An introduction to FUSES

This ppt describes about,
introduction of fuses, construction, Important terms, advantages and disadvantages, desirable characteristics of fuse element, Current time characteristics, Fuse types - Low voltages fuses and High voltage fuses, Semi enclosed rewirable fuse, HRC cartridge fuses - parts, operation, pros and cons, High voltage fuses and its types, selection of fuses, discrimination

1. FUSES Done by, Nithyapriya-15E134 Prashanna – 15E136 PraveenKumar – 15E137 Preethi – 15E138 Priyaa – 15E139 Rajesh – 15E141 Rakshit Rajan – 15E142 Ranjith – 15E143

2. INTRODUCTION Fuse  is an electrical safety device that operates to provide overcurrent protection of an electrical circuit. Its essential component is a metal wire or strip that melts when too much current flows through it, thereby interrupting the current. It is a sacrificial device; once a fuse has operated it is an open circuit, and it must be replaced or rewired, depending on type. There are thousands of different fuse designs which have specific current and voltage ratings, breaking capacity and response times, depending on the application.

3. The time and current operating characteristics of fuses are chosen to provide adequate protection without needless interruption. Wiring regulations usually define a maximum fuse current rating for particular circuits.  Short circuits , overloading, mismatched loads, or device failure are the prime reasons for fuse operation. A fuse is an automatic means of removing power from a faulty system; often abbreviated to ADS (Automatic Disconnection of Supply).  Circuit breakers can be used as an alternative design solution to fuses, but have significantly different characteristics.

4. CONSTRUCTION A fuse consists of a metal strip or wire fuse element, of small cross-section compared to the circuit conductors, mounted between a pair of electrical terminals, and enclosed by a non-combustible housing. The fuse is arranged in series to carry all the current passing through the protected circuit. The resistance of the element generates heat due to the current flow. The size and construction of the element is determined so that the heat produced for a normal current does not cause the element to attain a high temperature. If too high a current flows, the element rises to a higher temperature and either directly melts, or else melts a soldered joint within the fuse, opening the circuit.

5. The fuse element is made of zinc, copper, silver, aluminium, or alloys to provide stable and predictable characteristics. The fuse elements may be shaped to increase heating effect. In large fuses, current may be divided between multiple strips of metal. Fuse elements may be supported by steel or nichrome wires, so that no strain is placed on the element, but a spring may be included to increase the speed of parting of the element fragments. The fuse element may be surrounded by air, or by materials intended to speed the quenching of the arc. Silica sand or non-conducting liquids may be used.

6. ADVANTAGES Fuse is cheapest type of protection in an electrical circuit Fuse needs zero maintenance Operation of fuse is simple and no complexity is involved Fuse has the ability to interrupt enormous short circuit current without producing noise, flame, gas or smoke The operation time of fuse can be made much smaller than operation of circuit breaker. It is the primary protection device against short circuits It affords current limiting effect under short-circuit conditions Fuse inverse time current characteristic has the ability to use for over-load protection

7. DISADVANTAGES During short circuit or overload once fuse blows off replacing of fuse takes time. During this period the circuit lost power When fuses are connected in series it is difficult to discriminate the fuse unless the fuse has significant size difference

8. IMPORTANT TERMS 1) Current rating of fuse element The current which the fuse element can normally carry without overheating or melting. Depends upon the temperature rise of the contacts of the fuse holder, fuse material and the surroundings of the fuse. 2) Fusing current The minimum current at which the fuse element melts and thus disconnects the circuit protected by it. Its value will be more than the current rating of the fuse element. For a round wire, the approximate relationship between fusing current I and diameter d of the wire is I = k d (3/2) where k is a constant, called the fuse constant. Its value depends upon the metal of which the fuse element is made

9. The value of k for different materials as given in the table below : S.NO MATERIAL Value of constant K in d in cm 1 Tin 405.5 2 Lead 340.5 3 Iron 777 4 Aluminium 1870 5 Copper 2530 S.W.G Diameter in cm Rated current in A Approximate fusing current in A 20 0.09 34 70 25 0.05 15 30 30 0.031 8.5 13 35 0.021 5 8 40 0.012 1.5 3

10. The fusing current depends upon the various factors such as : (a)material of fuse element (b)length (c)size and location of terminals (d)type of enclosure used 3) Fusing factor It is the ratio of minimum fusing current to the current rating of the fuse Its value is always more than one For a semi-enclosed or rewirable fuse,the fusing factor is usually 2 Lower values of fusing factor can be employed for enclosed type cartridge fuses using silver or bimetallic elements

11. 4) Prospective Current The fault current would normally have a very large first loop, but it actually generates sufficient energy to melt the fusable element well before the peak of this first loop is reached. Prospective current can be defined as the r.m.s value of the first loop of the fault current obtained if the fuse is replaced by an ordinary conductor of negligible resistance. 5) Cut-off current The maximum value of fault current actually reached before the fuse melts On the occurrence of a fault, the fault current has a very large first loop due to a fair degree of asymmetry The heat generated is sufficient to melt the fuse element well before the peak of first loop is reached

12. The cut off value depends upon : (a) current rating of fuse (b) value of prospective current (c) asymmetry of short-circuit current It may be mentioned here that outstanding feature of fuse action is the breaking of circuit before the fault current reaches its first peak. Therefore, the circuits protected by fuses can be designed to withstand maximum current equal to the cut-off value 6) Pre-arcing time The time between the commencement of fault and the instant when cut off occurs The pre-arcing time is generally small : a typical value being 0·001second

13. 7) Arcing time The time between the end of pre-arcing time and the instant when the arc is extinguished 8) Total operating time The sum of pre-arcing and arcing times It may be noted that operating time of a fuse is generally quite low (say 0·002 sec.) as compared to a circuit breaker (say 0·2 sec or so) A fuse in series with a circuit breaker of low-breaking capacity is a useful and economical arrangement to provide adequate short-circuit protection 9) Breaking capacity It is the r.m.s. value of a.c . component of maximum prospective current that a fuse can deal with at rated service voltage.

14. DESIRABLE CHARACTERISTICS OF FUSE ELEMENT The function of a fuse is to carry the normal current without overheating but when the current exceeds its normal value, it rapidly heats up to melting point and disconnects the circuit protected by it. The fuse element should have the following desirable characteristics : ( i ) low melting point e.g., tin, lead. (ii) high conductivity e.g., silver, copper. (iii)free from deterioration due to oxidation e.g., silver. (iv)low cost e.g., lead, tin, copper.

15. Current- Time characteristic of fuse

16. FUSE ELEMENT MATERIALS The most commonly used materials for fuse element are lead, tin, copper, zinc and silver For small currents upto 10 A, tin or an alloy of lead and tin (lead 37%, tin 63%) is used for making the fuse element For larger currents, copper or silver is employed. It is a usual practice to tin the copper to protect it from oxidation Zinc (in strip form only) is good if a fuse with considerable time-lag is required i.e., one which does not melt very quickly with a small overload The present trend is to use silver despite its high cost

17. Silver is preferred due to the following reasons: ( i ) It is comparatively free from oxidation. (ii) It does not deteriorate when used in dry air. (iii) The coefficient of expansion of silver is so small that no critical fatigue occurs. Therefore, the fuse element can carry the rated current continuously for a long time. (iv) The conductivity of silver is very high. (v) Low specific heat (vi) Quick interruption of short circuit current

18. TYPES OF FUSES In general, fuses may be classified into : ( i ) Low voltages fuses (ii) High voltage fuses It is a usual practice to provide isolating switches in series with fuses where it is necessary to permit fuses to be replaced or rewired with safety If such means of isolation are not available, the fuses must be so shielded as to protect the user against accidental contact with the live metal when the fuse carrier is being inserted or removed. ( i ) Low voltage fuses: Low voltage fuses can be subdivided into two classes namely ( i ) Semi-enclosed rewireable fuse (ii) High rupturing capacity (H.R.C.) cartridge fuse.

19. Semi-enclosed rewireable fuse Rewireable fuse (also known as kit- kat type) is used where low values of fault current are to be interrupted Consists of ( i ) a base and (ii) a fuse carrier The base is of porcelain and carries the fixed contacts to which the incoming and outgoing phase wires are connected The fuse carrier is also of porcelain and holds the fuse element (tinned copper wire) between its terminals The fuse carrier can be inserted in or taken out of the base when desired. When a fault occurs, the fuse element is blown out and the circuit is interrupted The fuse carrier is taken out and the blown out fuse element is replaced by the new one. The fuse carrier is then reinserted in the base to restore the supply.

20. Disadvantages ( i ) There is a possibility of renewal by the fuse wire of wrong size or by improper material (ii) This type of fuse has a low-breaking capacity and hence cannot be used in circuits of high fault level. (iii) The protective capacity of such a fuse is uncertain as it is affected by the ambient conditions. (v) Accurate calibration of the fuse wire is not possible because fusing current very much depends upon the length of the fuse element. (vi)Risk of external flame and fire

21. (vii) Unreliable operation: a. Reduction in wire section due to exposure to atmosphere b. Local heating caused by loose connection (viii) Lack of discrimination (ix) Small time lag Semi-enclosed rewireable fuses are made upto 500 A rated current, but their breaking capacity is low e.g., on 400 V service, the breaking capacity is about 4000 A. Therefore, the use of this type of fuses is limited to domestic and lighting loads.

22. High-Rupturing capacity (H.R.C.) cartridge fuse

23. Introduction Objective: To overcome the, low and uncertain breaking capacity of semi-enclosed rewireable fuses. They are extensively used on low-voltage distribution system against over-load and shortcircuit conditions.

24. Parts of a H.R.C. fuse It consists of a heat resisting ceramic body having metal end-caps and a silver current-carrying element. The space within the body surrounding the element is completely packed with a filling powder. The filling material may be silica, quartz or marble dust and acts as an arc quenching and cooling medium.

25. Operation Under normal load conditions, the fuse element is at a temperature below its melting point. Therefore, it carries the normal current without overheating. When a fault occurs, the current increases and the fuse element melts before the fault current reaches its first peak. The heat produced in the process vapourises the melted silver element. The chemical reaction between the silver vapour and the filling powder results in the formation of a high resistance substance which helps in quenching the arc.

26. Advantages & Disadvantages ( i ) They are capable of clearing high as well as low fault currents. ( ii ) They provide reliable discrimination. ( iii ) They require no maintenance. ( iv ) They are cheaper than other circuit interrupting devices of equal breaking capacity. ( v ) Inverse time-current characteristics ( vi ) They do not deteriorate with age. ( vii ) They permit consistent performance. Disadvantages ( i ) Requires replacement after each operation. ( ii ) Overheating of adjacent contacts.

27. H.R.C. fuse with tripping device. When the fuse blows out under fault conditions, the tripping device causes the circuit breaker to operate. At one end is a plunger which under fault conditions hits the tripping mechanism of the circuit breaker and causes it to operate. The plunger is electrically connected through a fusible link, chemical charge and a tungsten wire to the other end of the cap as shown.

28. When a fault occurs, the silver fuse elements are the first to be blown out and then current is transferred to the tungsten wire. The weak link in series with the tungsten wire gets fused and causes the chemical charge to be detonated. This forces the plunger outward to operate the circuit breaker. The travel of the plunger is so set that it is not ejected from the fuse body under fault conditions.

29. Advantages ( i ) In case of a single phase fault on a three-phase system, the plunger operates the tripping mechanism of circuit breaker to open all the three phases and thus prevents “single phasing”. ( ii ) The effects of full short circuit current need not be considered in the choice of circuit breaker. This permits the use of a relatively inexpensive circuit breaker. ( iii ) The fuse-tripped circuit breaker is generally capable of dealing with fairly small fault currents itself. This avoids the necessity for replacing the fuse except after highest currents for which it is intended.

30. High voltage fuses High Voltage Fuses are generally used in power systems and are typically rated for voltages above 1500V and up to 138000 V. High Voltage (HV) Fuses are used to protect transformers, either small power transformers or instrument transformers, where circuit breakers might not guarantee the protection. The fuse element in High Voltage (HV) Fuses are made up of either Silver or Copper (sometimes even Tin is used), in order to provide reliable and stable performance.

31. Liquid type fuses Consists of a glass tube filled with carbon tetrachloride solution and sealed at both ends with brass caps. The fuse wire is sealed at one end of the tube. Other end of the wire is held by a strong phosphor bronze spiral spring fixed at the other end of the glass tube. When the current exceeds the prescribed limit, the fuse wire is blown out. As the fuse melts, the spring retracts part of it through a baffle (or liquid director) and draws it well into the liquid. The small quantity of gas generated at the point of fusion forces some part of liquid into the passage through baffle and there it effectively extinguishes the arc.

32. Expulsion fuses The Fuse Link Chamber is filled with Boric Acid. Under a fault condition, arc heat decomposes the boric acid into water vapor . The water vapor blast de-ionizes the arc path preventing arc re-ignition after a natural current zero. Boric acid expulsion power fuses are divided into two types, refillable and replaceable.

33. Contd … Refillable fuses - Constructed such that the consumable refill unit can be removed and replaced after a fuse operation. Because the fuse holder and spring and shunt assembly components are reused, they can be constructed with a heavy duty design that also allows the unit to have a high interrupting capacity. It is easy to change fuse current rating by simply changing the refill unit. Replaceable fuses- Have a lower initial installed cost by providing a more cost effective construction. Replaceable fuses generally offer faster reconnection, but with higher replacement cost and lower interrupting ratings.

34. Cartridge type Some designs employ fuse elements wound in the form of a helix to avoid corona effects at higher voltages. On some designs, there are two fuse elements in parallel ; one of low resistance (silver wire) and the other of high resistance (tungsten wire). High voltage cartridge fuses are used upto 33 kV with breaking capacity of about 8700 A at that voltage. Rating of the order of 200 A at 6·6 kV and 11 kV and 50 A at 33 kV are also available.

35. Metal clad fuses Metal clad oil-immersed fuses have been developed with the object of providing a substitute for the oil circuit breaker. Such fuses can be used for very high voltage circuits and operate most satisfactorily under short-circuit conditions approaching their rated capacity.

36. Comparison between fuse and circuit breaker

37. DROP OUT FUSES It is also an expulsion type high voltage fuse with one pole in closed position. When the fuse element gets fused, the fuse chamber drops down under gravity about its lower hinged. It also provides additional isolation. The operation of fuse can be spotted from the distance. These fuses are used for protection of outdoor transformers.

38. When the fuse carries normal current , the fuse element does not melt. But, When fault occurs the fuse element melts,it vaporizes and disperses. The chemical reaction between metal vapour and quartz powder forms a high resistance substance which helps in quenching the arc.

39. HRC FUSE WITH INDICATOR

40. APPLICATIONS Protection of low voltage distribution systems against overloads and short circuits. Back up protection to circuit breaker. Used in meter board in residential applications. It protects devices like transformer , Generator and Motor. Protection of bus bars. Protection of cables.

41. Selection of Fuses

42. Points to be taken into account for selection Withstand momentary over-current and transient current surges Ensured operation on the course of short circuit and overload Provision of proper discrimination with other protective devices Selection should be based on the load circuits which are of two types

43. Steady load circuits Load does not fluctuate much from it’s normal value Current rating of the fuse will be equal to little higher than the anticipated steady load current A more higher rating than the normal load current selected in the case of short circuit protection

44. Fluctuating load circuits Wide fluctuations of loads and peaks in a short duration while starting and switching Main criterion is that the fuse should not blow under transient overloads The time-current characteristics of the fuse must be above the transient characteristics of the load with a sufficient margin

45. Fuses for motors Starting current , its duration and frequency must be monitored Knowing the starting current of the motor , the surge current is assumed to persist for about 20 seconds and suitable cartridge fuse selected When the starting current is not known , it is assumed to be five times that of the full load current

46. Fuse for capacitors Protection is difficult due to arise of transient surge currents during switching operation Therefore , cartridge fuse links having rated current 50% greater than the rated currents of the capacitors are selected

47. Fuses of 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 25-50% greater than the normal full load current of the apparatus are selected

48. DISCRIMINATION Discrimination (or selectivity) is the selection of protective devices so that the device nearest to a fault will operate rather than any upstream device. The purpose is to ensure that the fault is isolated and supply is maintained to other parts of the installation without disruption. In order to obtain proper discrimination between two adjacent fuses carrying the same current, the pre- arching time of the major fuse must exceed the total operating time of the minor fuse.

49. Forms of discrimination There are three principal aspects to discrimination: Overload discrimination relates to the magnitude of the fault current - for this the upstream device must always have a higher continuous current rating and a higher instantaneous pick-up value than the next device downstream. Short-circuit discrimination: This occurs where the earth fault loop impedance is low, for example if the installation is close to the local transformer substation. Time discrimination relates to the time during which the circuit-breaker 'sees' the fault current. This requires the use of adjustable time delay settings in upstream devices.

50. DISCRIMINATION BETWEEN TWO FUSES If the fuses used do not have discriminative character, F1 will blown out and thus supply to the whole line will be interrupted. When a fault occurs beyond F2, only F2 should operate and F1 should remain unaffected. This is called proper discrimination.

51. DISCRIMINATION BETWEEN FUSES AND OVER CURRENT PROTECTIVE DEVICES

52. In motor circuits, fuses provide short circuit protection and the overcurrent relay provides overcurrent protection. Overcurrent relay operates for current within the breaking capacity of the circuit breaker and the fuses operate for faults of larger current. The characteristics of overcurrent protective device should be below the characteristics of the fuse. So, the fuse provides back up protection to the motor.

53. Current Carrying Capacity of Fuse Element When the fuse element attains steady temperature,

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