, reduces the short-circuit current and finally breaks the circuit.
High voltage cartridge fuses are used up to 33 KV, with a rupturing capacity of about 8,760 A at the voltage (500 MVA three phases). Rating of the order of 200 A at 6.6 KV and 11 KV and 50 A at 33 KV are also available.
Liquid Type HV HRC Fuse
Such fuses are filled with carbon tetrachloride and have the widest range of application in high voltage circuits. They may be employed for voltage transformer protection or circuit up to about 400 A rated current on systems up to 132 KV or higher and may have the breaking capacity of 6100 A at 33 KV (350 MVA, 3-phase).
It consists of a glass tube filled with carbon tetrachloride and selected at both ends with brass caps. The fuse wire is sealed at one end of the tube, and the other end of the element is held by a strong phosphorus bronze spiral fixed at the other end of the glass tube.
When the fault occurs the currents exceed the permissible limit, the fuse element is blown out and the spring retracts part it through a liquid detector draws it well into the liquid. The small quantity of gas generated at the point of fusion forces some part of liquid into the passage through the baffle, and there it effectively extinguishes the arc.
High voltage fuses are used for the backup protection to circuit breakers, whose short circuit capacity has been increased owing to the extension of generating plant or interaction to a value beyond their rated MVA. The fuses are provided to deal with short circuit exceeding a certain magnitude while the circuit breakers continue to interrupt short circuit within their breaking capacity.