Fuses and Types of Fuses

What is a Fuse?

A Fuse or an Electric Fuse is an Electrical / Electronic device that protects the circuit from different electrical faults like over current and overload. Fuses can be considered as a sacrificial element in the circuit as they act as a weak link in the entire circuit.

This is because, a fuse scarifies itself and reliably opens the circuit when there is an excessive current in the circuit or the circuit is under over load and if there is any short circuit.

The principle of a fuse is based on the heating effect of the electric current. A simple fuse consists of a small conductive material with low resistance and it is placed in series with the circuit.

The cross section area of this conducting material is designed such that it allows a certain amount of current that is permitted to flow in the circuit.

When the current in the circuit exceeds this permitted value (which may be caused due to overload, short circuit or load mismatch), this excessive current will melt the conductive element in the fuse and opens the circuit.

This will disconnect the power supply and thus, the rest of the circuit is protected from being damaged. The following image shows a block diagram of how a fuse is connected in the circuit.

Fuses are very simple and cheap devices that are being used for over hundred years as a protective equipment. For electrical drawings and circuits, there are three symbols of fuses we can use. The following image shows the symbols of fuse along with their standards.

Characteristics of a Fuse

There are different types of Fuses available in the market for different types of applications like residential, industrial, automotive, etc. All the fuses are often characterized by the following characteristics.

• Current or Ampere Rating
• Melting Time
• Voltage Rating and
• Interrupting Rating or Breaking capacity
• I²T Value of the Fuse
• Packaging
• Temperature

The first two i.e. Current Rating and Melting Time of the Fuse are generally associated with the Thermal Characteristics of the fuse whereas the Voltage and Interrupting Rating are classified under Interrupting Characteristics of the fuse.

As the amount of current in the circuit increases, the melting time of the conducting element in the fuse decreases. This is because, as the current increases, the power dissipation (determined by I2R) will increase and the temperature of the element increases rapidly.

If there are any inductive components in the circuits, then melting of the conductive element in the fuse is not enough for interrupting the current. Even though the element in the fuse melts, there is a chance of arc in the fuse before the current is completely disconnected.

During this period, the fuse must hold the transient voltages and hence, any fuse must be given a clearing time.

Till now, we have spoken only about the current ratings of the fuse but haven’t brought up the voltage rating. All the fuses are rated with a maximum voltage that they can be operated with.

Current Rating or Current Capacity of Fuse

Current Rating or the Current Capacity of a Fuse defines the maximum amount of current a fuse can hold without blowing or melting. This is usually mentioned in Amperes i.e. 2A, 4A, 600A etc.

Voltage Rating of Fuse

Along with Current Rating, a fuse will also be specified with the maximum voltage it can be supplied with. Based on the Voltage Rating, Fuses are again classified in to Low Voltage (LV) Fuses and High Voltage (HV) Fuses (and even miniature fuses).

I²T (Ampere Squared Seconds)

I²T value of a Fuse measures the heat energy in the fuse. This heat energy is due to current flow and also the arc produced when the fuse is blown.

Breaking Capacity of Fuse

Breaking Capacity of the Fuse is also known as the Interrupting Rating or Short Circuit Rating. Breaking Capacity will specify the maximum safe current that the fuse can interrupt at a voltage less than the maximum rated voltage.

Classification of Fuses

Even though the job of a fuse seems simple, there are different methods to classify different types of fuses. The basic classification is the usability i.e. One Time Only Fuses and Resettable Fuses.

One Time Only Fuses, when blown out due to over current in the circuit, has to be manually replaced. These types of fuses are frequently used in electrical and electronics system in houses, industries, consumer products, etc.

Resettable Fuses on the other hand will be automatically reset after the fault has occurred by changing its resistance.

The other classification is based on the current limiting and non – current limiting fuses. Current Limiting Fuses introduces high resistance in the circuit for the short period of time. In non – current limiting fuses, as soon as the excess current flow, the gases in the fuse produce arc that interrupts the current.

Types of Fuses

There are many types of fuses available for a variety of applications. The main category of Fuses are based on the type of circuit they are used in i.e. AC Fuses and DC Fuses. Again, AC Fuses are divided in to High Voltage (HV) Fuses and Low Voltage (LV) Fuses.

High Voltage (HV) AC Fuses are used for voltages above 1000V and Low Voltage (LV) AC Fuses are used for voltages less than 1000V. Low Voltage (LV) Fuses are again classified in to Cartridge Fuses (Totally Enclosed Type), Rewirable Fuses (Semi – Enclosed Type), Switch Fuses, Drop out Fuses and Striker Fuses.

High Voltage (HV) Fuses are further divided in to Cartridge Type HRC (High Rupturing Capacity) Fuses, Liquid Type HRC Fuses and Expulsion type Fuses.

The following image shows the chart of fuses divided based on the AC and DC currents.

We will now see the different types of Fuses in general, irrespective of the above classification.

DC Fuses

The main difference between the DC Fuses and AC Fuses is the size of the fuse. In a DC circuit, when the current exceeds the limit, the metallic wire in the fuse melts and disconnects the rest of the circuit from power supply.

Since DC is constant value and is always above 0V, there is a chance of an electric arc between the melted wires, which will be difficult to avoid and turn off. Hence, usually, the electrodes of the DC Fuses are placed at larger distance when compared to the AC Fuses.

This will minimize the chance of arc and since the distance between the electrodes is increased, the size of the DC Fuses is comparatively large.

AC Fuses

We know that AC current (and voltage) oscillates as 50 or 60 times per second and in this the amplitude of the signal varies from minimum to maximum. At one point of this oscillations, the AC Voltage touches the 0V and hence the arc between the melted electrodes can be easily terminated.

As a result, the size of the AC Fuses can be much smaller when compared to the size of DC Fuses.

Rewirable Fuses

Rewirable or Kit – Kat Type Fuses are a type of Low Voltage (LV) Fuses. They are most commonly used in house wiring, small industries and other small current applications.

Rewirable Fuses consists of two main parts: a Fuse Base, which contains the in and out terminal, and a Fuse Carrier, which holds the Fuse Element. The Fuse Base is generally made up of Porcelain and the Fuse Element is made up of Tinned Copper, Aluminium, Lead, etc.

The Fuse Carrier can be easily plugged in or removed from the Fuse Base without the risk of any electric shock. When the fuse is blown due to over current, we can easily remove the Fuse Carrier and replace the fuse wire. This is the main advantage of Rewirable Fuses.

Cartridge Type Fuses or Totally Enclosed Type Fuses

As the name indicates, Cartridge or Totally Enclosed Fuses have a completely closed structure with the Fuse Links enclosed in the container. This type of design and construction will help in keeping the arc with in the container at the event of blown fuse.

Cartridge Type Fuses are a very important category of fuses that are used in almost all types of applications like Low Voltage (LV), High Voltage (HV) and miniature fuses.

Cartridge Type Fuses are again further divided in to D Type Cartridge Fuses and Link Type Cartridge Fuses.

D – Type Cartridge Fuse

This type of fuses consists of a Cartridge, fuse base, cap and adapter ring. The cartridge with the fuse element in it is fitted with the fuse cap and is inserted in to the fuse base through the adapter ring and the connection is complete only when the tip of the cartridge touch the conductor.

D – Type Fuses are non – interchangeable and have an advantage of being highly reliable.

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

High Rupturing Time or HRC Fuses are a type of Cartridge Fuses. In HRC Fuses, the current flows through the fuse element under normal conditions.

In case of a fault, the high current due to short circuit (or any other fault) will be allowed to flow through the fuse for a short but known period of time. If the fault is cleared in this time, the fuse will not blow or the fuse element doesn’t melt.

If the fault continues even after some time i.e. short circuit current for longer duration than allowed, the fuse blows by melting the fuse element.

Since HRC Fuses are designed for high current rupturing, a special method must be used to control the arc produced in the event of blown fuse. Usually, the body of the fuse is made up of Porcelain or Ceramic and the fuse element chamber is filled with Silica Sand.

There are two types of HRC Fuses: Blade Type and Bolted Type. Blade Type Fuses are also known as Plug – in Type Fuses.

The body of the Blade Type Fuse is generally made up of plastic and the two conducting Blade Type Plates are fixed to the fuse element. Blade Type Fuses are generally used in Automobiles.

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. In Expulsion type High Voltage (HV) Fuses, the Fuse Link Chamber is filled with Boric Acid.

Resettable Fuses

Resettable Fuses are also called as Self Resetting Fuses. They can be used even after a short circuit fault has occurred (even after multiple faults) without any hassle of replacement.

The Fuse Element in Resettable Fuses is a Thermoplastic Conductive type Thermistor with Polymeric Positive Temperature Coefficient (PPTC).

If there is any fault in the circuit, the current increases and as a result the overall temperature of the Fuse increases. Since it has a Positive Temperature Coefficient, the Resistance of the Fuse Element increases with the rise in temperature (which is caused due to short circuit).

This will limit the current in the rest of the circuit and if the fault is cleared after some time, the temperature drops and the fuse is reset to allow the normal operation of the circuit.

Resettable Fuses are often used in applications where replacing fuses is difficult like in Military Applications or Aerospace Applications.

Thermal Fuses

Thermal Fuses are one – time only fuses and are basically a temperature sensitive fuses. Thermal Fuses are also called as Thermal Links or Thermal Cutouts (TCO). The Fuse element in a Thermal Fuse is made up of a temperature sensitive alloy.

The fuse element in a Thermal Fuse holds a mechanical spring contact, which is normally closed. When the temperature in the fuse element increases (due to over current or surrounding conditions), the fuse element alloy melts and releases the spring mechanism. This will open up the circuit and prevents fire in the device.

Thermal Fuses are available in compact sizes at very low cost making then useful in heat sensitive applications like hair dryers, water heaters, coffee maker machines etc.

Surface Mount Fuses or Chip Fuses

Surface Mount Fuses or SMD Fuses are often used in DC Power Applications like cell phones, Hard Disk Drives, Cameras, DVD Players etc. where space is a constrain. There are different types of Chip or Surface Mount Fuses like

• Fast Acting Chip Fuses
• Very Fast Acting Chip Fuses
• Slow – Blow Chip Fuses
• Pulse Tolerant Chip Fuses
• High Current Rated Chip Fuses
• Telecom Fuses

Automotive Fuses

Fuses play an important role in the electrical connection of an automobile. An over load or short circuit in a car or bike (or any automobile for that matter), can cause a catastrophic damage to the vehicle as well as the individual.

Blade Type fuses are the most frequently used fuses in automotives while other fuses like Glass Tube (or Bosch Fuse), Fuse Limiters etc. are also used.

The voltage ratings of Automotive Fuses will be low when compared to other fuses. Typical voltage ratings are: 12V, 32V and 42V.

Applications of Fuses

Electrical or Electronic Fuses are one of the main components in almost all electrical or electronic circuits, systems and applications. Some of the commonly known applications of Fuses are mentioned below.

• Power Transformers
• Home Electrical Wiring
• All Electrical Appliances (Air Conditioners, Washing Machines, TV, Music Systems, etc.)
• Motor Starters
• Mobile Phones
• Laptops
• Power Adapters
• Cameras
• Printers, Scanners and Photocopiers
• All Automobile (Cars, Bikes, Trucks, Buses, etc.)
• All Electronic Devices (Hard Disks, DVD Writers, DVD Players, etc.)
• Gaming Consoles

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