Análisis microestructural

These fuselinks have short elements and include means for lengthening the short arcs initially set up when the pre-arcing period ends. Even so, the arc voltage is much lower than the system voltage and for this reason the current limitation produced tends to be insignificant. They provide ‘full range’ performance and operate, as do all current-interruption devices, by preventing arc re-ignition after a current zero.

Because the breaking performance is not greatly affected by the element para-meters, the latter may be varied to provide different time/current characteristics. As an example, elements of different lengths could be used; a very short element, because of the high conduction of heat energy to its ends, has a small cross-sectional area for a given long operating time, at a particular current, but it would, as a result, operate

very quickly at high currents, heat conduction in the short time available then being low. In practice, elements are made of tin wire in many cases, but tinned-copper wire is often used when fast operation is required.

The elements are subject to spring tension and therefore many designs incorporate a fuse wire in parallel with a strain wire of high tensile strength and high resistivity.

Details of the two main types of non-current-limiting fuses in use in the UK are given in the following sections.

5.1.1 Expulsion fuses

These fuselinks contain a short element of tin or tinned-copper wire in series with a flexible braid. These items are mounted in a fuse carrier incorporating a tube of organic material, usually closed at the top with a frangible diaphragm, and containing a liner of gas-evolving material such as fibre. The fuse element carries a closely fitting sleeve of gas-generating fabric. The flexible braid is brought out of the open lower end of the tube and held in tension, by a spring attached to the lower end of the fuse base.

A cross-section through a typical fuselink is shown in Figure 5.1 and three popular physical connection arrangements, designated button-head, double-tail and universal, are illustrated in Figure 5.2. It will be seen that these all have a braid at the lower end, it being the upper terminations which differ.

The fuse carrier has pins at the lower end which act as a hinge when it is mounted in the lower contact of the fuse mount. In the service position the fuse carrier is tilted from the vertical, as can be seen from Figure 5.3, which shows a complete assembly.

When the fuse element melts during operation, the release of the spring tension disengages a latch that allows the fuse carrier to swing down by gravity. This provides

solder

Figure 5.1 Sectional view of an expulsion fuselink

Figure 5.2 Terminations of expulsion fuselinks

Figure 5.3 Expulsion fuse assembly

isolation and prevents discharges along the tube which could lead to tracking. It also indicates that operation has occurred.

These fuses are able to break a wide range of fault currents. When interrupting a small fault, the arc is extinguished within the fibre sleeve around the element, but at high currents this sleeve bursts and the arc is extinguished within the liner of gas-evolving material. The diaphragm at the upper end of the fuse carrier ruptures when the current being interrupted is sufficiently high and thus double-end venting is provided, so relieving the fuse-carrier tube of excessive pressure.

These fuses are available for use in three-phase circuits with current and line voltage ratings up to 100 A and 72 kV, respectively. Their maximum breaking capacity is typically limited to 150 MVA.

Expulsion fuses are for outdoor use only and they may be replaced using a pole from the ground. The pole can also be used to swing down the carrier of a healthy fuse for the purpose of isolation. It is necessary to check that the cir-cuit is not carrying current when this latter operation is to be performed, unless the equipment is arranged so that load current may be interrupted by the contacts.

In some designs this is done by including circuit-breaker-type arc chutes around the contacts which separate. An alternative, which is sometimes adopted, is to use fuselinks which are so designed that the element may be snapped mechanically by an operator inserting a pole into it from the ground. The fuselink then functions in the normal way to clear the circuit, after which the carrier may be swung down safely.

5.1.2 Liquid fuses

In the earliest non-current-limiting fuses, the arcs were quenched in a liquid and this principle has been used for many years to produce fuselinks. Many of them are in use in the UK. They have a body consisting of a glass tube with metal ferrules at each end and within it is the element. This is normally of silver strip or wire, with a strain wire across it except for low current ratings, say 10 A and below, in which a wire is used as the element and this is made strong enough to make a separate strain wire unnecessary. In all designs the element is positioned near the top of the tube so that it is shielded from corona discharge by the upper ferrule. The element is held in tension by a spring anchored to the lower ferrule and the tube is filled with an arc-extinguishing liquid, usually a hydrocarbon. When the element melts during operation, the spring collapses and the arc is extinguished in the liquid. To relieve the tube of excessive pressure, a diaphragm on the upper ferrule is ejected, except under very moderate conditions. Provision is usually made to allow the user to recharge by inserting a new fuse-element assembly and new liquid. Figure 5.4 shows a cross-sectional view of a typical liquid fuselink. These fuses are only in use outdoors and provision is made for removing from and replacing the fuselinks into their mountings by pole operation from the ground, a bayonet-fixing arrangement being usual. They are mostly used for the protection of 11 and 33 kV pole- or pad-mounted transformers on rural systems and also for spurs feeding a number of transformers.

diaphragm

element

arc-extinguishing liquid

spring

glass tube

Figure 5.4 Sectional view of a liquid fuse

The breaking capacity of these fuses is limited to values below those of expulsion fuses and although they have given, and indeed still are giving good service, they are no longer recommended for new installations.