A Comprehensive Guide to Fuses

Figure 1. Various Fuses
Source: Littelfuse

Fuses are components that are used to protect electrical systems and devices from damage caused by overcurrent or short circuits. They are an important part of most electrical systems. When choosing fuses, it is important to consider the following factors:

Current rating: The current rating of a fuse is the maximum amount of current it can safely carry.

Voltage rating: The voltage rating of a fuse is the maximum voltage it can safely handle.

Breaking capacity: The breaking capacity of a fuse is the amount of current that it can safely interrupt in the event of an overcurrent or short circuit.

Time-delay: Some fuses are designed with a time-delay feature, which allows them to carry current for a short period of time before tripping. These fuses are useful in applications where temporary overcurrents or inrush currents are expected, such as when starting a motor.

Type of fuse: There are several types of fuses available, including cartridge fuses, plug fuses, and rewirable fuses. Cartridge fuses are the most common and are available in a wide range of current and voltage ratings. They are easy to install and replace, and are ideal for residential and light commercial applications. Plug fuses are also common and are used in older homes and buildings. They are easy to install and replace, but are not as common as cartridge fuses. Rewirable fuses are used in industrial and commercial applications and are ideal for high-current applications. They are more difficult to install and replace, but offer the advantage of being able to be rewired if the fuse blows.

Environmental conditions: Fuses are rated for specific environmental conditions, such as temperature and humidity.  Some electronics need to function in harsh environments and proper fuses must be selected to support that.

How does a fuse work?

A fuse consists of a fuse element (a metal wire or filament) that is encased in a housing. When the current flowing through a circuit is within the rated value of the fuse, the fuse element remains intact and allows the current to flow through the circuit. If an overcurrent condition occurs, such as a short circuit or an overloaded circuit, the increased current causes the fuse element to heat up. If the current exceeds the current rating, the fuse element melts, breaking the circuit and stopping the flow of current.

By breaking the circuit, the fuse prevents damage to the equipment or creating a fire hazard. Once a fuse has "blown," it must be replaced before the circuit can be restored.

A fuse will only work if the current rating is appropriate for the circuit. If the fuse rating is too low, it will blow too frequently, if it's too high, it won't protect the circuit from overcurrent.

Do all fuses work in the same way?

Most fuses have a similar method of operation, with the fuse element designed to melt or "blow", however there are other devices that use different methods to accomplish this.

Resettable fuses, also known as polymeric positive temperature coefficient (PPTC) fuses, use a special polymer that changes its resistance when heated, this causes the fuse to trip, once the device cools down the resistance goes back to normal and the fuse is reset.

Circuit breakers are another type of overcurrent protection device that work differently from fuses. Circuit breakers use an electromechanical mechanism to open or close the circuit, this allows the circuit breaker to be reset and reused multiple times, unlike a fuse which has to be replaced.

What types of fuses do I need for various applications?

The type of fuse you need will depend on the specific application. These are some common types of fuses and their typical applications:

Fast-acting or standard fuses are the most common type and are used in a wide range of applications, including homes, businesses, and industrial settings. They have a quick response time and are designed to protect against short-circuits and overcurrents.

Slow-blow or time-delay fuses are designed with a time-delay feature, which allows them to carry current for a short period of time before tripping. They are used in applications where temporary overcurrents or inrush currents are expected, such as when starting a motor.

High-speed or semiconductor fuses are designed to protect semiconductor devices such as diodes, transistors, and thyristors. They have a very fast response time and are used in electronic and industrial applications.

High-voltage fuses are designed to handle high voltage and are used in electrical distribution systems, power plants, and other industrial applications.

Current-limiting fuses are designed to limit the amount of current that can flow through the circuit in the event of a short circuit.  Current-limited fuses are designed to open in less than half a cycle. They are used in applications where it is important to minimize damage to equipment and structures in the event of a fault.

Low-voltage fuses are designed to handle low voltage and are used in applications such as lighting and control circuits. Low voltage fuses are any fuses rated less than 1500V.

AC or DC Fuses are designed specifically for use in either AC or DC circuits.

How do you check if a fuse is still working?

There are a few different ways to check if a fuse is still working. Many fuses can be inspected visually to determine its state. If the filament inside looks intact and not melted, the fuse is likely working. If the filament is melted or broken, the fuse is likely blown and needs to be replaced.

Figure 2. A good fuse vs. a blown fuse
Source: University of Illinois

One of the most common methods is to use a multimeter set to the "continuity" or "resistance" setting. To use this method, first turn off the power to the circuit. Then, remove the fuse from the holder and touch the multimeter probes to the metal ends of the fuse. If the multimeter displays a low resistance reading, the fuse has not blown. If the multimeter displays "infinity" or no reading at all, the fuse is likely blown and needs to be replaced.

Another method is using a continuity tester, a simple device that indicates when the circuit is closed. To use this method, first turn off the power to the circuit. Remove the fuse from the holder and touch the continuity tester leads to the metal ends of the fuse. If the continuity tester lights up or beeps, the fuse is working. If the continuity tester does not light up or beep, the fuse is likely blown and needs to be replaced.

How do you replace a fuse?

Replacing a fuse is typically a relatively simple process, but care must be taken to ensure safety.

• Turn off the power.
• Locate the fuse holder or fuse box.
• Remove the old fuse from the fuse holder. This is usually done by grasping the fuse and pulling it straight out.
• Inspect the fuse element visually to determine if the fuse is blown. Sometimes this determination cannot be made visually and other testing devices (such as a multimeter) must be used (see previous section).
• Choose a replacement fuse with the appropriate current and voltage rating for the application.
• Insert the new fuse into the fuse holder, making sure it is secure and with the metal ends are making good contact.
• Turn the power back on to the circuit and test the electrical system.

Make sure to follow the manufacturer's instructions when replacing a fuse. If you are unsure about how to replace a fuse, contact a qualified electrician for assistance.

What is a resettable fuse?

A resettable fuse, also known as a polymeric positive temperature coefficient (PPTC) fuse or a multimeter fuse, is a type of fuse that is able to automatically reset itself after it has tripped. Resettable fuses use a special polymer that changes its electrical resistance in response to changes in temperature. When the current through the fuse exceeds its rated value, the heat generated by the current causes the polymer to expand, increasing its resistance and reducing the current flowing through the fuse. This causes the fuse to open the circuit. Once the current is reduced, the polymer cools down and contracts to its original size, resetting the fuse and allowing the current to flow through the circuit again.

Figure 3. Resettable fuses

Resettable fuses are used in applications where repeated overcurrents or short circuits are likely to occur, such as in electronic devices and appliances. They are also used in industrial, automotive and medical applications.

What are the some of the different types of fuses?

Figure 4. Cartridge fuses

Cartridge fuses are the most common type of fuses and are available in a wide range of current and voltage ratings. They are easy to install and replace, and are well-suited for residential and light commercial applications. They are cylindrical in shape and have metal or ceramic end caps.

Figure 5. Plug fuses

Plug fuses are also common, and typically used in older homes and buildings. They have a similar shape to cartridge fuses but have a different mechanism to install and remove them.

Resettable fuses, also known as polymeric positive temperature coefficient (PPTC) fuses or multimeter fuses, use a material that changes its electrical resistance in response to changes in temperature. When the current through the fuse exceeds the rated value, the heat generated by the current causes the polymer to expand, increasing its resistance and reducing the current flowing through the fuse.

Figure 6. Automotive Fuses

Automotive fuses are specifically designed for use in automobiles and are available in a wide range of current and voltage ratings.

Low-voltage fuses are specifically designed for use in low-voltage electrical systems, such as telecommunications systems, security systems, and control systems.

Figure 7. Circuit Breakers

Circuit breakers are an alternative to fuses, and are designed to break a circuit automatically when the current exceeds a certain limit. Circuit breakers can be reset, and do not need to be replaced after tripping. They are available in different types like magnetic and thermal-magnetic circuit breakers.

Frequently Asked Questions

What do fuses do?

Fuses are devices that protect electrical systems and devices from damage caused by overcurrent or short circuits. They are an essential component in any electrical system. Fuses work by interrupting the flow of current in the circuit when it exceeds a certain limit, known as the fuse's rating. Fuses come in different types and ratings; the proper fuse must be selected for each application.

How can you tell if a fuse has blown?

There are a few ways to tell if a fuse has blown. Details are in the “How do you check if a fuse is still working?” section above.

What happens if you don't fix a blown fuse?

If a blown fuse is not replaced, the circuit stays open; no electrical current will be able to flow through it. The device or equipment connected to the circuit will not work properly or at all. Blown fuses should be replaced as soon as possible.

What is the main reason for a blown fuse?

The main reason for blown fuses is overcurrent, which occurs when the electrical current flowing through a circuit exceeds the rated current of the fuse. This can happen for a few reasons:

• Overloaded circuit: When too many devices are connected to a circuit, the total current can exceed the rated current of the fuse.
• Short circuit: A short circuit occurs when the electrical current flows through an unintended path, bypassing the intended load and creating an excessive current. This can happen due to a malfunction or damage.
• Inrush current: An inrush current is a large initial current that flows through a circuit when a device is first turned on. Examples include when motors start up, or when a transformer is energized.
• Aging of the fuse: A fuse may become less effective over time and may blow even if the current is not excessive.

Is it expensive to fix a fuse?

Fuses are typically inexpensive. The cost of replacing a fuse will depend on the type of fuse, the current and voltage rating, and the quantity of fuses that need to be replaced. Standard household fuses range in cost from a few cents to a few dollars. Costs increase for more specialized types of fuses. For fuses that are difficult to replace, the cost of hiring a professional must be factored in.

How do you reset a blown fuse?

A blown fuse cannot be reset; it must be replaced. When a fuse is "blown," the metal filament inside the fuse has melted, breaking the circuit and preventing any current from flowing through it. Once this happens, current can no longer run through the fuse. Note: there are types of fuses that this does not apply to; see the next question for an example.

Are there fuses that can be reset?

Resettable fuses, also known as polymeric positive temperature coefficient (PPTC) fuses, use a polymer that changes its electrical resistance in response to changes in temperature. If the current exceeds the fuse’s rating, the heat generated by the current causes the polymer to expand, increasing its resistance and protecting the electrical system from damage. When the polymer cools down, current is able to flow through the fuse again.

Is a circuit breaker different from a fuse?

A circuit breaker and a fuse are both devices used to protect electrical systems from damage caused by overcurrent or short circuits, but they work in slightly different ways.

A fuse is a device that is designed to "blow" or interrupt the flow of current in the circuit when the current exceeds a certain limit. Once a fuse has "blown," it must be replaced with a new one.

A circuit breaker is a device that is designed to "trip" or open the circuit when the current exceeds a certain limit. Unlike a fuse, a circuit breaker can be reset after it has tripped, allowing the current to flow through the circuit again. Circuit breakers typically work via electromagnetism. A simple circuit breaker works as follows: the current running through the system also energizes an electromagnet. The greater the current, the greater the strength of the magnet. At unsafe levels, the magnet is strong enough to pull down the metal lever connected to the switch linkage, shifting the linkage, and breaking contact and cutting power.

What happens when a fuse goes bad?

When a fuse goes bad, it can no longer protecting a circuit from overcurrent or short circuits. This can happen for several reasons, such as age, environmental conditions, or excessive heat.

How long should a fuse last?

The lifespan of a fuse can vary depending on the type of fuse, the current and voltage rating, and environmental conditions. Some types, such as cartridge fuses and plug fuses, have a relatively short lifespan and may need to be replaced after a few months or a year. A typical lifespan for a standard household fuse is around 3 to 5 years. Resettable fuses have a longer lifespan, but have a limited number of cycles before they need to be replaced.

Fuses should be checked periodically and replaced if they are showing signs of wear.

How do I choose a fuse?

There are different types of fuses available, such as cartridge fuses, plug fuses, resettable fuses, automotive fuses, and low-voltage fuses. The specific needs and requirements of the application will dictate which type of fuse to use. The criteria for choosing the fuse include:

• Current rating: the maximum amount of current that the fuse can safely handle
• Voltage rating: maximum amount of voltage that the fuse can safely handle
• Time-current characteristics: the fuse's ability to interrupt the current at a specific time and current level
• Size and shape: the fuse must fit properly in the holder and should not interfere with other devices or equipment.
• Standards and certifications: a fuse should meet relevant safety and industry standards.

A fuse should also have the ability to withstand the environmental conditions where it will be installed, including harsh environments, such as high temperatures or vibration.

What are some of the relevant safety standards related to fuses?

There are several safety standards related to fuses, including:

• UL (Underwriters Laboratories): UL is an independent, non-profit organization that tests and certifies products. Fuses are covered by UL standard 248-14.
• CSA (Canadian Standards Association): CSA is a Canadian organization that tests and certifies products. Fuses are covered by CSA standard C22.2 No. 248.14-14.
• IEC (International Electrotechnical Commission): IEC is an international organization that develops and publishes international standards for electrical and electronic equipment and systems. Fuses are covered in IEC standard 60269.
• RoHS (Restriction of Hazardous Substances) is a directive established by the European Union, which restricts the use of certain hazardous substances in the production of electronic equipment.
• CE (Conformité Européene) is a mark that indicates that a product has met EU safety, health, and environmental protection requirements.

Fuses from Littelfuse 

Littelfuse offers a variety of fuses for a wide range of applications.