Electric Fuse Types
In 1847 French physicist and watch maker Louis François Clément Breguet recommended a reduced strip conductor to protect telegraph station from lightning strikes. He thought by melting smaller wires would protect the apparatus and wiring inside the building. By 1864 a variety of wires and foil fusible apparatus were used for the purpose of protection of telegraph cables and lighting systems. In 1890 Thomas Alva Edison patented a fuse as a part of his electrical distribution system. Electrical fuses are sacrificial elements in an electrical circuit which protect the system from over current. Fuses are designated to open circuit during an occurrence of excessive current due to the presence of an overload or fault and prevent the further damage of the system.
Various components of a fuse are: melt fuse element, set of contacts and supporting body. Fuse element are generally made of materials with low melting point, high conductivity and less deterioration due to oxidation. They are inserted in series with the circuit to be protected. Under normal operating conditions the fuse element in kept at a temperature below its melting point so that it carries normal current without overheating. During the occurrence of short circuit or overload, the current in the fuse element increases beyond the rated value. This raises the temperature leading to melting of fuse element thus disconnecting the circuit protected by it. The magnitude of the over current decides the time required for the blow out of the fuse. Greater the current, smaller the time required for the fuse to blow out or it can be said that fuse has an inverse time-current characteristics.
Figure1: Time-Current Characteristics of Different Fuses
Desirable Characters of Fuse Elements
The function of the fuse is to carry normal current without interruption and during an over current situation it rapidly heats up to melting point and isolates the circuit protected by it. For the satisfactory performance of the fuse, the following desired characters of the fuse element is necessary.
- Low melting point, eg. Tin, Lead
- High conductivity, eg. copper, Silver
- Free from deterioration due to oxidation, eg. Silver
- Low cost, eg. Tin, Lead, Copper
No material has all the desired characters in common so compromise must be made in the selection of fuse elements.
Materials for Fuse Elements
The commonly used materials for fuse elements are Tin, Lead, Copper, Zinc and Silver etc. For currents up to 10A Tin or and alloy of Tin and Lead (63% Tin and 37% Lead) is used. For higher voltages copper or silver is used. Copper is usually tinned to prevent it from oxidation. Zinc strips are used when a considerable time-lag is required as it does not melt quickly for small overloads.
Nowadays despite of its high cost silver is used for the following reasons.
- High conductivity. So for a given rating of the fuse element, the mass of silver metal required is much less than other elements.
- Free from oxidation
- Silver does not deteriorate when used in dry air.
- The coefficient of expansion of copper is very low so it can carry the rated current continuously for a long time without interruption.
- Silver vaporises at a temperature much lower than the one at which its vapour will rapidly ionise. So when an arc is formed through the vaporised portion of the element, the arc path has high resistance so that the short circuit current is quickly interrupted.
- The resistance of silver increases abruptly as the melting temperature is reached, making the transition from melting to vaporisation occurs instantaneously.
Current rating of fuse element
It is the current which the fuse element can carry without overheating or melting. It depends on the temperature rise in the contacts of the fuse holder, fuse material and the surrounding of the fuse.
It is the minimum current at which the fuse element melts. Its value will be higher than the current rating of the fuse element.
For a round wire the approximate relationship between the fuse current I and the diameter d of the wire is given by
I = kd3/2
k is a constant called the fuse contact. The value of k depends on the material used for making fuse element.
The value of k for various elements as found by Sir William Henry Preece is given below
||Value of k
|d in cm
|| d in mm
Table1: Value of k for different elements
The fusing current depends on various factors such as:
- Material of the fuse element
- Length: smaller the length greater the current because a short fuse can conduct away all the heat
- Size and location of the terminals
- Previous history
- Type of enclosure used
It is the ration of fusing current to the current rating of the fuse element.
Fusing Factor = Fuse current/Current rating of fuse
The value of fusing factor is always greater than 1. The small the value of fusing factor, greater is the chance of deterioration of fusing element due to overheating and oxidation. For a semi enclosed or rewirable fuse which employ copper as the fuse element, the value of fuse factor is usually 2. Lower values of fusing factor can be employed for enclosed type cartridge fuse which uses silver or bimetallic elements.
Figure 2. AC Current Cut-off by Fuse
From Figure 2 the fault current would normally have a large first loop but it generate sufficient energy to melt the fuse element before the peak value of the first loop is reached. The rms value of the first loop of fault current is known as prospective current. Prospective current is defined as the rms value of first loop of fault current is the fuse is replaced by an ordinary conductor of negligible resistance.
It is the maximum value of fault current reached when the fuse melts. The current corresponding to the point ‘a’ is called cut-off current. The cut-off value depends on
- Current rating of the fuse
- Value of prospective current
- Asymmetry of the fault current
It is the time between commencement of the fault current and the instant when cut-off occurs. When a fault occurs, the fault current rises rapidly and as the fault current reaches a cut-off value the fuse melts and an arc is initiated. The time between the occurrence of the fault and the instant the arc is initiated is called the pre-arcing time. The value of pre-arcing time is generally small and a typical value is 0.001 second.
The time between the pre-arcing time and the instant at which the arc is extinguished.
Total Operating Time
The sum of pre-arcing and arcing times. It may be noted that the operating time of a fuse (0.002seonds) is much lower than that of a circuit breaker (0.2 seconds). A fuse in series with a circuit breaker of low breaking capacity is useful and economical way of providing adequate short circuit protection. In that case the fuse will blow fast even before the circuit breaker begins to operate.
It is the rms value of the AC component of maximum prospective current that a fuse can deal with at a rated service voltage.
Types of Fuses
Since its invention, a lot of improvements have been made and now a variety of fuses are available. Some fuses even have arrangements to extinguish the arc that appears when the fuel element melts. Fuses are generally classified into two:
- Low voltage fuses
- High voltage fuses
Low Voltage Fuses
Low Voltage Fuses are of two types
- Semi Enclosed Rewirable Fuse
- High Rupturing Capacity (HRC) Cartridge Fuses
Semi Enclosed Rewirable Fuse
Rewirable fuses also known as kit-kat type fuses are used to interrupt fault currents of lower magnitude. It consists of two parts: a base and a fuse carrier. The base is made of porcelain and carrier the fixed contacts to which the incoming and outgoing phase wires are connected. The fuse carrier is also made of porcelain and fuse elements between the terminals. The fuse carrier can be inserted or removed at any time.
Figure 3: Semi Enclosed Rewirable Fuses
When a fault occurs the fuse element is blown out and the circuit is interrupted. The fuse carrier can be taken out and the blown out fuse element can be replaced by a new one. It is then inserted to the base to restore the supply.
- Detachable fuse carrier permits the replacement of fuse element without coming in contact with live parts.
- Cost of replacement is negligible.
- Possibility of renewal with fuse wire of wrong size and improper material.
- Low breaking capacity, so cannot be used in circuits with higher fault current levels.
- The fuse element is subjected to deterioration by oxidation through the continuous heating up of the element. This reduces the current rating of the fuse. Thus the fuse operates at a current lower than the rated value.
- Uncertainty in the protective capacity of the fuse since it is affected by ambient conditions.
- Accurate calibration of the fuse is not possible as the fusing current depends on the length of the fusing element.
Semi enclosed rewirable fuses have capacity up to 500A but their breaking capacity is much lower so their use is limited to domestic and lighting applications only.
High Rupturing Capacity (HRC) Cartridge Fuse
The shortcomings of uncertain and low breaking capacity of semi enclosed rewirable fuse are overcome in HRC cartridge fuse. It consist of a heat resisting ceramic body with metal end caps to which is welded silver current carrying element. The space surrounding the element is completely packed with a filling powder. The filling powder can be of chalk, plaster of paris, quartz or marble dust and act as an arc quenching and cooling medium.
Figue 4: Parts of a typical HRC Fuse
Figure 5: HRC Fuses
The HRC cartridge fuse carried the normal current without overheating. During the occurrence of a fault, the current increases and the fuse element melts before the fault current reaches its first peak. The heat produced in the process vaporises the melted silver element. The chemical reaction between the silver vapours and the filling powder results in a high resistance substance that helps in quenching the arc.
- Capable of clearing high as well as low fault currents
- Does not deteriorate with age
- High speed of operation
- Provide reliable discrimination
- Require no maintenance
- Cheaper than other circuit interrupting devices of equal breaking capacity
- Must be replaced after operation
- Heat produced by the arc may affect the associated switches
High Rupturing Capacity Fuse with Tripping device
In some case, the HRC fuse is provided with a tripping device. During the occurrence of a fault the fuse is blown out and the tripping device causes the circuit breaker to operate. The body of the fuse is made of a ceramic material with a metal cap attached rigidly to both ends. The caps are connected by a number of silver fuse elements. At one end of the fuse there is a plunger. Under fault condition it hits the tripping mechanism of the circuit breaker and causes it to operate. The plunger is electrically connected by means of a fusible link, chemical charge and a tungsten wire to the other end of the cap.
Figure 6: HRC Fuse with Trigger Mechanism
When fault occurs, the silver fuse elements are the first to be blown out and the current is then 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 to move outward to operate the circuit breaker. The travel of the plunger is so set that it is not ejected from the fuse body under the fault conditions.
Advantages over the fuse without triggering device
- 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 phase and thus prevents ‘single phasing’
- The effect of full short circuiting need not be considered in the choice of circuit breaker. This prevents from the use of expensive circuit breakers
- The fuse tripped circuit breaker is generally capable of dealing with fairly small fault current itself. This avoids the necessity for replacing the fuse except after highest currents fir which it is intended
High Voltage fuses
Due to the fact that the low voltage fuses has low current rating and breaking capacity they cannot be used for modern high voltage systems. With the advancement in technology researches found a way to protect the high voltage circuits; the high voltage fuses.
Figure 7: High Voltage Fuses
Some of the high voltage fuses are:
Cartridge type fuses
The construction of high voltage cartridge fuse is similar to that of a low voltage cartridge fuse except that special features are incorporated in the former. On some design the fuse element is wound in the form of a helix so as to avoid the effects due to corona at high voltages. In another design, there are two fuse elements in parallel, one low resistance (silver wire) and one high resistance (tungsten wire). Under normal operating conditions the low resistance element carries the normal current. During the occurrence of a fault the low resistance element is blown out and the high resistance element reduces the short circuit current and finally breaks the circuit. High voltage cartridge fuses are used up to 33kv with breaking capacity of about 8700A. Ratings in the order of 200A at 6.6kv and 11kV and 50A at 33kV are also available.
Liquid type fuses
These fuses are filled with carbon tetrachloride and they have wide range of application in high voltage systems. They may be used in circuits with 100A rated current in 132kV system. The breaking capacity is of the order of 6100A. It consists of a glass tube filled with carbon tetrachloride solution with both ends sealed with brass caps. The fuse wire is sealed at one end of the tube and the other end is held strongly by a phosphor bronze spiral spring fixed at one end of the glass tube.
Figure 8: Parts of High Voltage Liquid fuse Fuse
Figure 9: High Voltage Liquid Fuse
Fig 8 shows the essential parts of a liquid fuse. When the current exceeds the predetermined limit the fuse wire is blown out. As the fuse wire melts the spring retracts the part of the wire through liquid director and draws it completely into the liquid. The small quantity of gas generated at the point of fusion forces part of the liquid into the passage through liquid director and it effectively extinguish the arc.
Metal clad fuses
Metal clad oil immersed fuses were developed as a substitute for oil circuit breakers. They operate satisfactorily under fault conditions in high voltage circuits.
Other Types of Fuses
Self-resetting fuses are Polymeric Positive Temperature Coefficient(PPTC) thermistor which uses thermoplastic conductive element. It breaks the circuit as it increases device resistance during an over current condition. When the fault current is cleared the device then cools and return to lower resistance or in other words PPTC thermistor is self resetting. They are ofthen used in nuclear or aerospace applications where the replacement of the fuse is difficult. They are also used in computer mother boards so that any short circuit in keyboard or mouse does not damage the mother board.
Figure 10: Resettable Fuses
They are single operation devices that work mainly as temperature sensitive protective device. Their reduced size and cost effectiveness make them useful as a protective device in systems where thermal variations are experienced during ordinary working conditions. The thermal fuse consists of a contact spring which is enclosed into a wax pellet. The pellet is constructed to melt at a set temperature. When the wax get heated it melts and the spring get stretched till it open the circuit. Another type of thermal fuse uses a specially composed solder that melts at a predetermined temperature. Thermal fuses are also called thermal links or thermal cutouts. A thermal fuse can be found in consumer equipment such as water heaters, hair dryers or small transformer poweredelectronics equipments.
Figure 11: Thermal Fuses
Surface Mount fuses
Fast Acting Chip Fuses
They are mainly used in DC power applications where quick clear is required during an over current condition. Reduce in fuse aging, improve reliability and resilience and enhance high temperature performance are the main highlights.
High Current Rated Fuses
They are used when high current is required and space is critical. Due to their high reliability and strong arc suppression property they are used for over current protection in power supplies servers, communication equipment etc.
Figure 12: High Current Rated Fuses
Slow Blow Chip Fuses
They are used to protect from damages caused by over current on systems that experience large and frequent surges as part of their normal operation.
Telecom fuse offers low temperature rise performance under sneak current fault to prevent damages to circuit trace or multilayer board. The low profile and small footprint make them suitable for high density and space constraint applications.
Figure 13: Telecom Fuses
Pulse Tolerant Chip Fuses
They provide over current protection to systems using DC power up to 63V. The monolithic, multilayer design provides the highest hold current in the smallest foot print, reduces diffusion related aging, improve product reliability and resilience and high temperature performance in a wide range of circuit design.
Very Fast Acting Chip Fuses
Very fact acting at 200% and 300% overloads. Provides Over current protection to systems using DC power up to 63V
They are used to protect the wiring and equipment in vehicles. Automotive fuses can be mounted in fuse block, inline fuse holders or fuse chips. Some automotive fuses are also used in non-automotive applications. Standards for automotive fuses are published by Society of Automotive Engineers (SAE) International. Automotive fuses are classified into Blade fuses, Class tube or Bosch type, Fusible links and Fuse limiters. Most automotive fuses rated 32V are used on circuits rated 24V DC and below. Certain vehicles use dual 12/24V DC electrical system will require fuse rated at 58V DC.
Figure 14: Automotive Fuses
Coordination of Fuses in Series
Several fuses are connected in series at various levels of a power distribution system so that only the fuse electrically closest to the fault is blown out. This process is called coordination or discrimination and may require the time current characteristics of two fuses plotted on a common current basis. Fuses are selected so that the minor branch fuse disconnects its circuit before the supplying major branch fuse starts to melt. So only the fault circuit is interrupted with minimum disturbance to other circuit fed by a common supplying fuse.
The current carrying capacity of a fuse depends on the material used, the cross sectional area, length of the element, the state of the surface and the surrounding of the fuse.
Heat produced per second = Heat lost due to conduction, convection and radiation
Where d is the diameter, l is the length and a is the area of cross section of the fuse element
This is the fuse law.
Advantages and Disadvantages of Fuse
- Cheapest form of protective device
- No maintenance required
- Operation is automatic unlike circuit breakers which require various other devices. to initiating automatic action
- Breaks heavy short circuit current without noise or smoke
- Smaller size of fuse element can limit large short circuit currents
- Minimum time of operation can be made much smaller than the circuit breaker
- Time loss in rewiring or replacing fuse
- During heavy short circuits, discrimination between fuses in series cannot be obtained unless there is a size difference between the fuses
- The time-current characteristics of the fuse cannot always be co-related to that of the protected apparatus.