The electrical fuse is a device used to protect load or source from overcurrent. It is a simple, less resistive, self-sacrificial and cheapest device used to interrupt a circuit under short circuit, excessive overload or over-current conditions.
Electrical fuse can be found everywhere from the generating stations to small home appliances. It is used for overload and short circuit protection in high voltage (up to 66 kV) and low voltage (up to 400V) installations. In high voltage circuits, their use is confined to those applications where their performance characteristics are particularly suited for their current interruption.
The functions of use wire are to carry the normal working current without interrupting and to break the circuit when the current exceeds the limiting current. The main purpose of using a fuse in a circuit is to limit the excessive damage to the equipment.
Construction and Operation of Fuses
A fuse consists of two main components: one is a fusible element in the form of a metal conductor along with a set of contacts between which it is fixed and the other is a case or cartridge to hold the fusible element. A cartridge is sometimes provided with arc extinction arrangements within it.
The principle behind the working of a fuse is the heating effect of electric current. When current flows through a conductor having a certain resistance, the loss due to the resistance of the conductor is dissipated in the form of heat. In normal operating conditions, the heat produced in the fuse element due to the flow of current through it is easily dissipated to the environment. Therefore the fuse element remains at a temperature below its melting point. Whenever some faults such as short circuit occur, the flow of current through the fuse element exceeds the prescribed limits. This produces an excess of heat that melts the fuse element and breaks the circuit. Thus the machine or apparatus is protected from severe damage caused due to excessive current.
Usually, isolating switches are provided in series with the fuses in order to permit the fuses to be replaced or rewired safely. In the absence of isolation switches, proper shielding must be provided to prevent electric shock hazard.
The Fuse Should Be Connected in Series to the Supply
The time for blowing out of fuse depends upon the magnitude of the excessive current. Larger the current, the more rapidly the fuse is blown. Hence the blow time of fuse is inversely proportional to the current flowing through the fuse element.
Symbol of fuse
Fuse Element Materials
The material used as fuse element must have the following properties.
- Low melting point
- Low ohmic resistance
- High conductivity
- Low cost
- It should be free from deterioration.
There is no such material that satisfies all the above properties. The materials commonly used for fuse elements are tin, lead, silver, copper, zinc, aluminum, and alloys of lead and tin. An alloy of lead and tin (lead 37% and tin 63%) is used for fuses with a current rating below 15 A. For current exceeding 15A copper wire fuses are employed. The higher melting point of copper is a serious disadvantage. Zinc in strip form is good if a fuse with desirable time delay is required.
The present trend is to use silver as fuse element material despite its higher cost owing to the following advantages.
- It does not get oxidized and is oxide is unstable.
- The conductivity of silver is not deteriorated with oxidation.
- High conductivity.
- Fast operation.
- It remains unaffected by dry air but when exposed to humid air containing hydrogen sulphide a layer of silver sulphide is former over it and prevents it from further attack.
Either copper or lead-tin alloy is used in domestic fuses.
Melting point in degree celsius
Specific resistance in μΩ- mm
Value of fuse constant k for d in m
Types of Fuses
In general, fuses are classified into two types viz. (i) low voltage fuses and (ii) high voltage fuses.
Low Voltage Fuses
Low voltage is classified into two classes namely semi-enclosed or rewirable type and fully enclosed or cartridge type.
1. Rewirable Fuses
Rewirable fuse is the most commonly used fuse in house wiring. It is also known as kit-kat fuse. It consists of a base and a fuse carrier made of porcelain. The base contains the incoming and outgoing terminals. The fuse element is fixed on the fuse carrier. The fuse carrier is inserted into the base to close the circuit. The fuse wire may be of lead, tinned copper, aluminum or an alloy of tin – lead. On the occurrence of a fault, the fuse element blows off and the circuit is interrupted. Supply can be restored by replacing the fuse element with a new one. Standard ratings of rewireable fuses are 6A, 16A, 32A, 63A and 100 A.
2. Cartridge or Totally Enclosed Fuse
In this type of fuse, the fuse element is enclosed in a totally enclosed container and is provided with metal contacts at the both ends.
There are two types of Cartridge fuse viz. (i) D- link fuse (ii) Link type or High Rupturing Capacity (HRC) fuse.
(i) D- Link Fuse
It is a screw type fuse comprising of a fuse base, cartridge, and a fuse cap. The cartridge is pushed in the fuse cap and the cap is screwed on the fuse base. It is a noninterchangeable fuse. The standard ratings are 6A, 16A, 32A, 63A. The breaking capacity of 6A, 16A fuse is 4 kA and that of 32A, 63A is 16 kA.
(Ii) Link Type or High Rupturing Capacity (HRC) Fuse
HRC cartridge fuses are designed and developed to provide high known breaking capacity, to be used in the modern distribution system. The fuse element is enclosed in a chamber made up of stealite, a ceramic material having a good mechanical strength or epoxy resins. Fuse contacts are welded with the end caps made up of brass or copper. The fuse is designed so as to withstand the pressure developed during a short circuit. The chamber is filled with pure quartz power, which acts as arc extinguishing agent. The commonly used fuse elements are silver and copper wires.
Preferred rating of HRC fuses are 2, 4, 6, 10, 16, 25, 30, 50, 63, 80, 100, 125, 160, 200, 250, 320, 400, 500, 630, 800, 1000, and 1250 amperes.
There are two types of HRC fuses: (i) Knife blade type (ii) Bolted type.
Drop Down Fuse
- These fuses are employed in the protection of outdoor transformers. In this fuse, once the fuse element melts, it falls down due to gravity, hence providing an additional isolation.
- It is a set of renewable fuses placed inside a metal enclosure. The ratings of switch fuses available are in the range of 30, 60, 100, 200, 400, 600 and 800 Amperes.
High Voltage Fuses
1. Cartridge Type Hv HRC Fuse
This is similar in construction to the low voltage HRC fuse except that a few special features are incorporated. In this type of fuse, in order to prevent corona effect at high voltages, the fuse element is wound in the shape of a helix or two fuse elements are employed in parallel.
HV HRC fuses are available with a rating of 33kV with rupturing capacity of 8700A.
2. Liquid Type High Voltage HRC Fuse
In liquid fuse, carbon tetrachloride is used for arc extinction. Liquid type HRC fuse consists of a carbon tetrachloride 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 fault, the fuse element melts and the springs pull it into the carbon tetra chloride solution, thereby extinguishing the arc.
The thermal fuse is used in protect electrical appliances from the damages caused due to overheating. It consists of a fusible metal holding with a stretched spring. Overheating appliances melts the fusible material. Hence it releases the spring and the contact opens. Thermal fuses are used in coffee makers, refrigerators, hair dryers and other such equipment in which thermostats are used to protect the devices during the times of thermostat malfunction.
Important Terms and Definitions Related to Electric Fuse
The following are a few important definitions related to electric fuse.
Electric fuse is a self sacrificial device used to interrupt a circuit under short circuit, excessive overload or over current conditions by melting the fuse element.
The part of the fuse that melts when an excessive current flows in the circuit is known as the fuse element.
The RMS value of current that the fuse wire can carry without being deteriorated, within specified temperature limits, is known as the current rating. Current rating is specified by the manufacturer.
Fusing current is defined as the minimum value of current at which the fusing element melts.
For a round wire, the appropriate amount of fusing current is given by
I = kd3/2
Where k is a constant called fusing constant.
The fusing current depends on the following factors:
- Type of material used.
- Length of the element.
- Size and location of the terminals.
- Diameter of the wire.
- Type of enclosure used.
Fusing factor is the ratio of minimum fusing current and the current rating of the fuse element.
Fusing factor = Minimum fusing current / Current rating of the fuse element.
The voltage rating of fuse must be greater than or equal to the open circuit voltage.
Breaking capacity of a fuse is the rating corresponding to the RMS value of ac component of maximum prospective current.
The current that would flow in the circuit under fault condition when the fuse is replaced by a link of negligible impedance is called prospective current.
Advantages of Electric Fuse
- It is the cheapest form of protection available
- It does not need any maintenance.
- Short circuit currents are interrupted without causing smoke flame or gasses.
- Time required for operation is minimum.
- It works automatically.
- Inverse time current characteristics enables over current protection