High Voltage Fuse Technology For the protection of equipment operating above 1000 volts AC Current Limiting Current limiting HV fuse links split into three Current limiting back-up fuse links are the closest in internationally recognised types: back-up (or partial construction to LV cartridge types. However, a longer strip range) fuse links, which will interrupt any current element with many more restrictions is necessary to from their rated breaking capacity down to a minimum produce the large number of series arcs needed to breaking current specified by the manufacturer and interrupt a high voltage. Fuse manufacturers achieve general purpose fuse links, which will interrupt all this by coiling the element around a ceramic core with a currents from rated breaking capacity down to a star shaped cross section. In this way, a 1m long element current that will melt the elements within one hour. can be fitted into a 250mm long fuse body. If the adjacent A third type is the full range fuse. This term applies coils are too close, there will be a flashover between to fuse links that can interrupt any current below them and this sets a practical limit on how long an the rated breaking capacity that melts the fuse element can be crammed into a particular cartridge, and elements satisfactorily. how many elements can be used in parallel. Cross Section Silver ceramic End cap Porcelain barrel Star core Moisture tight seal point contact Star core Striker coil Granular Quartz Fuse elements Porcelain barrel Fuse elements Granular Quartz Striker coil Expelled striker Like an LV cartridge fuse link, an HV fuse link of this in parallel with the elements, so a current flows type has a ceramic body. through it when the elements melt. The current heats up the wire and detonates the explosive Many HV current limiting fuse links are also fitted with charge, or melts the wire andreleases the spring, a striker mechanism. This not only provides the user pushing the striker pin out of the fuse links end with a visual indication that the fuse link has operated, cap. A lock washer mechanism makes it impossible but can also be used to operate other switchgear. In to push the pin back into the fuse body. this way, a fuse on a single phase system can cut off all three phases if a fault occurs. There are several variations on this design for specific applications. For example, the elements in three- Striker mechanisms are driven by explosive charges or phase motor protection fuse links are corrugated to compressed springs. Both are triggered by a thin fuse withstand the cyclical mechanical stresses induced wire running the length of the fuse, usually through the by the high starting and stopping currents drawn centre of the star shaped core. The wire is connected by the motor. 1 For more information visit our website www.bussmann.com High Voltage Fuse Technology For the protection of equipment operating above 1000 volts AC Non- Current Limiting Essentially, non-current limiting fuse links have short downwards under gravity, breaking the circuit. As the elements and incorporate some means of lengthening fuse swings downwards, the arc is lengthened, the arc after the element melts, extinguishing the arc extinguished and prevented from restriking. and preventing re-ignition. There are two distinct types: Liquid fuse links are based on early non-current expulsion fuse links and liquid fuse links. limiting fuse links which use liquids to quench the arc. Expulsion fuses are an effective way of protecting The fuse element is anchored to the top ferrule of a overhead distribution lines and transformers. They are glass tube filled with a quenching liquid - usually a designed for outdoor use only and comprise a tin or hydrocarbon. The rest of the tube is filled with a copper fuse element in series with a flexible braid in a spring that holds the element - or a strain wire in very tube. The tube forms one side of a triangle, with a high voltage fuse links - in tension. latched connection at the top and a hinge at the When the element melts, the spring pulls the two bottom. The braid emerges from one end of the fuse parts of the element apart, extending the arc and link and is held in tension by a spring. quenching it in the liquid. When the element melts, the braid is no longer under tension, the latch is released and the fuse swings M- effect Bussmann high voltage fuse links include M-effect as a design feature to enhance performance. M-effect (named after its discoverer, Prof. Metcalf, in the late 1930s) consists of applying overlay spots of special low melting alloy onto the fuse element strips. Its effect is to ensure that the fuse link runs cooler in service and that the maximum temperature reached during fuse operation is kept down to a modest 160 deg. C (compared with 300 - 400 deg C for fuse links which do not employ this feature). M-effect has been in widespread use in both low voltage and high voltage fuse links of British design for many years and is now coming into use in many other countries. Cooler running M-effect fuse links have longer service life since lower surface temperature on the elements ensures against long term embrittlement of the material. The M-effect spots are stable and have been proved by many tests to remain inert for periods of 30 years or more. The low maximum operating temperature of M-effect fuse links ensures safe operation of Back-up types in striker tripped switchgear under low fault conditions. Whereas non M-effect types are known to have a risk of body fracture because of their much higher body temperature under such conditions. The low maximum operating temperature also ensures against damage to heat sensitive fuse enclosures such as cast resin fuse pods. In short, M-effect fuse links are safer, give better protection and are longer lasting than alternative designs which do not employ this valuable feature. 2 For more information visit our website www.bussmann.com