Working Principle and Types of Electric Fuse

Catalog

I Fuse in Electric Circuit

An electric fuse is a current interrupting device that protects the electrical circuit, which is located by generating an open circuit in response to excessive current.

When the item in the fuse receives too much heat, the current is melted and disrupted. Fuses are typically used as a conduit between an electrical power source and an electrical component or as a mixture of components organized in an electrical circuit. A fuse link is connected between the fuse terminals. This means that when the electrical current passing through the fuse goes beyond what the device can handle, the fuse link is melted and the circuit is opened to prevent damage to the electrical component.

Figure 1. Electric Fuse

The fuses are usually made for single-time use. In other words, once the system is powered down, it must be replaced. You can get overcurrent protection from a variety of sources, such as circuit breakers, switches, and relays. Each type of equipment has different ratings, service requirements, and costs. Fuses are usually the most economical means of supplying automatic high-voltage current protection against single overcurrent failures.

Fuses are part of electrical systems for automobiles, buses, boats, motorcycles, and other forms of vehicles. Such fuses are designed to avoid electricity from flowing to a particular section of the device by producing an open circuit due to a hazardous electrical situation. In the utility industry, the fuses used in distribution transformers, cables, capacitor banks, and other equipment are found to cause damage to currents. The fuses are used in such a way that disconnection happens before damage can harm the equipment. Fuses are used quite a bit in high voltage electrical grids to protect electrical equipment in the network from damage caused by system surges.

II What Does an Electric Fuse Do?

The fuse consists of two main parts: one is a fuse element in the form of a metal conductor, with a pair of contacts between which it is attached, and the other is a case or cartridge to carry the fuse element. A cartridge is often adjusted with an arc-extinguishing device inside it.

The principle behind the operation of the fuse is the heating effect of the electrical current. If the current passes through a conductor with a certain resistance, the loss due to the conductor's resistance is dissipated in the form of heat. Under normal operating conditions, the heat produced in the fuse element is easily dissipated into the environment due to the flow of current through it.

As a result, the fuse part remains at a temperature below its melting point. Whenever some faults, such as short circuits, occur, the current flow through the fuse element exceeds the prescribed limits. This creates an excess of heat that melts the fuse part and breaks the circuit. The computer or device is thus safe from serious damage caused by excess current.

In general, insulation switches are provided in series with fuses in order to allow the fuses to be replaced or rewired safely. In the absence of insulation switches, adequate protection must be provided to prevent electrical shock from occurring.

The fuse should be connected to the supply in series

The time to come out of the fuse depends on the amount of the excess current. The greater the current, the faster the fuse is blown. The blow time of the fuse is therefore inversely proportional to the current flowing through the fuse part.

III Electric Fuse Symbol

Figure 2. Electric Fuse Symbol

IV Materials of Fuse Component

The material used as a fuse part includes the following properties.

● The low point of melting

● Low Ohm Resistance

● High degree of conductivity

● Low Costs 

It's meant to be safe from deterioration. There are no materials that satisfy all of the above properties. The materials widely used for fuse elements are tin, lead, silver, copper, zinc, aluminum, and lead and tin alloys. Lead and tin alloys (lead 37% and tin 63%) are used for fuses with a current rating below 15 A. For a current exceeding 15A copper wire fuses are used. The higher melting point of copper is a major drawback. Zinc in strip form is good when a fuse with a desirable time delay is needed.

The new trend is to use silver as a fuse element material even with its higher cost due to the following advantages.

● It doesn't get oxidized, and the oxide is volatile.

● The conductivity of silver does not deteriorate with oxidation.

● It's high conductivity.

● Fast service.

It remains unaffected by dry air, but when exposed to humid air containing hydrogen sulfide, a layer of silver sulfide is produced over it and prevents the further attack.

Copper or lead-tin alloy is used in domestic fuses.

V Types of Electric Fuses

In general, the fuses are divided into two groups:  low voltage fuse and high voltage fuse.

1. Low Voltage Fuses

Low voltage is classified into two types, the semi-enclosed or rewirable type and the fully enclosed or cartridge type.

(1) Rewirable Fuse

The rewirable fuse is the most widely used fuse for house wiring. It's also known as the kit-kat fuse. This consists of a foundation and a porcelain fuse carrier. The base includes both the incoming and outgoing terminals. The fuse part is attached to the fuse carrier. To close the connection, the fuse carrier is inserted into the base. The fuse wire can be made of lead, tinned copper, aluminum, or tin-lead alloy. If a fault occurs, the fuse part blows off and the circuit is disrupted. Supply can be restored by replacing the new fuse part. The regular ratings of the rewirable fuses are 6A, 16A, 32A, 63A, and 100A.

Figure 3. Rewirable Fuse

(2) Cartridge or Fully Enclosed Fuse  

For this type of fuse, the fuse part is enclosed in a tightly sealed container and has metal contacts at both ends.

There are two types of fuse cartridge,  D-link fuse and Link type or High Rupturing Capacity (HRC) fuse.

1) D-Link Fuse

It is a screw-type fuse consisting of a fuse base, a cartridge, and a fuse cap. The cartridge is pushed into the fuse cap and the cap is screwed onto the fuse base. It's a non-interchangeable fuse. Standard ratings are 6A, 16A, 32A, and 63A. Breaking capacity of 6A, 16A fuse is 4 kA and 32A, 63A is 16 kA.

2) Link Type or High Rupturing Capacity (HRC) Fuse

The HRC cartridge fuses are designed and built to have high-established breaking power for use in the modern distribution system. The fuse part is enclosed in a chamber made of stealite, a ceramic material with a strong mechanical strength or epoxy resins. Fuse contacts are welded with end caps made of copper or brass. The fuse is designed to withstand the pressure produced over a short circuit. The chamber is filled with pure quartz energy, which acts as an arc-extinguishing agent. The widely used components of the fuse are silver and copper wires.

Preferred ratings 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 fuse, knife blade type and bolted type.

Figure 4. Knife Blade Type Fuse

Figure 5. Bolted Type Fuse

(3) Drop Down Fuse

Such fuses are used for the safety of outdoor transformers. In this fuse, once the fuse element melts, it falls due to gravity, thus providing additional insulation.

(4) Switch Fuse

This is a series of recycled fuses within a metal enclosure. Switch fuse ratings are available in the range of 30, 60, 100, 200, 400, 600, and 800 amperes.

Figure 6. Switch Fuse

2. High-voltage Fuses

(1) Cartridge Type Hv HRC Fuse

It is close to the low voltage HRC fuse, except that a few unique features are added. In this type of fuse, the fuse element is wound in the shape of a helix, or two fuse elements are used in parallel to prevent corona effects at high voltages.

HV HRC fuses are available with a voltage of 33kV and a rupturing power of 8700A.

(2) High Voltage Liquid HRC fuse

Carbon tetrachloride is used for arc extinction in a liquid fuse. Liquid HRC fuse consists of a carbon tetrachloride filled glass tube sealed on both ends with a brass cover. One end of the fuse part is sealed with the cap, and the other end is kept by a powerful bronze phosphorous spring fixed at the other end of the tube. When a fault occurs, the fuse element melts and the springs pull it into the carbon tetrachloride solution and the arc is extinguished.

(3) Thermal Fuse 

The thermal fuse is used to protect electrical equipment from damage caused by overheating. It consists of an electric fuse holder with a stretched spring. The fusible material is burned by overheating devices. And the spring is released and the link is opened. Thermal fuses are used in coffee makers, refrigerators, hairdryers, and other appliances where thermostats are used to secure the machines at the time of thermostat failure.

VI Electric Fuse Properties

The following are some relevant parameters of the electric fuse.

1. Fuse 

An electric fuse is a self-sacrificial device used to interrupt the circuit under short circuit, overload, or overcurrent conditions by melting the fuse element.

2. Fuse Components

The part of the fuse that melts when the excessive current flows through the circuit is known as the fuse component.

3. Current Rating

The RMS value of the current that the fuse wire will bear without degrading within the specified temperature limits is known as the current rating. The current rating is given by the manufacturer.

4. Fusing Current  

Fusing current is known as the minimum current value at which the fusing element melts.

In the case of a round wire, the proper amount of fusion current is calculated by

I = kd3/2

Where k is the fusing constant.

The fusion current depends on the following factors:

● Form of material.

● Length of the component

● Size and location of the terminals

● Wire diameter

● Type of enclosure 

5. Fusing factor

The fusing factor is the ratio of minimum fusing current and fuse element current rating.

Fusing factor = Minimum fuse current / Fuse element current rating 

6. Voltage Rating 

The voltage level of the fuse shall be greater than or equal to the voltage of the open circuit.

7. Breaking Capacity 

A fuse's breaking capacity is the rating that corresponds to the RMS value of the ac component of maximum prospective current.

8. Prospective Current

Once the fuse is replaced by a link of negligible impedance, the current that would flow in the circuit under a fault condition is called prospective current.

VII Advantages and Disadvantages of Electric Fuse

1. Advantages

● Cheapest form of protection available 

●    No need for repairs

● Short circuit currents are disrupted without producing smoke or gas

● Operating time is minimal

● Work automatically

● Inverse time-current characteristics allow current protection

2. Disadvantages

● After the operation, considerable time should be taken to replace the fuse.

● The current-time characteristic of the fuse can not always be correlated with that of the protection device.

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