Fuse and Types of Fuses

What Are the Different Types of Fuses ? . A fuse is probably the simplest electrical device, but its function is critical in protecting electrical circuits from damage. Fuses are found in every circuit in one form or another in various shapes, sizes, and ratings. In this article, we will learn how a fuse works and about the different types of the fuse.

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What is a Fuse and How does a fuse work?

The primary job of a fuse is to break the circuit if a current higher than desired is drawn by the circuit, thus preventing damage due to short circuits.

The simplest kind of fuse consists of a resistive element, selected carefully for its melting point. The working principle of a fuse is as follows, when a current passes through this element, a small voltage drop (small enough so the circuit downstream won’t be affected) is created across the element, and some power is dissipated as heat. The temperature of the element thus increases. For normal currents, this temperature increase is not enough to melt the filament. However, if the current draw exceeds the rated current of the fuse, the melting point is quickly reached. The resistive element melts and the circuit is interrupted. The thickness and length of the resistive element determine the rated current.

Fuse elements are made of zinc, copper, silver, aluminium or other alloys to provide predictable trip currents. The element must not oxidize or corrode over time.

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How to Select Proper Rating Size of Fuse?

While selecting the proper fuse and its rated size for electrical appliances is based on different factors and environments. But the following basic formula shows how to choose the right size of fuse?

Fuse Rating = ( Power / Voltage ) x 1.25

For example, you have to find the right size of fuse for 10A two pin socket.

(1000W / 230V ) x 1.25 = 5.4A

In the above example, 1kW is the power rating which can be controlled through the 2 pin socket and the main supply voltage is single phase 230V AC (120V AC in US).

But you should go for the max i.e. 6A fuse rating instead of 5.4A for safe and reliable operation of the circuit.

Symbol of a Fuse

The standard IEEE/ANSI symbols for the fuse is as follows:

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However, the IEC fuse is slightly different:

Types of Fuses

What is AC fuse and DC fuse?

As the name itself suggests, AC fuses are the ones we use in an AC circuit and the DC fuses are the ones we use in a DC circuit. The AC fuses are commonly rated for 120V or 240V depending on the grid voltage it is used with. they are designed to handle the alternating nature of the AC voltages. Like AC fuses, DC fuses are rated for particular voltage levels, but because DC voltage might be less forgiving than AC voltage, they must be more voltage-specific.

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DC Fuses

1. CARTRIDGE FUSES

This is the most common type of fuse. They are also called glass fuse because the fuse element is encased in a glass envelope that is terminated by metal caps. The fuse element is encased in a glass envelope that is terminated by metal caps. The fuse is placed in an appropriate holder. Since the glass envelope is clear, it is easy to visually determine if the fuse is blown.

There are many variants of this design, including slow blow fuse and fast blow fuse. Slow blow fuses have a larger element that can handle overcurrent for a relatively short period of time and are unaffected by spikes in the appliance. Fast blow fuses react instantly to current spikes.

Some variants of this fuse are encased in ceramic to withstand high temperatures. Fuses for high voltage applications are filled with sand or oil. This is to prevent arcing between the two ends of the fuse after it has blown. SMD variants of cartridge fuses also exist for direct PCB mounting.

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D – Type Cartridge Fuse

It consists of an adapter ring, cartridge, base, and cap. The fuse base is connected to the fuse cap and the cartridge is kept inside the fuse cap through the adapter ring. The circuit gets completed when the tip of the cartridge makes contact with the conductor.

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Link-type Fuse

Link-type cartridge fuses are also known as High Rupturing Capacity fuses (HRC). HRC fuse has a high breaking capacity. It has two metal ends on both sides. The Filling of the fuse with powdered pure quartz acts as an arc extinguishing agent. The fuse element uses silver or copper for its construction.

The fuse element carries the short circuit current for a long period of time. During this time, an uncertain fault will melt and open the circuit. The chemical reaction between a silver vapor and the filling powder forms a high resistance which helps in quenching the arc.

The breaking capacity of the fuse is increased by using two or more silver wire in parallel. This type of fuse is very reliable and divided into two types: Blade type fuses and Bolt down type fuses.

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2. AUTOMOTIVE FUSES

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These fuses are specifically designed for automotive systems that run up to 32V and occasionally 42V. They are also called blade fuse because they come in ‘blade’ form (a transparent plastic envelope with flat contacts) and are colour-coded according to rated current. Some of these types are also used in other high-power circuits. The most common automotive refuse types are micro2, micro3, LP-min(APS), mini (ATM/APM), regular(APR / ATC / ATO / ATS) and maxi(APX). This classification is based on the physical dimensions of the fuse.

3. RESETTABLE FUSES/POLYFUSE 

Like their name suggests, these fuses are self-resetting. They contain carbon black particles embedded in organic polymers. Normally, the carbon black makes the mixture conductive. When a large current flows, heat is generated which expands the organic polymer. The carbon black particles are forced apart, and conductivity decreases to the point where no current flows. Conductivity is restored as temperature decreases. Thus, the fuse does not have to be physically replaced. This kind of fuse is also called a PTC, meaning positive temperature coefficient, since resistance increases with temperature.

PTC Fuse is ubiquitous in computer power supplies and phone chargers. They are particularly handy here since replacement is difficult. For the same reason, they are used in aerospace devices.

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PTCs are easily identified by their yellow-orange colour and disc (and occasionally rectangular) shape in their through-hole variants. SMD poly fuses usually come in green with white markings or black with gold markings. PTCs are available in virtually every current rating.

4. SEMICONDUCTOR FUSES 

The power dissipated by a semiconductor increases exponentially with current flow, and hence semiconductors are used for ultrafast fuses. These fuses are usually used to protect semiconductor switching devices that are sensitive to even small current spikes.

5. OVERVOLTAGE SUPPRESSION

Sometimes voltage spikes can be harmful to circuits too, and often an overvoltage protection device is used with a fuse to protect against both voltage and current spikes.

NTCs (negative temperature coefficient) are placed in parallel with the supply. When the supply voltage spikes, NTC Fuses decrease resistance due to higher current flow and ‘absorb’ spikes.

AC FUSES

HIGH VOLTAGE FUSES:

These fuses are used in high voltage AC transmission lines where voltages can exceed several hundreds of kilovolts. Some of the most popular high-voltage AC fuses are AJT125, ATQR4, TRS150R and AJT50.

HRC (High Rupture Current) fuses: HRC fuses are cartridge type fuses consisting of a transparent envelope made of steatite (magnesium silicate). The fuse is filled with quartz powder (and in the case of a liquid-filled HRC fuses, a non-conducting liquid like mineral oil) that acts as an arc extinguishing agent.

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These fuses are used for very high fault currents.

Expulsion Fuses: These fuses are filled with chemicals like boric acid that produce gases on heating. These gases extinguish the arc and are expelled from the ends of the fuse. The fuse element is made of copper, tin or silver.

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LOW VOLTAGE FUSES: 

These fuses are used in the relatively low voltage distribution networks.

Cartridge fuses: They are very similar to cartridge DC fuses. They consist of a transparent envelope surrounding the fuse element. They can be plugged in (blade type) or screwed into a fixture (bolt type).

Drop out fuses: They contain a spring-loaded lever arm that retracts when a fault occurs and must be rewired and put back in place to resume normal operation. They are a type of expulsion fuse.

Rewireable fuses: They are a simple reusable fuse used in homes and offices. They consist of a carrier and a socket. When the fuse is blown, the carrier is taken out, rewired and put back in the socket to resume normal operation. They are somewhat less reliable than HRC fuses.

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Striker fuse: These fuses are provided with a spring-loaded striker that can act as a visual indicator that the fuse has blown and also activate other switchgear.

Switch fuse: A handle that is manually operated can connect or disconnect high current fuses.

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Thermal Fuses

As mentioned above, thermal fuse is a one time used only fuse. They are temperature sensitive fuse and the fuse element is made of temperature sensitive alloy. They are known as Thermal Cutouts (TCO) or Thermal Links.

In a thermal fuse, the fuse element holds a mechanical spring contact which is normally closed. When high currents due to over current and short circuit flow through the elements of the fuse, the fuse elements melts down which lead to release the spring mechanism and prevent the arc and fire and protect the connected circuit.

SMD Fuses (Surface Mount Fuse), Chip , Radial, and Lead Fuses

SMD Fuses (Surface Mount Device and the name derived from SMT = Surface Mount Technology) are chip types of fuses (also known as electronic fuse) are used in DC power applications like Hard Drive, DVD players, Camera, cell phones etc where space playing an important role because SMD fuses are very tinny in size and hard to replace as well.

Below are some additional types of SMD Fuses and Leaded fuses.

• Slow – Blow Chip Fuses
• Fast Acting Chip Fuses
• Very Fast Acting Chip Fuses
• Pulse Tolerant Chip Fuses
• High Current Rated Chip Fuses
• Telecom Fuses
• Through-hole styles fuses
• Radial Fuse
• Lead Fuse
• Axial Fuse

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Characteristics of a Fuse

Different types of fuses can be categories on the following characteristics.

• Current Rating & Current Carrying Capacity of Fuse
• Voltage Rating of Fuse
• Breaking Capacity of a Fuse
• I²t Value of Fuse
• Response Characteristic
• Rated voltage of Fuse
• Packaging Size

Below is the brief explanation of the above categories.

Fuse Current Carrying Capacity

Current carrying capacity is the amount of current which a fuse can easily conduct without interrupting the circuit.

Breaking capacity:

The value of maximum current that can safely be interrupted by the Fuse is called Breaking Capacity and should be higher than the prospective short circuit current.

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Rated Voltage of Fuse

Expect the current capacity of current, there is the maximum voltage rating a fuse can handle safely. Each fuse has maximum allowed voltage rating, for example, if a fuse is designed for 32 volts it cannot be used with 220 volts, different amounts of isolation is required in different fuses working on different voltage levels. Bases of voltage rating, a fuse can be HV (High Voltage) LV (Low Voltage),  and Miniature Fuses.

 I²t Value of Fuse

The I²t  terms related to fuse normally used in short circuit condition. It is the amount of energy which carries the fuse element when the electrical fault is cleared by the fuse element.

Response Characteristic of a Fuse

The speed at which the fuse blows, depends on the amount of current flowing through its wire. The higher the current flowing through the wire, faster will be the response time.

Response characteristic shows the response time for an overcurrent event. Fuses which respond rapidly to the over current situation is called ultra fast fuses or Fast fuses. They are used in Many semiconductor devices because semiconductor devices are damaged by over current very rapidly.

There is another fuse which Is called a slow burn fuse, switch fuses do not respond rapidly to the over current event, but blow after several seconds of over current occurrence. Such fuses found their application in motor control electronics systems because motors take a lot more current at starting than running.

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Packaging Size

As we have mentioned above, AC and DC fuses have a little bit different packaging type, in the same way different applications require different packages to be used accurately in the circuit.

other factors and parameters are marking, temperature derating, voltage drop and speed etc.

Classification of Fuses

Fuses can be classified as “One Time Only Fuse”, “Resettable Fuse”, “Current limiting and non – current limiting fuses” based on the usage for different applications.

One time use fuses contain a metallic wire, which burns out, when an over current, over load or mismatched load connect event occurs, the user has to manually replace these fuses, switch fuses are cheap and widely used in almost all the electronics and electrical systems.

On the other hand, the Resettable fuse automatically reset after the operation when fault occurs at the system.

In the Current limiting fuse, they produce high resistance for a very short period while the non – current limiting fuse produces an arc in case of high current flow to interrupts and limit the current in related and connected circuits.

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Fuse Applications

Some of the applications of fuse for industrial applications are:

• Used to protect transformers, motors and power system from over-current conditions
• In feeders, power transformers, and solar circuits
•  Electrical appliances and house distribution boards use fuse for domestic purposes.
• Used in automotive cars, electric vehicles, racing cars, rails
• Fuses are present in laptops, hard disk drives, printers/scanners, and electronic devices.
• Used in gaming systems and smartphone

Advantages of Using Fuses

• Cost-Effectiveness: Fuses are affordable and readily available.
• Simplicity: Easy to install and replace.
• Reliability: Provide consistent protection without requiring maintenance.

Limitations of Fuses

Single-Use Nature

Once blown, fuses need replacement, unlike circuit breakers that can be reset.

Limited Protection Against Arc Faults

Fuses may not detect certain faults, such as arcs, which require specialized devices.

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Frequently Asked Questions (FAQs)

What is the main function of a fuse?
A fuse protects electrical circuits from overcurrent and prevents damage to devices.

How do fuses differ from circuit breakers?
Fuses are single-use devices, while circuit breakers can be reset after tripping.

What are the most common types of fuses?
Cartridge, blade, and plug fuses are among the most commonly used types.

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