A fuse is a part of the circuit which consists of a conductor which melts easily and breaks the connection when the current exceeds the predetermined value. An electrical fuse is the weakest part of an electrical circuit that breaks when more than predetermined current flows through it. Fuses are used for overload and short circuit protection in high voltage up to 66KV and low voltage up to 400v. The principle of fuse is based on the heating effect. In the normal operating condition, when the current flows through the circuit, the heat developed in the fuse element carrying current is readily dissipated in the surrounding air, and the fuse element remains at a temperature below its melting point.
But when the faults occur in the circuit, the current exceeds the limiting value, the heat generated due to excessive current cannot be dissipated fast and the fuse element gets heated, melts, and breaks the circuit and protects the circuit from damage.
Components of Fuse
The fuse consists of the fusible element in the form of a metal conductor of a small cross-section area and the case is there to hold the fusible element. The part which melts and opens the circuit is the fuse element.
The function of fuse wire is to carry the normal current without excessive heating but more than normal current when passing through fuse wire, it rapidly heats up and melts. The materials used for fuse wires are mainly tin, lead, copper, aluminum, etc.
Rated fuse current
It is the rms value of current that the fuse wire can carry continuously without deterioration and with temperature rise within specified limits.
• It is the minimum value of current at which the fuse wire melts. The value of the Fusing current will be more than the Rated fuse current. For a round wire, the approx value of fusing current is given by
I = kd^3/2
Where, k = fuse constant; d = diameter of wire
The value of fuse constant “k” depends upon various factors such as; the type of material used, cross-section area i.e. round or rectangular, length of wire, enclosure type, the diameter of the wire, etc.
The ratio of the minimum fusing current and the current rating of the fuse element is known as the fusing factor.
Fusing factor =Minimum fusing current/Current rating of fuse wire
In the case of a small fusing factor, the fuse element can be deteriorated due to overheating at the rated current. For a copper rewirable fuse, the fusing factor is equal to 1.9 to 2. For the cartridge fuses, it is equal to 1.45.
Before melting, the fuse element has to carry the short circuit current through it. The prospective current is defined as the value of current which would flow through the fuse immediately after a short circuit occurs in the network.
The melting of the fuse element before the current reaches the prospective peak is called the cut-off. The value of the current at which the cut-off occurs is called the cut-off value.
Pre-arcing Time of Fuse
This is the time taken by a fuse wire to be broken by melting. It is counted from the instant; the over current starts to flow through the fuse, to the instant when the fuse wire is just broken by melting.
Arcing Time of Fuse
After the breaking of the fuse wire there will be an arcing between both melted tips of the wire which will be extinguished at the current zero. The time accounted from the instant of the arc initiated to the instant of the arc being extinguished is known as the arcing time of the fuse.
Operating Time of Fuse
- Operating time of fuse = pre-arcing time + arcing time of fuse.
- The operating time of the fuse is the time gap between the instant when the overrated current just starts to flow through the fuse and the instant when the arc in the fuse is finally extinguished. Operating time of fuse = pre-arcing time + arcing time of fuse.
Advantages and Disadvantages of Fuses
- Cheapest form of protection available.
- Needs no maintenance.
- Interrupts enormous short-circuit currents without noise, flame, gas, or smoke.
- Less time of operation than in circuit breaker.
- Its inverse time-current characteristics enable its use for overload protection.
- Considerable time is lost in rewiring or replacing a fuse after operation.
Components of Fuse Unit
A fuse unit consists of;
– Metal fuse element or link
– Set of contacts between which the fuse element is fixed
Categories of Fuses
There are two categories of fuses in general
– Low voltage fuse
– High voltage fuse
Low voltage fuse
Low voltage fuse can be further subdivided into two classes:
– Rewirable or semi-enclosed type
– Cartridge or enclosed type
Rewirable or semi-enclosed type fuses
It consists of a porcelain base carrying the fixed contacts and a porcelain fuse carrier holding the fuse element.
- The fuse carrier can be taken out or inserted in the base without risk, even without opening the main switch.
- The fuse wire can be replaced in the case of damage. It means the fuse base and the fuse holder need not change in the case of the fuse wire being burnt.
- The fuse wire may be of lead, tinned copper, aluminum, or an alloy of tin-lead.
- Generally the fusing current is twice the rated current.
- When two or more fuse wires are used, the wires should be kept apart and a derating factor of 0.7 to 0.8 should be employed to arrive at the total fuse rating.
The specifications are covered by IS: 2086-1963. Standard ratings are 6, 16, 32, 63, and 100 A. Fuse wire of lower rating can be used in higher rating fuse base and holder but not vice versa.
– easy removal or replacement of fuse element without any danger
– negligible replacement cost
- the possibility of replacement with the wrong size
- due to deterioration, the fuse wire may operate at the current below-rated capacity
- As the fusing current is about 2 times of rated current the apparatus may get damaged during fault conditions before melting the fuse wire
- Low accuracy
- Accurate calibration of the fuse wire is impossible as a longer fuse operates earlier than one of a shorter length.
- Single phasing may occur in the operation of the motor when fuses are used in the motor circuit
Lack of discrimination
- Due to unreliable operation, discrimination cannot be always ensured.
Small time lag
- Can blow with large transient currents which are encountered during the starting of motors and switching on the operation of transformers, capacitors, etc.)
Low rupturing capacity
- Rewirable fuses have low rupturing capacity. A fuse of 16 A rating has a breaking capacity of 2 kA and those up to 200 A have 4 kA
Slow speed of operation
- Arcing time is high because no special means are there for arc extinction.
Risk of flame and fire
- The spark may be converted into a fire in case of delay in arc extinction
Cartridge or enclosed-type fuses
- The fuse element is enclosed in an enclosed container and is provided with metal contacts on both sides. The fuses are further classified as
- D type
- Link type
D-type cartridge fuse
- This is a fuse comprising a fuse base, adaptor ring, cartridge, and fuse cap.
- The cartridge is pushed in the fuse cap and the cap is screwed on the fuse base.
- The standard ratings are 6, 16, 32, and 63 A.
- The breaking or rupturing capacity is of the order of 4 kA for 2 and 4 A fuses and 16 kA for 6 to 63 A fuses.
- These types of fuses do not have the drawbacks of the rewirable fuses.
- These are reliable and discrimination and coordination are achieved to a reasonable extent.
Link-type Cartridge fuses or HRC fuses
- In that type of fuse, the fuse wire or element can carry a short circuit heavy current for a known period.
- During this time if the fault is removed, then it does not blow off otherwise it blows off or melts.
- The enclosure of the HRC fuse is either glass or some other chemical compound. This enclosure is fully airtight to avoid the effect of the atmosphere on the fuse materials.
- The ceramic enclosure has a metal end cap at both heads, to which fusible wire is joined.
- The space within the enclosure, surrounding the fuse wire or fuse element is completely packed with a filling powder.
- This type of fuse is reliable and has inverse time characteristics, which means if the fault current is high then the rupture time is less and if the fault current is not so high then the rupture time is long.
- These are available in different ratings up to 1250 A.
Operation of HRC fuses
- When the overrated current flows through the fuse element of a high rupturing capacity fuse the element is melted and vaporized.
- The filling powder is of such a quantity that the chemical reaction between the silver vapor and the filling powder forms high electrical resistance substances which very much help in quenching the arc.
Knife Blade Type HRC Fuse:– This kind is easily replaceable in the circuit– For this purpose, especially insulated fuse pullers are used.
Bolted Type HRC Link Fuse:– In this type, the conducting plates are bolted to the base of the fuse.
Time current characteristics
Time current characteristics
- In the above figure, the curve for three Fuses of rating 60 A, 100 A, and 200 A is given. Let’s select Fuse of rating 60 A for the sake of understanding.
- See, if the current flowing through the Fuse element is around 300 A then the Fuse element will melt in .03 sec i.e. 30 ms while if the current is around 200 A then it will take 70 ms to melt.
- Thus we see that the Fuse characteristic is Inverse Time.
- Higher the current, the lower will be the time to melt.
- Laboratory tests are conducted on each fuse design to determine the amount of energy required to melt the fusing element. This energy is described as nominal melting I2t and is expressed as“Ampere Squared Seconds” (A2 sec.).
- A pulse of current is applied to the fuse, and a time measurement is taken for melting to occur. If melting does not occur within a short duration of about 8 milliseconds (0.008 seconds) or less, the level of pulse current is increased. This test procedure is repeated until the melting of the fuse element is confined to within about 8 milliseconds.
- All of the heat energy (I2t) is used, to cause melting. Once the measurements of current (I) and time (t) is determined, it is a simple matter to calculate melting I2t. When the melting phase reaches completion, an electrical arc occurs immediately before the “opening” of the fuse element.
Clearing I2t = Melting I2t + arcing I2t
- This term is normally used in short circuit conditions and the values are used to perform coordination studies in electrical networks.
- I2t parameters are provided by charts in manufacturer data sheets for each fuse family. For coordination of fuse operation with upstream or downstream devices, both melting I2t and clearing I2t are specified.
- The melting I2t, is proportional to the amount of energy required to begin melting the fuse element. The clearing I2t is proportional to the total energy let through by the fuse when clearing a fault.
- The I2t rating of the fuse is proportional to the energy it lets through, it is a measure of the thermal damage and magnetic forces that will be produced by a fault.
Cut-off current characteristics
- The ability of the HRC fuse to limit the short circuit current is known as a cut-off as illustrated in Fig below.
- Due to this cut-off property the short circuit current does not attain the prospective current.
- Hence the circuit is not subjected to electrodynamic stresses corresponding to peak prospective current. Because of this property of the HRC fuse the operating time is as low as one-fourth of a cycle.
- On the occurrence of a fault, large current flows through the fuse, and the energy produced melts and vaporizes the fuse element before the fault current reaches the peak. The chemical reaction between the fuse element vapor and filling powder results in high-resistance material which helps in extinguishing the arc
- The HRC fuse interrupts the short circuit current long before it attains the max value. The short circuit current is interrupted within the first quarter of a cycle.
Non deteriorating characteristics
- This property of the HRC fuse is achieved by sealing of the silver element within the fuse body with the help of special cementing and the soldering of the end caps.
- To prevent overheating of associated contacts this property is very essential.
- The magnitude of voltages induced due to the interruption of inductive circuits depends upon the magnitude of the short circuit and the circuit constants. HRC fuses are designed within safe limits of these overvoltages.
- It is the product of service voltage in kV and the rms value of the prospective current in kA it can handle. It is expressed in MVA.
- Because of the cut-off characteristics of the HRC fuse, for the given rupturing capacity the actual current to be interrupted by an HRC fuse is much less in comparison to other interrupting devices hence it is less expensive in comparison to other interrupting devices.
- Discrimination means the breaking of the faulty section of the line to an extent restricted to minimum, healthy sections of the circuits remaining in operation.
Advantages and disadvantages of HRC link fuses
- Cheaper compared to the other types of circuit interrupters of the same breaking capacity
- Simple and easy installation
- No maintenance is required
- High breaking capacity
- Consistency in performance
- Quick and sure
- Do not deteriorate with time
- Inverse time characteristics; which makes them suitable for overload protection
- Capable of clearing high as well as low current
- Low fusing factor
- Low thermal and dynamic stresses on equipment to be protected
- Time-consuming for a replacement for each operation
- Interlocking is not possible
- It causes overheating of adjacent contacts
Drop out fuse
- The melting of the fuse causes the fuse element to drop under gravity about its lower support which creates visible isolation.
- Such fuses are employed for the protection of outdoor transformers.
- On blowing off the fuse the complete tube can be lifted from the hinge using a special insulator rod and brought down for replacement of the fuse element.
- After replacing the element the tube is placed in the hinge and the device is closed in a way similar to the closing of isolators.
High voltage HRC fuses
Cartridge-type HV HRC fuse
- This is similar in general construction to the low voltage type except that some special design features are incorporated. In some designs, the element is wound in the shape of a helix to avoid the corona effect at higher voltages.
- In some designs two fuse elements in parallel are employed; one of low resistance carries the normal current, the other is of high resistance (tungsten wire), and after the low resistance has blown off, reduces the short circuit current and finally breaks the circuit.
- HV cartridge fuses are used up to 33 kV, with a rupturing capacity of about 8760 A at that voltage (500 MVA 3 phase). Ratings of the order of 200 A at 6.6 kV and 11 kV, and 50 A at 33 kV are also available.
Liquid-type HV HRC fuse
- In liquid fuse carbon tetrachloride is used for arc extinction.
- Liquid-type HRC fuse consists of a carbon tetra chloride-filled glass tube sealed at both ends with brass caps.
- One end of the fuse element is sealed with the cap and the other end is held by a strong phosphor bronze spring fixed at the other end of the tube.
- On the occurrence of a fault the fuse element melts and the springs pull it into the carbon tetra chloride solution, thereby extinguishing the arc.