10.1 Introduction

A fuse in its simplest form, is a small piece of thin metal wire (or strip) connected in between two terminals mounted on an insulated base and forms the weakest link in series

with the circuit. The fuse is the cheapest, simplest and oldest protective device and is used as current interrupting device under overload and/or short circuit conditions. It is designed so that it carries the working current safely without overheating under normal conditions and melts due to sufficient i²R heating when the current exceeds a certain predetermined value in abnormal conditions, caused by overloads or short circuits and thus interrupts the current. A fuse, being a thermal device, possesses inverse time- current characteristic, i.e. the operating time decreases as the fault current increases. Fuses are used to protect cables, electrical equipment and semiconductor devices against damage from excessive currents due to overloads and/or short circuits in low-voltage and medium voltage circuits. They are relatively economical, they do not require relays or instrument transformers and they are reliable. They are available in a large range of sizes and can be designed “one shot” or as reusable devices with replaceable links. Modem HRC (High Rupturing Capacity) fuses provide a reliable discrimination and accurate characteristics. Recently, HRC fuses of voltage ratings up to 66 kV have been developed for application to distribution systems. The initial cost of a fuse is small but its installation, maintenance (against ‘corrosion and deterioration) and replacement (periodically and after blowing) can be quite expensive (for example, on remote rural distribution networks).

10.2 Definitions

(i) Fuse A fuse is a protective device used for protecting cables and electrical equipment

against overloads and/or short-circuits. It breaks the circuit by fusing (melting) the fuse element (or fuse wire) when the

current flowing in the circuit exceeds a certain predetermined value. The term fuse in general refers to all parts of the device.

(ii) Fuse element (or fuse wire) It is that part of the fuse which melts when the current flowing in the circuit

exceeds a certain pre determined value and thus breaks the circuit. Materials commonly used for fuse elements (or fuse wires) are tin, lead, zinc,

silver, copper, aluminum, etc. Practical experience has shown that the materials of low melting point such as

tin, lead and zinc are the most suitable materials for the fuse element. But these materials of low melting have high specific resistance as can be seen

from Table 10.1. For a given current rating, the fuse element made from low melting point metal

of high specific resistance will have greater diameter and hence, greater mass of the metal than those made from high melting point metal of low specific resistance.

Thus, the use of low melting point metals for fuse elements introduces the problem of handling excessive mass of vaporized metal released on fusion.

For small values of current, an alloy of lead and tin containing 37 per cent lead and 63 per cent tin is used.

Either lead-tin alloy or copper is mostly used as ordinary fuse wire for low range current circuits.

For currents above 15 A, lead-tin alloy is not found suitable as the diameter of the fuse wire will be large and after fusing,

the vaporized metal released will be excessive. It has been found practically that silver is quite a suitable and satisfactory

material for fuse elements, because it is not subjected to oxidation and its oxide is unstable and there is no deterioration of the material.

The only drawback is that it is a costlier material. Despite being costlier, the present trend is to use silver as the material for fuses meant for reliable protection of costly and precious equipment.

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(iii) Rated Current The rated current of a fuse is the current it can carry indefinitely without fusing.

(iv) Minimum fusing current It is the minimum current (rms value) at which the fuse element will melt. The minimum fusing current depends upon various factors, such as the material

length, shape and area of cross-section of the fuse element, size and location of the terminals, the type of enclosure employed and the number of strands in the stranded fuse wire. \

The minimum fusing current for a stranded fuse wire will be less than the product of the minimum fusing current for one strand and the number of strands. For example, the typical values of fusing current for stranded fuse wires having 2, 3, 4 and 7 strands will be 1.66, 2.25, 2.75 and 4 times respectively of the fusing current for one strand.

An approximate value of the minimum fusing current for a round fuse wire is given by

(vi) Prospective current Figure 10.1 shows the first major loop of the fault current. The prospective current (shown dotted) is the current which would have flown in

the circuit if the fuse had been absent, i.e. it had been replaced by a link of negligible resistance.

It is measured in terms of the rms value of the a.c. (symmetrical) component of the fault current in the first major loop.

In Fig. 10.1, Ip is the peak value of the prospective current.

(vii) Cut-off current The current at which the fuse element melts is called the cut-off current (i in Fig.

10.1) and is measured as an instantaneous value. It is thus possible for a prospective current (rms value) to be numerically less than

the cut-off current. Since the fault current normally having a large first loop generates sufficient heat

energy, the fuse element melts well before the peak of the prospective current is reached.

Figure 10.1 Illustrates the cut-off action. On occurrence of a fault, the current starts increasing. It would have reached the

peak of the prospective current (Ip), if no fuse were there to protect. But the cut-off action of the fuse does not allow the current to reach Ip. Because of the sufficient heat energy generated by the fault current, the fuse

element is cut-off at Ic and an arc is initiated. After a brief arcing time, the current is interrupted. The cut-off current depends upon the rated current, the prospective current and the asymmetry of the fault current waveform.

The cut-off property has a great advantage that the fault current does not reach the prospective peak.

(viii) Pre-arcing time or melting time

This is the time taken from the instant of the commencement of the current which causes cut-off to the instant of cut-off and arc initiation. In Fig. 10.1, oa is the pre-arcing time (tpa).

(ix) Arcing time This is the time taken from the instant of cut-off (arc initiation) to the instant of

arc being extinguished or the current finally becoming zero. In Fig. 10.1, ab is the arcing time (ta).

(x) Total operating time It is the sum of the pre-arcing time and the arcing time. In Fig. 10.1, ob is the total

operating time, i.e. tpa +ta(xi) Rupturing capacity (or Breaking capacity)

It is the MVA rating of the fuse corresponding to the largest prospective current which the fuse is capable of breaking (rupturing) at the system voltage.

A fuse is never required to pass an actual current equivalent to its rupturing or breaking capacity.

When a particular rupturing capacity is specified, it is expected that the fuse will successfully operate in a circuit having prospective current equivalent to its rupturing capacity;

but the fuse melts much earlier due to cut-off action. Hence a fuse never allows to pass a current equivalent to its rupturing capacity.

10.3 Fuse Characteristics Figure 10.2 shows a typical time-current characteristic of a fuse, the current scale

being in multiples of the rated current (= 1) and the time scale being logarithm. In practice, this graph is usually given in terms of pre-arcing time and prospective

current.

By observation of the characteristic, it is clear that as the prospective current increases, the pre-arcing time decreases.

It is also clear that the characteristic becomes asymptotic and there is a minimum current below which the fuse does not operate.

This current is called the minimum fusing current. The operating time of the fuse for currents near the minimum fusing current is

long.

10.4 Types of FUSESFollowing are two main types of fuses:(i) Rewirable type

This type of fuse is rewirable, i.e. the blown-out fuse element can be replaced by a new one.

The fuse element can be either open or semi-enclosed. Therefore, this type of fuse is further of two types.

(a) Open type, and (b) Semi-enclosed type.

(ii) Totally enclosed or cartridge fuse

The fuse element of this type of fuse is enclosed in a totally enclosed container and

is provided with metal contacts on both sides.

this type of fuse is also of two types. (a)D-type cartridge fuse, and (b)Link type cartridge fuse or high rupturing capacity (HRC) cartridge fuse. Different types of fuses are described in the following sections.

10.4.1 Open Fuse An open fuse element is a thin piece of wire of tin, lead or copper inserted

directly in a circuit. It is the simplest and cheapest form of protection but due to fire hazard, and

unreliable operation caused by oxidation, it is seldom used any longer.

10.4.2 Semi-enclosed Rewirable Fuse This fuse is most commonly used in house wiring and small current circuits.

Figure 10.3 shows various components of this fuse. The fuse wire is fitted on a porcelain ‘carrier’ which is fitted in the porcelain base. Whenever the fuse wire blows off due to overload or short-circuit,

the fuse carrier can be pulled out, the new wire can be placed and service can be restored.

The fuse wire may be of lead, tinned copper or an alloy of tin-lead. The fuse wire should be replaced by a wire of correct size and to the specification,

otherwise it may prove dangerous with the possibility of the equipment burning out.

Because of its simplicity and low cost, this type of fuse is extensively used as a protective device on low voltage circuits.

But, since the fuse wire is exposed to atmosphere, it is affected by ambient temperature.

As the fuse is affected by ambient condition, its time-current characteristic is not uniform and gets deteriorated with time, resulting in unreliable operation and lack of discrimination.

Therefore, these fuses are mainly used for domestic and lighting loads. For all important and costly equipment operating at comparatively lower voltages

(up to 33 kV), cartridge fuses are used because they provide more reliable protection.

Rewirable fuses suffer from the following disadvantages. (i) Unreliable operation The operation of these fuses is unreliable because of the following factors. (a) Since the fuse wire is exposed to the atmosphere, it gets oxidised and deteriorated, resulting in a reduction of the wire section with the passage of time. This increases the resistance, causing operation of the fuse at lower currents (b) Local heating caused by loose connection, etc.(ii) Lack of discriminationProper discrimination cannot be ensured due to unreliable operation. (iii) Small time lag

Because of the small time lag, these fuses can blow with large transient currents encountered during the starting of motors and switching-on operation of transformers, capacitors and fluorescent lamps, etc. unless fuses of sufficiently high rating are used.

(iv) Risk of external flame and fire.

10.4.3 D-type Cartridge Fuse Figure 10.4 shows a typical D-type cartridge fuse

which comprises a fuse base, a screw type fuse cap, an adapter ring and a cartridge-like end contact.

The cartridge is pushed into the fuse cap. The fuse-link is clamped into the fuse base by the fuse carrier. The fuse cap is screwed on the fuse base. After complete screwing, the cartridge tip touches the conductor and circuit

between two terminals is completed through the fuse link. These fuses have none of the disadvantages of the rewirable fuse. Due to their reliable operation, coordination and discrimination to a reasonable

extent can be achieved with them.

10.4.4 High Rupturing Capacity (HRC) Cartridge Fuse

The HRC fuses cope with increasing rupturing capacity on the distribution system and overcome the serious disadvantages suffered by the semi-enclosed rewirable fuses.

In an HRC fuse, the fuse element surrounded by an inert arc quenching medium is completely enclosed in an outer body of ceramic material having good mechanical strength.

The unit in which the fuse element is enclosed is called ‘fuse link’. The fuse link is replaced when it blows off. In its simplest form (Fig. 10.5), an HRC fuse consists of a cylindrical body of

ceramic material usually steatite, pure silver (or bimetalic) element, pure quartz powder, brass end-caps and copper contact blades.

The fuse element is fitted inside the ceramic body and the space within the body surrounding the element is completely filled with pure powdered quartz.

The ends of the fuse element are connected to the metal end-caps which are screwed to the ceramic body by means of special forged screws.

End contacts (contact blades) are welded to the metal end-caps. The contact blades are bolted on the stationary contacts on the panel.

The fuse element is either pure silver or bimetalic in nature. Normally, the fuse element has two or more sections joined together by means of

a tin joint. The fuse element in the form of a long cylindrical wire is not used, because after melting, it will form a string of droplets and an arc will be struck

between each of the droplets.

Later on these droplets will also evaporate and a long arc will be struck. The purpose of the tin joint is to prevent the formation of a long arc. As the melting point of tin is much lower than that of silver, tin will melt first

under fault condition and the melting of tin will prevent silver from attaining a high temperature.

The shape of the fuse element depends upon the characteristic desired.When the fuse carries normal rated current, the heat energy generated is not sufficient to melt the fuse element But when a fault occurs, the fuse element melts before the fault current reaches its first peak. As the element melts, it vaporizes and disperses. During the arcing period, the chemical reaction between the metal vapour and quartz powder forms a high resistance sub stance which helps in quenching the arc. Thus the current is interrupted.

Figure 10.6 shows an HRC fuse developed by General Electric Company.

The cylindrical body made of ceramic material is closed by metal end-caps which carry the copper terminal tags.

The brass end-caps and the copper terminal-tags are electro-tinned. The fuse element made of pure silver is surrounded by silica as the arc quenching

medium. In order to increase the breaking capacity of the fuse, two or more widely

separated fuse elements are used in parallel. The breaking capacity is increased due to a greater surface area of the fuse

element in contact with the silica filler. An indicating device consisting of a fine resistance wire connected in parallel

with the fuse elements and led through a small quantity of mild explosive held in a pocket in the side of the fuse and covered by a label is also provided.

It indicates whether the fuse has blown out or not

When the fuse operates on occurrence of a fault, the fine wire is automatically fused resulting in the combustion of the explosive material.

The combustion of the explosive material chars the label, and thus indicates that the fuse has blown out.

Advantages of HRC fusesThe HRC cartridge fuses possess the following advantages.

(1) Capability of clearing high values of fault currents.

10.4.5 Expulsion Type High-Voltage Fuse

In an expulsion type fuse, the arc caused by operation of the fuse is extinguished by expulsion of gases produced by the arc.

An expulsion fuse contains a hollow tube made of synthetic-resin-bonded paper in which the fuse element is placed and the ends of the element are connected to suitable fittings at each end.

The length of the tube is generally longer than the conventional enclosed fuses. On the occurrence of a fault, the arc produced by fusing of the fuse element

causes decomposition of the inner coating of the tube resulting in the formation of gases

which assist arc extinction. Such fuses are developed for 11 kV, 250 MVA and are commonly used for protection of distribution transformers, overhead lines and cables terminating with overhead lines.

10.4.6 Drop-out Fuse

This is also an expulsion type high voltage fuse with one pole in closed position (Fig.10.7).

When the fuse element gets fused on the occurrence of a fault, the fuse element-carrying tube drops down under gravity about its lower hinged support, and hence the operation of the fuse can be readily spotted from a distance.

As the fuse element-carrying tube drops down on operation of the fuse, it provides additional isolation.

The blown fuse element is replaced by lifting the complete tube from the hinge and bringing it down by means of a special insulated rod.

After replacing the fuse element, the tube is replaced in the hinge and the device is closed in a way similar to closing of isolators.

These fuses are extensively used for protection of outdoor transformers, and the fuse- isolator combination is generally pole mounted.

10.5 Applications of HRC Fuses The applications of HRC fuses are enormous but some very important

10.6 Selection of FusesThe following points should be taken into account while selecting a fuse. (i) It should be able to withstand momentary over-current due to starting a motor and transient current surges due to switching on transformers, capaci tors and fluorescent lighting, etc. (ii) Its operation must be ensured when sustained overload or short-circuit occurs.

(iii) It should provide proper discrimination with the other protective devices. (iv) Its selection should depend upon the load circuit. For the purpose of protection, electric circuits are broadly classified as: steady load circuits and fluctuating load circuits.

(a) Steady load circuits: In these circuits, the load does not fluctuate much from its normal value (e.g. circuit of heating devices). If both overload and short-circuit protection of these circuits is required, fuses having a current- rating equal to or a little higher than the anticipated steady-load current are selected. However, if the fuse has to provide protection only against short-circuit, then a fuse of much greater rating than the normal load-current can be selected.

(b) Fluctuating load circuits: In these circuits there are wide fluctuations of load and peaks of a comparatively short duration occur during starting or switching on. Motors, transformers, capacitors and fluorescent lighting are examples of this category of load. The main criterion for selection of a fuse for fluctuating loads is that the fuse should not blow under transient overloads. Hence the time-current characteristics of the fuse must be above the transient current characteristic of the load, with a sufficient margin.

10.6.1 Fuses for Motors The fuses for protection of motors are selected keeping in view the starting current, its duration and frequency. When the starting-current of the motor is known, it is assumed that the starting-current surge will persist for about 20 seconds and a suitable cartridge fuse which will withstand the starting current for this time is selected.When the starting current is not known, it is assumed to be about five times the full-load current.

10.6.2 Fuses for Capacitors Protection of capacitors is a particularly difficult task due to the presence of transient current surges during switching operations. TherefOre, cartridge fuse links having rated current about 50% greater than the rated currents of the capacitors are selected.

10.6.3 Fuses for Transformers and Fluorescent Lighting The selected fuse should be capable of withstanding the transient current surges during switching on. Generally fuses having rated current about 25 to 50% greater than the normal full load current of the apparatus are selected.

10.7 Discrimination When two or more protective devices, e.g. two or more fuses, a fuse and a circuit breaker, etc. are used for the protection of the same circuit, the term discrimination concerns the correct operation of the correct device on occurrence of a fault. For proper discrimination, there should be coordination between the protective devices. In order to obtain proper discrimination be tween two adjacent fuses carrying the same current, the pre-arcing time of the major fuse (nearer the source) must exceed the total operating time of the minor fuse (far from the source).

10.7.1 Discrimination between Two Fuses In order to understand this type of discrimination, consider a radial circuit as