Switchgear-Construction of cartridge fuses

Construction of cartridge fuses

Fuse elements

from theoretical and other studies it is evident that fuse elements should generally be made from materials with low resistivities. If possible the materials should also possess the following properties:

(a) low specific heat

(b) low melting and vaporization temperatures

(c) low latent heats

(d) low density

(e) high thermal conductivity

Construction of cartridge fuses.properties (a) to (d) are desirable when rapid operation at high currents is necessary. At The rated current level, steady-state equilibrium conditions must be established in which the electrical power input to an element is conducted away from it and dissipated by the connecting cables and the outer surfaces of the fuse link. This is the reason for property (e). Of course, not only must the element material be of high thermal conductivity but so must the surroundings filling material and the body. In addition, because the power which can be dissipated from a particular fuse link is limited, it is necessary to limit the power input to the element by using elements of the adequate cross-sectional area.

After extensive studies of the available materials and taking the above factors into account, it has been found that silver and copper are the most suitable materials for elements and they are used in the majority of fuse links. Notched elements are produced in various forms a few being shown in figure 1.11. 

The strip thickness is usually in the range of 0.05 to 0.5 mm and the widths seldom exceed 10 mm.

Occasions do arise when the necessary performance cannot be made using notched elements of a single material with a relatively high melting point. In these circumstances, advantage may be taken of the so-called' effect in which a low melting point material is deposited adjacent to one or more of 

the restrictions on the notched strip, an example shown in figure 1.12. This method resulted from work done by Metcalf who published an article entitled 'A new fuse phenomenon' in 1939.

Fuselink bodies

bodies must possess good electrical insulating properties and should not allow the Ingress of moisture. They should be reasonably good thermal conductors and have an adequate emissivity constant so that they can transmit and radiate heat energy emanating from the elements within them. They should also be physically robust.

Filling material

The filling material invariably quartz of high chemical purity and grain sizes in the region of 300 microns, conducts some of the heat energy away from the fuse element. To ensure consistency of performance it is necessary that the packing density of the filling material be maintained at a high constant level during production. It has also been found that this factor may have a very significant effect on the fuse performance at high current levels. A low packing density is undesirable as it may allow arcs to expand more rapidly and thus adversely affect the extinction process.

Mountings and ratings

Low voltage cartridge fuse links of standardized performance and dimensions are available with rated currents ranging from 2 to 1250 ampere to provide overcurrent protection on three phase 415 volt a.c systems and up to 250-volt dc systems. Designs are also available at certain ratings for a.c circuits up to 660 volt and d.c circuits up to 500 volts.

Fuse links with rated currents up to 200 amperes are usually fitted into fully shrouded fuse holders comprising a carrier and base. such fuses are produced in large numbers and many are fitted in distribution fuse boards.

Fuse links with the current rating above about 200 amperes are large and produced in small quantities. They are normally installed directly in convenient positions without their own special or standardized housings. They are often incorporated in fuse switch units and example being shown in figure 1.13.

High Voltage cartridge fuse links are produced for use in a.c systems operating at frequencies of 50 hertz and 60 hertz with rated voltages exceeding 1000 volt. They are of the same basic design as low voltage fuses. Those with low rated currents have silver wire elements whilst those for higher current applications have elements of the silver strip with restricted sections along their lengths. All elements have low melting point metals applied to them to enable the 'M' effect referred to in section 1.5.1 to be obtained. The elements are long because to achieve satisfactory current interruption, the total voltage across the fuse link must be high and this is best achieved by having many restrictions and therefore many arcs in series. To accommodate such elements in a body of acceptable length, the elements are accommodated in a helical form on insulating formers with a star-shaped cross-section.

Whilst some high voltage fuse links are mounted in air, it is nevertheless common practice for them to be immersed in oil in the pieces of equipment they are protecting. This is advantageous because the cooling effect of the oil allows a given current rating to be achieved in a smaller fuse link then that required for use in the air. The smaller fuse link also operates more rapidly at very high current levels.

Fuse links suitable for use in systems operating at voltages up to 33kv at a range of rated current values are produced. As an example, voltage ratings of 3.6 to 7.2 KV with current ratings up to 500 A are available.