FUSES AND FUSE PROTECTION

BY

ENGR.W.A.ASONMWONRIR

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Introduction

Definition of a fuse:A fuse is a device that, by the fusion of one or more of its specially designed and proportioned components, opens the circuit in which it is inserted and breaks the current when this exceeds a given value for a sufficient time. The fuse comprises all the parts that form the complete device (BS 88).We see in the definition that the fuse is the complete device, consisting of a fuse-holder (which comprises a fuse base and fuse carrier) and a fuselink.

A fuselink is defined as follows:

Definition of a fuselink:

A fuselink is a device comprising a fuse element or several fuse elements connected in parallel enclosed in a cartridge, usually filled with an arc-extinguishing medium and connected to terminations, the fuselink is the part of a fuse which requires replacing after the fuse has operated. (BS 88) We see from this definition that the 'fuselink' is the part of the fuse popularly but incorrectly referred to as 'a fuse', and that the internal part which melts is called the 'element'.

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Categories of fuse

The powder-filled fuse is the most advanced type of fuselink, with great advantages in limitation of short circuit currents and very high breaking capacity . 'Miniature fuses' are used in electronic and similar apparatus, and 'semi enclosed fuses' are the rewirable type still extensively used in consumer units and other applications where only limited breaking capacity is required. Other types of fuse, such as the liquid-quenched and the expulsion fuse, which employ some mechanical feature to assist the circuit-breaking process, are available. These are used mainly for high voltage overhead-line networks .A fuse is a weak link in a circuit, and, as such, possesses one important advantageover mechanical interrupting devices such as circuit-breakers. Because the element in the fuse is of much smaller cross-sectional area than the cable it protects(assuming, of course, that they are of the same material) the element will reach its melting point before the cable. The larger the current the quicker the element melts. If deterioration of the element should occur it operates even faster; a fuse is therefore a device that fails safe'. A comparison of the modem fuse with mechanical interrupters shows that the fuse has one outstanding property not possessed by the latter, namely the ability toInterrupt very large currents in a much shorter time - so short in fact that the current will be 'cut off' before it reaches its peak value, which in a 50 Hz system implies operation in less than 5 ms. serious overheating and

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electromagnetic forces in the system will thus be avoided. It is not unusual for a fuse to be used as 'backup' protection for a circuit breaker that might, by itself, have inadequate breaking capacity. In addition, the sealed cartridge fuse is silent in operation and does not emit flame. Being sealed, it is tamper-proof and with fuse elements of, say, silver, itis non-deteriorating and gives a consistent and reliable performance. The fuse has two disadvantages. The first is the inconvenience of replacement, because it takes longer to replace a fuse than to reclose a circuit breaker. This drawback is often exaggerated, especially with modern cartridge fuses, some of which can be replaced quite quickly. It may, in practice, take longer to trace a blown fuse, if it is not fitted with an indicating device, than to replace it; usually far more time is consumed in checking the faulty circuit. The other disadvantage, which has now lost much of its former significance, is the hitherto poor protection against small overcurrents. A circuit-breaker can be set to trip on as little as 5% overcurrent whilst the semi-enclosed fuse has a fusing factor (that is the ratio of the minimum fusing current to the rated current) of about 1.75. Modern cartridge fuses can, however, be obtained with fusing factors as low as 1.25; they can be supplied with even lower fusing factors, but experience indicates that values lower than 1.25 are not to be recommended if unwanted blowing due to momentary system abnormalities is to be avoided. The earlier criticism that the fuse could not give the kind of performance obtained from a circuit-breaker fitted with a time-lag relay has lost much of itsforce because reliable time-lag or 'surge resisting' cartridge fuses of high breaking capacity are now readily obtainable,

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at least for domestic and industrial use. As regards capital cost, fuses are cheaper than circuit breakers of similar rating and breaking capacity, especially for use at high voltages (over lkV) where the saving in cost is considerable. In addition, the cost of maintenance is much less in the case of the fuse. Against these savings, however, must be set the cost of replacement fuses after operation onfault.

Fuse design:

Fuses differ considerably in design, dependent upon the voltage, a.c. or d.c, at which they are to operate, and the breaking capacity required. For this reason it is always very important to ensure that a blown fuselink is replaced by the correct type, as otherwise damage to the installation or personal injury may result. The following are the principal types:

Powder-filled cartridge fuse

The filler used is almost invariably high purity sand or powdered quartz (both forms of silicon oxide (SiO2)). The sand must be very pure (some impurities like iron compounds or organic matter are particularly troublesome) as otherwise the fuse is liable to failure. The arc produced when the fuse 'blows' creates a tube of melted sand around it, which withdraws energy from the arc and extinguishes it. The spray of metal from the arc roots is also entrained in the filler. Excessive pressures can be reached if the filler is

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very fine or very coarse; an intermediate grain size provides the optimum cooling. It is desirable to reduce the volume of metal in an element as far as possible,since this reduces the pressure in the cartridge. When using filler the heat is conducted away from the element more rapidly than in air and thus a smaller element can melt at a larger minimum fusing current. Thus a thinner wire can beused for a given current rating. If the wire is flattened into a tape, its heat dissipation is even faster, and if it has increased width at sectors along its length, the heatdissipation from the constrictions is faster still.

High-voltage powder-filled fuses: The high-voltage (above 1 kV rating) fuselink uses the same basic principles of construction as the low voltage (below1 kV), but in a manner refined to produce the special characteristics and additional features needed in high voltage applications. Since the length of the fuse element, and the number of constrictions in it must increase roughly proportionally to its voltage rating, many designs of high voltage fuselink would become unmanageably long if the elements were connected straight down the barrel, as in low-voltage fuselinks. For this reason they are frequently wound in the form of a helix on a star core in the barrel, which requires great care and precision in manufacture, and careful design to ensure that the arc from one turn of a helically wound element does not merge with arcs from the next turn on the helix of the same element or of another nearby, which could lead to destruction of the cartridge by sustained arcing between its end-caps. Manufacturers of high-voltage fuselinks consequently employ many more methods of

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nondestructive testing of their products during production, and this makes the high voltage cartridge fuselink a particularly safe and reliable product, when used withinits limits of rating and breaking capacity.

Miniature fuselink:This is the small cartridge fuselink used in apparatus, electronic equipment, radio and t.v. sets, etc. They come in two main categories: filled and unfilled.

Semi-enclosed fuse

The semi-enclosed or rewirable fuse consists of a base, a carrier, the fuse element and some form of protection, such as an arc resistant tube, to limit the emission of flame. The element is usually of an easily procurable material suchas tinned copper. The melting point of copper is 1083°C but it cannot be run for any length of time at temperatures approaching the melting point, and rapid oxidation occurs at temperatures above 250°C, even if the wire is tinned. Hence the semi-enclosed fuse requires a relatively large overcurrent to blow it. Care should be taken to ensure that the wire is not kinked or otherwise damaged in rewiring and that the correct size of fuse wire is used.

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Expulsion fuse

The expulsion fuse consists of a tube of insulating material into which the fuse element is inserted, in some cases one end of the tube being closed whereas in others both ends are open. When the element melts and arcing takesplace, the resultant gas pressure causes the arc to be blown out of the ends of the tube and thus be extinguished. In certain designs this process is assisted by fining the interior of the tube with a material such as boric acid which produces gas when heated by the arc. In order to accelerate the process of arc extinction, the element is held under spring tension and when the element melts the spring rapidly separates the two sections. The operation of the expulsion fuse is violent, especially with large fault currents, and it is usually pole mounted out-of-doors, increased phase spacing being employed to avoid flashovers due to the nature of its operation. The walls of the tube may be contaminated by carbon and other arc products after blowing, and in order to prevent leakage along this path the tube is arranged to be isolated from thecircuit contacts after operation. This is achieved by utilizing the spring normally holding the element under tension effectively to shorten the length of the fuse carrier when released. This allows the carrier tube to disengage from the upper contact and to fall, under the influence of gravity, about the lower hinged contact.

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Mechanism of fuse operation:All fuses operate in the same way. A conductor of limited cross-section is heated by current passing through it until it melts. This takes time, represented in a time/current characteristic for the fuse. On melting, a break is caused in the element, at which an electric arc is established, which burns in the fuse until the current returns to zero. Thus there are two stages in fuse operation:(a) The pre-arcing time(b) The arcing time.

Operation on small overcurrents:When a fuse is blown by a current not much larger than the minimum fusing current so that the melting time is measured in minutes, the temperature distribution along the element is not uniform, the hottest point being near the centre. It isat this point that melting first occurs, giving rise to a short arc, and because the arcing voltage is then about 20 V, the effect in reducing the current is inappreciable in circuits at 240 V or more. An arc has a negative volt-ampere characteristic, which, balanced against the positive volt-ampere characteristic of the circuit resistance tends to produce stable sustained arcs at small overcurrent level. These will continue to burn until the arc has lengthened sufficiently, and developed sufficient pressure within the lumen of molten sand in order to raise the arc voltage enough to give sufficient back e.m.f, to exceed the circuit voltage and cause extinction. Fuses of higher voltage rating thus need longer elements or more constrictions than fuses of lower voltage rating.

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It is unwise to connect fuses in series to obtain a higher voltage rating except perhaps for back-up protection where they will be subjected only to large fault currents. If they are subjected to a small sustained overcurrent only one may melt because it is unlikely that they will be exactly matched. The fuse that operates will do so at the full circuit voltage and may fail. Fuses may, however, be connected inparallel, provided that each takes its proper share of the load (otherwise premature blowing will take place).

Design of the protection of a Typical distribution transformer

A knowledge of the magnitude of the primary and secondary current of the distribution transformer will help us choose the correct rating for J&P fuses and HRC catridge fuses. Generally a fuse is designed to rupture when the current flowing exceeds the current rating of the fuse element. Hence when properly selected, fuses offer adequate protection over distribution transformers against overload and shortcircuit.Fuses are rated in amperes .

Given: 300kVA, 11KV Distribution Transformer

Required: Rate the J&P and HRC fuses required to protect the transformer.

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Solution

Ip = KVA = 300 = 15.745A 3xKV 3x11

Is = KVA = 300 = 400A 3xKV 3x0.415

Fuse selection:J&P Fuse =15A HRC Fuse =400AWhat is the effect of using a 30A J&P Fuse? Also what is the effect of using 800A HRC fuse?

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