Domestic Wiring Chapter 6

 FUSES AND HRC FUSES

A fuse is a small piece ofwire connected in between two terminals mounted on insulated base and is connected in series with the circuit. 

The fuse is perhaps the cheapest and simplest form of protection and is used for protecting low voltage equipments against overloads and/or short circuits.

The fuse is expected to carry the normal working current safely without overheating and during overloads or short circuits it gets heated up to melting point rapidly. 

The materials used normally are tin, lead, silver, zinc, aluminium, copper etc. For small values of currents an alloy oflead and
tin in the ratio of 37 per cent and 63 per cent respectively is used. 

For currents more than 15 amperes this alloy i s not used a s the diameter o fthe wire will b e large and after fusing, the metal released will be excessive. 

Silver is found to be quite satisfactory as a fuse material because it is not subjected to oxidation and its oxide is unstable. 

The only drawback is that it is a relatively costlier material. 

Therefore, for low range current circuits either lead-tin alloy or copper is used.

Fuse

Fuse is a device used in circuit for protecting electrical equipments against overloads and/or short circuits.

Fuse element or fuse wire. It is that part ofthe fuse device which melts when an excessive current flows in the circuit and thus isolates the faulty device from the supply circuit.

Minimum fusing current

Minimum fusing current is a value corresponding to operation in an arbitrary time obtained under prescribed test conditions. 

Alternatively, it is a value of current corresponding to a chosen value of time indicated on a time/current curve which is itself obtained from prescribed testing condition.

Fuse rating

It is that value of current which when flows through the element, does not

melt it. This value of the fuse is less than the minimum fusing current.

Fusing factor

This is the ratio of minimum fusing current to the current rating of fusing

element, i.e.

Fuusing factor = Minimum fusing current/ fuse rating 

The fusing factor is always greater than unity.

Prospective current

 It is defined as the rms value of current which would flow in a circuit immediately following the fuse when a short circuit occurs assuming that the fuse has been replaced by a link of negligible resistance.

Melting time or pre-arcing time

The time taken from the instant the current that causes a break in the fuse wire starts flowing, to the instant the arc is initiated.

Arcing time

The time taken from the instant of arc initiation to the instant of arc being

Extinguished Total operating time

It is the sum of the pre-arcing and the arcing time.

The most commonly used fuse in 'house wiring' and small current circuits is the semienclosed or rewirable fuse. 

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 the supply can be restored. 

This looks simple and is really very simple only ifthe wire is replaced by the correct size. 

For a layman this may prove dangerous ifhe replaces the fuse wire by some copper wire not to the specification

and there is a possibility of burning the equipment. 

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

The time-current characteristics of such fuses get deteriorated with time and hence are not reliable for discrimination purposes. 

These fuses are, therefore, mainly used for domestic and lighting loads. For all important and costly equipments

operating at low voltages (upto 33 kV) another class of fuse is used which is known as cartridge fuse. 

HRC (High Rupturing Capacity) 

Cartridge fuse link was first introduced, it was designed to satisfy two important requirements. 

The first was to cope up with the increasing rupturing capacity on the supply system and the second was nondeterioration to overcome the serious disadvantages suffered by the types of semi-enclosed fuses.

Construction of HRC fuse

The HRC fuse consists of a ceramic body usually of steatite, pure silver element, clean silica quartz, asbestos washers, porcelain plugs , brass endcaps and copper tags Fig.

The brass end-caps and copper tags are electro-tinned. The metal end-caps are screwed to the ceramic body by means of special forged screws to withstand the pressure developed under short 

circuit condition. 

The contacts are welded to the end-caps. The assembly also includes solder of various types, cement and indicator devices. 

Deterioration of the fuse must involve a change in one or more of these meterials or a change in their structure.

Normally the fuse element has two

or more sections joined by means of a tin joint. The fuse wire is not in the form of a long cylindrical wire as after it melts, it will form a string of droplets and will result into an arc between the

droplets. 

Afterwards these droplets will also evaporate and a long arc will be struck. The purpose of the tin j oint is to limit the temperature of fuse under small overload conditions. 

The melting point of silver is 960°C while that ofpure tin is 230°C. As the circuit is overloaded the melting of

tin prevents the silver element from attaining high temperature. 

The shape ofthe fuse element depends upon the time-current characteristic required.

Fuse operation

When an HRC fuse operates, the element absorbs energy from the circuit and heats until it melts. 

The heat produced during operation is given by J i2R dt where R is the instantaneous resistance and i the instantaneous current during the operating time. 

The fuse element melts before the fault current reaches its peak value. As the element melts , it vaporizes and disperses.

This action is then followed by a period of arcing during which chemical reaction between the

silver vapour and the quartz powder takes place, which further results into building up a high resistance and reduces the current to zero. Thus the arc is quenched. 

Generally, the filling powder used is quartz-sand as it can absorb heat at a very high rate and it does not evolve appreciable amount of gas.

Cut-off Current

When an HRC fuse interrupts a heavy fault it exhibits an ability to limit the short circuit current. 

This ability is referred to as a 'cut off as shown in and has the effect of reducing the magnetic and thermal stresses both in the system and within the fuse itself under fault conditions. 

Cut-off is in fact one of the main reasons why HRC fuse is so successful as a protective device and it is at times preferred over the circuit breaker of low ratings . Due to this property of the HRC fuse the operating time is as low as I/4th of a cycle. The maximum to which the fault current reaches before the fuse melts is called the cut-off current.

Properties of HRC Fuses

The careful designs of HRC fuses have overcome the disadvantages of the conventional rewritable fuses and their properties are described as follows :

(i) Fast operation

The HRC fuse interrupts the short circuit current long before its maximum value is attained which is not true in case of CBs. This property of the HRC fuse reduces

both thermal and magnetic stresses on the equipment to be protected and the short circuit fault is interrupted well within the first quarter of a cycle.

(ii) Rupturing capacity

The rupturing capacity of a fuse is expressed in terms of MVA and is equal to the product of service voltage and the rms value of the prospective current it can

handle. Since the fuse melts much before the current reaches prospective value due to its cut-off property, it is clear that a fuse is never called upon to carry a current equal to its rupturing capacity. It is to be noted here that the rupturing capacity of a breaker is different from the

rupturing capacity of a fuse.

(iii) Non-deterioration

This means that all the characteristics ofthe fuse are maintained throughout its life. As the fuses are called upon to function only once in a while, it is most essential that they should preserve their characteristics throughout their useful life. 

This also implies that they should not operate inadvertently when carrying normal load currents as so often happens with a rewirable fuse which may fail due to oxidation and reduction of crosssectional area. 

This property of HRC fuses is very important and is achieved by the hermetic sealing of the silver element within the fuse body with the help of special cementing and the

soldering ofthe end caps. It has been found that HRC fuses maintain non-deterioration property unimpaired even after approximately 20 years oftheir manufacture.

(iv) Low-temperature operation

This is required to eliminate the deterioration ofthe fuses and to prevent overheating of associated contacts. This is achieved by employing fabricated elements of pure silver which are specially designed to give low temperature rise when carrying

their full-rated current.

(v) Accurate discrimination

By this characteristic is meant that an HRC fuse on a distribution system will isolate the faulty section from the healthy section whenever a fault takes place. 

In case ofan HRC fuse it is found that the time ofoperation is inversely proportional to the prospective short circuit current over a much wider range offault condition and, within practical limits, while the values of prospective short circuit current increase, the time of operation will continue to decrease without reaching a definite minimum. 

This means that a fuse oflow current

rating will blow before a fuse of a higher rating, no matter how heavy the fault. It is, therefore, desirable while designing the installation from the view point of discrimination to use fuses of the

same design and characteristics throughout, which will ensure that time-current characteristics of each succeeding current size will not cross and the characteristics will be parallel to one another upto the maximum values offault current.

Arc voltage within safe limits

Whenever an inductive circuit is interrupted, there is likelihood of large voltages induced. The magnitude of such voltages depend upon the magnitude of

the short circuit to be interrupted and the circuit constants. 

A careful design of the HRC fuse controls these over voltages and keeps them within safe limits.

Low cost

It is known that because of the cut-off characteristics of the HRC fuse, for the

same rupturing capacity the actual current to be interrupted by an HRC fuse is much less as compared to any other interrupting device and hence it is less expensive as compared to other interrupting devices. 

It is, therefore, usual to employ a circuit breaker of low rupturing capacity backed up by an HRC fuse where circuit breakers are necessary for other reasons. A combination

of these two circuit interrupting devices works as follows. 

Whenever there is an overload the CB

trips whereas for short circuits the HRC fuse operates.

Applications of HRC Fuses

The applications ofHRC fuses are enormous but a few very important are: 

(i) protection of cables, 

(ii) protection of bus bars, 

(iii) protection of industrial distribution system, 

(iv) contactor gear for motor control, 

(v) earth faults-both of low and high magnitude

(vi) semi-conductor rectifiers and 

(vii) aircraft.

It is to be noted that the HRC fuses cover a very wide range of applications. This involves the principles of fuse design in varying degrees. 

For special application, the parameters of the fuse are defined to close limits. The design of HRC fuses for the same rupturing capacities for protecting an SCR are different from the one for protecting cables.

An HRC fuse rated for 150 amps continuous rating and 200 kA rupturing capacity at 400

V used for protecting a semi-conductor device weighs about 30 gm whereas an industrial application HRC fuse rated for 100 amps and 250 kA rupturing capacity weighs about 200 gms. 

The HRC fuses have been used for protecting aircraft equipments and offer many advantages not available by alternative means.

Within wide limits HRC fuses are not affected by frequency. For practical purposes, a fuse tested and rated at 50 Hz is satisfactory for 60 Hz duty and vice versa. As frequency tends towards d.c., the interrupting capability ofthe fuse at the lower and medium overcurrents may be

less, because it is in these zones that d.c. duty is the more onerous. 

A fuse which has been tested

and rated at a given frequency will almost invariably safely interrupt short-circuit faults of higher frequency.