Fuselink elements

Cartridge fuselinks, as stated earlier, contain one or more parallel-connected elements which are made of materials of low resistivity. The materials, if possible, should also

Figure 4.3 Assembled and exploded views of cartridge fuselinks

possess the following properties if rapid operation is required: (a) low specific heat

(b) high thermal conductivity

(c) low melting and vaporisation temperatures (d) low latent heats

(e) low density

(f ) ease of connection to other conductors.

It will be appreciated that rapid operation is usually required during short-circuit conditions. Taking a wire element, of length l and cross-sectional area A, and for simplicity considering operating times short enough for heat movement within the fuselink to be neglected, it is clear that the following energy balance must be satisfied at the instant when vaporisation of the element occurs:

1 A

 tv 0

i2ρ dt = Al × element density {specific heat(θv− θi) + latent heat of fusion}

in which θvand θiare the vaporisation and initial temperatures, respectively, and ρ is the resistivity of the element.

The time taken (tv)for vaporisation to occur is therefore proportional to the square of the cross-sectional area multiplied by the element density [specific heat (θ )v−θi)+ latent heat of fusion] and it varies somewhat inversely with the resistivity. This tends to confirm that the time to vaporising and the energy let-through to the protected circuit (tv

0 i2dt) are both minimised when materials with the desirable properties (a) and (c), listed above, are employed. Short operating times at high currents are not the only requirement, particular operating time/current characteristics being necessary to

enable fuselinks to fully protect the devices or circuits with which they are associ- ated. In addition it is usually desirable that they should have a reasonably low fusing factor, that is the ratio of the minimum fusing current to the rated value should not be greatly in excess of unity, so that adequate protection is given against prolonged overloads. The term minimum fusing current is no longer used in fuse standards, despite its descriptive nature. For general purpose fuselinks, type gG to IEC 60269-1 (BS88-1) fuses are subjected to a conventional non-fusing current, Inf, and a conven- tional fusing current, If, for a time equal to the conventional time. The conventional time relates to the thermal time constant of the whole fuse and can vary from 1 to 4 h. Typical values for Inf and If are:

Inf = 1.25In I_f = 1.6I_n

In addition, a cable overload test is made at 1.45Iz, Iz being the associated cable rating, see Section 7.3.

At the rated current level and currents up to the minimum fusing level, steady-state equilibrium conditions must be established in which the electrical power input to an element is conducted away from it and dissipated from the fuselink outer surfaces. This is the reason for the property (b) listed above. Of course, not only the element must have a high thermal conductivity but so must the surrounding filling material, the body and other parts of the fuselink. In addition, because the power which can be dissipated from a fuselink of particular dimensions, with surface temperatures limited to acceptable levels, is fixed, it is necessary to limit the power produced by the element. This can only be achieved by having an element with a relatively large cross-sectional area and low resistivity. It will be seen that these requirements conflict with those which were shown to be needed to give rapid clearance of short circuits. Practical conditions, including cost, lead to acceptance of small cross-sectional areas of low-resistivity material.

It will further be appreciated that it is difficult to limit the temperature rises of fuselink surfaces to low levels at currents approaching the minimum fusing level if the element material has a high melting-point temperature and this is a further reason for attempting to satisfy property (c) above. A low fusing factor is very difficult to obtain when plain elements of materials with high melting points are employed, because the element temperature at rated current must be high and this must tend to lead to unacceptably high fuselink surface temperatures. Fortunately, acceptable fusing factors can be obtained when using elements with high melting points by alloying them to other metals to take advantage of the M-effect described in Section 2.3.

After extensive studies of all the available materials and taking the above factors into account, it has been concluded that silver and copper are the most suitable materials and certainly they are used in the vast majority of modern fuselinks.

Strip-type elements with restricted sections are produced in various forms by different manufacturers, a few typical examples being shown in Figure 4.4. The dimensions and numbers of restrictions depend on the current and voltage rat- ings. The strip thicknesses are usually in the range 0·05–0·5 mm. Fuselinks with

Figure 4.4 Various fuse element designs

single-strip elements are used for rated currents in the range of about 10–63 A and fuselinks with higher ratings contain two or more elements connected in parallel.

Although fuselinks containing several wire elements connected in parallel were produced in the past, this practice has now ceased and only single-element designs are produced for industrial fuselinks using wires up to approximately 0·2 mm diameter for ratings up to 10 A. It is of interest to note that ratings as low as 0·25A can be produced.