This British Standard relates to fuses for use indoors or outdoors on alternating current systems having line-to-line voltages above 1 000 volts at a frequency of 50 cycles per second, and with breaking-capacities corresponding to three-phase values from25 MVA to 750 MVA for power fuses and from50 MVA to 2 500 MVA for voltage-transformer fuses (or equivalent values for other systems).
In general, these fuses are suitable for short-circuit protection only, leaving other devices for protection against over-currents.
The requirements of this standard apply to fuses for use in air and to fuses for use immersed in oil.
This standard relates only to expulsion fuses.
Where fuses depend for their correct operation as fuses on the movement of some component to provide an insulating gap, this feature is considered as part of the fuse operation and comes within the scope of this standard.
On the other hand, there are fuse-links that contain tripping devices for initiating the opening of a switch. These fuse-links are required to comply with this standard, but the satisfactory operation of the equipment operated by the tripping devices is outside the scope of the standard.
The range of breaking-capacity ratings of 25 MVA to 750 MVA covers the usual applications of high-voltage fuses for power-system and voltage-transformer protection. It is recognized, however, that voltage-transformer fuses may also be required for use on systems having short-circuit capacities in excess of 750 MVA. Where such fuses are designed to operate in series with current-limiting impedances, the values of which are more than 100 times the impedance of the system, their performance is practically unaffected by the short-circuit current of the system, hence they may be safely used, with the appropriate impedance, on circuits having greater short-circuit capacities than 750 MVA. Voltage-transformer fuses without current-limiting impedances for use on systems with short-circuit currents greater than those corresponding to 750 MVA may be deemed to comply with this standard provided that all the requirements other than the tests set out in Clauses 27 and 28 are met; in such cases tests for breaking-capacity should be the subject of agreement between purchaser and manufacturer.