Current-interrupting abilities and categories of partial-range fuselinks

When a high-voltage fuselink with one or more conventional parallel-connected ele- ments is clearing a fault current, short breaks are formed at the element restrictions and short arcs result. The process continues until the burn backs have a total gap length which can withstand the recovery voltage present after the arcs have extin- guished at a current zero. The total gap lengths needed in HV applications are clearly considerable, and the times required to melt and vaporise sufficient element material are correspondingly long. Consequently, only at currents above a certain level is the rate of lengthening of the arcs in any particular fuselink sufficient to so limit the arcing time that the temperature of the filling material is prevented from reaching a level at which its arc-extinguishing properties are so reduced that the fuselink would fail to clear. There is thus a range of currents above the minimum fusing level within which satisfactory clearance will not be effected. This situation, which does not arise with low-voltage fuselinks, can be accepted for some applications as stated later, but it is nevertheless undesirable and manufacturers sought to lower the minimum

safe clearance currents. Experiments demonstrated that improvements were obtained by using large numbers of parallel-connected elements of small cross-sectional area rather than a smaller number of thicker elements. The improvement occurred because all the short arcs set up at each of the restrictions when a relatively low fault current is being cleared do not continue to burn in each of the parallel-connected elements. In practice they tend to commutate around the elements, the gaps in those elements, which are arcing, extending rapidly because of the high current densities in them. The voltages across these gaps rise to levels where the now shorter gaps in other elements ionise and cause the previously burning arcs to extinguish. This process shortens the time taken for current interruption to be effected. Performance was also improved by employing elements with long restricted sections of small cross-sectional area, which again increased the current densities.

It was nevertheless difficult to produce fuselinks which could safely clear all currents above their minimum fusing currents up to their rated breaking capacities, and therefore three internationally recognised categories of HV fuselinks were introduced, namely:

Back-up Fuses in this category must be able to interrupt all currents between a minimum value specified by the manufacturer and the full rated breaking capacity.

General purpose Fuses in this category must be able to interrupt currents from the rated breaking capacity down to the level at which the operating time is 1 h.

Full range Fuses in this category must be able to interrupt all currents from the rated breaking current down to the smallest current which causes the fuse elements to melt.

It is the practice of the UK Electricity Supply Industry and of supply authorities in numerous overseas territories influenced by UK practice, e.g. Australia, South Africa, India and the Middle and Far East, to use fuse-switch ring-main units.

In the UK, such fuse-switch ring-main units of 250 MVA rating at 11 kV are covered by Electricity Supply Industry standard ESI41-12, which stipulates that the fuselinks must be provided with strikers which trip the switch instantaneously when one or more fuselinks operate.

This not only prevents single-phasing of any motors fed from the transformers but, equally important, eliminates the possibility of trouble if the equipment is subjected to a fault current less than the minimum breaking current of the fuse.

Typically, the total time to trip the switch from inception of arcing in the fuselink may be only 30–50 ms whereas it will generally take at least ten times as long as this for any persistent low-level arc within the fuse to cause any trouble.

It is, of course, only for fault currents below the stated minimum-breaking current for the fuse that such external aid is necessary. At higher fault currents the more usual series-multiple-arcing mode of fuselink operation takes over and ensures easy circuit interruption.

The UK fuse-switch gear is thus fully self-protecting at all possible fault levels and consequently the fuselinks may be of the back-up variety. Such fuselinks having minimum breaking currents (MBC) in the region of 2·5 to 5·0 times their rated current are suitable. Satisfactory co-ordination may be achieved by making the switches capable of breaking at least seven times the rated current of the largest fuselink used. The statistical chances of faults occurring below the minimum safe breaking cur- rent of the fuse are, in UK practice, quite small in any case, since unearthed neutral points (which could result in small capacitive earth fault currents flowing) are not used and low-voltage secondary-feeder fuses ensure that low-voltage (LV) faults or overloads do not have to be cleared by the high-voltage fuses. Both back-up and general-purpose fuselinks are suitable for such circuits and the latter type has, in the past, been preferred in some applications where instantaneous striker-tripping facil- ities were not provided because the ratios of their minimum safe breaking currents to minimum fusing currents are lower than those of back-up fuselinks. The usage of general-purpose fuselinks is now reducing, however, because of recent developments which have led to the introduction of high-voltage current-limiting fuselinks capa- ble of clearing all currents between their minimum-fusing and rated-breaking levels (i.e. full-range fuses).

As early as 1965, Mikulecky published a paper entitled ‘Current limiting fuse with full-range clearing ability’ [35]. The term full-range has recently been included in fuse specification IEC 60282-1 and it is now widely used by manufacturers and users to describe any HV current-limiting fuse which can, unaided, safely interrupt currents from the rated breaking capacity down to the smallest current which will cause melting of the elements, even under conditions of restricted air circulation. Clearly such fuselinks may be used as the sole protection in circuits operating over a wide range of normal and abnormal conditions.