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What are bayonet fuses, and why are they important?

Bayonet fuses play a crucial role in protecting transformers as well as preventing damage to electrical systems. Learn what they are and why they are important.

Written by:
Mac Spiller & Ben Gulick

May 2, 2023

Maddox transformer tech pointing out bayonet fuses in padmount transformer

Padmount transformers are essential components of underground electrical distribution systems, and protecting them from damage caused by overcurrents is critical to ensure the reliability and safety of the system. Transformer fuses play a crucial role in protecting transformers from overcurrents and short circuits, as well as preventing damage to the electrical system from failed transformers.

In this article, we will explore the topic of transformer fusing, with a focus on bayonet transformer fuses. We will discuss what transformer fusing is, how it works, and we will also review the most common types of bayonet transformer fusing setups, providing insights into their unique features and benefits.

What are transformer fuses

Transformer fuses are electrical protection devices that guard the electrical system and/or transformer against destructive overcurrents.

How do transformer fuses work

When exposed to dangerously high currents or temperatures, fuses operate (blow) and break the circuit, isolating the individual transformer from the system.

Why do transformer fuses blow

Transformer fuses can blow due to a number of problems in the electrical system. Some of the leading causes of transformer fuses blowing are listed below:

Overloading a transformer

Transformers are designed to operate within certain limits of an electrical load. If the load exceeds allowed overload limits of the transformer, the excessive current flow can potentially cause the fuse to blow. Overloading can be caused by connecting too many electrical devices to a transformer, or by a sudden surge in demand for power.

External short circuits

Short circuits occur when a live part of a circuit gets in contact with another live part or with a conductive object it is not supposed to. When this happens, an unusually high amount of current is drawn from the power source into the circuit. It is this high flow of current which causes a transformer fuse to blow.

Short circuits can be caused by many things such as faulty equipment, rodents chewing through wires, damaged power lines, or aged/degraded insulation between nearby conductors. Most short circuits occur in or around the equipment the transformer is feeding–such as malfunctioning motors and faulty devices.

Internal short circuits / Transformer failure

When an internal transformer failure occurs, such as a breakdown in the insulation between the windings, it can cause a short circuit or a ground fault, which results in an excessive current flow through the transformer windings. If the transformer fuses are functioning correctly, they will detect this excess current and immediately blow, disconnecting the transformer from the electrical supply and preventing further damage to the transformer and surrounding equipment.

multiple transformer bayonet fuses on a table

How transformer fuses are sized

Transformer fuses are sized based on several factors including: rated voltage, transformer inrush current, transformer rated current, and the available short circuit current in the electrical system.

The first step in sizing a transformer fuse is to determine the rated voltage of the transformer. This is typically specified by the manufacturer and is used to select a fuse with a suitable voltage rating.

After selecting a fuse type which is compatible with the transformer’s rated voltage, a particular fuse will need to be selected which accommodates the transformer’s initial inrush current (a quick spike of current the fuse will see when the transformer is first energized). This is accomplished by selecting a fuse with a TCC (time current characteristic) curve which sits to the right of the transformer’s inrush curve.

The next step is to determine the rated current of the transformer. This is the maximum current that the transformer is designed to handle under normal operating conditions. The fuse must be selected to have a current rating that is equal to or greater than the rated current of the transformer.

Finally, the available short circuit current in the electrical system must be considered. This is the maximum current that can flow through the system in the event of a fault, such as a short circuit. The fuse must be able to safely interrupt this current to protect the transformer and other equipment in the system.

It's important to note that sizing a transformer fuse correctly is critical for ensuring proper protection of the transformer and the electrical system. A fuse that is too small may fail to protect the transformer adequately, while a fuse that is too large may not interrupt the current quickly enough, leading to damage or failure of the transformer or other equipment in the system.

What are Bayonet Fuses

Bayonet fuses are widely used in padmount transformers and are considered the industry standard for transformer fusing. These fuses are a type of expulsion fuse, which is commonly used in electrical distribution systems on utility poles and inside transformers.

Expulsion fuses, including bayonet fuses, have a thin, fusible element that melts when subjected to high current or temperatures. When the element melts, it creates an arc inside the fuse housing. The arc heat interacts with a special lining in the housing, generating gasses that fill the housing and quickly extinguish the arc, and prevent current flow.

Bayonet fuses are designed to handle most common electrical distribution system issues, such as overheating, overloading, secondary faults, and low level short circuits which might damage the transformer depending on their magnitude and duration. The cause of these issues can typically be identified, and resolved by experienced electrical workers, and (if quickly cleared by the bayonets) do not often cause lasting damage. Once the issue is resolved, the bayonet fuse is replaced, and the transformer can be energized.

The assembly of a bayonet fuse with inner holder, expulsion fuse, cartridge, and end plug

Why bayonet fuses are commonly used

Bayonet fuses are a popular transformer fuse choice because they are cost-effective and easy to replace in the field.

The actual fuse part of the assembly is located inside a cartridge at the end of a tube which extends down into the transformer tank beneath the oil and can be removed like a “bayonet” from outside the transformer.

Bayonet Fuse in a pad mount transformer being removed and replaced

Bayonet fuse replacement

Coordination with Current Limiting Fuses and Isolation Links

Bayonet fuses are an important part of the protection scheme in padmount transformers, but they are not used as the sole means of protection. Instead, they are usually connected in series with backup current-limiting fuses or isolation links to provide comprehensive protection.

Backup Partial Range Current Limiting Fuse

If the transformer could potentially see high fault currents exceeding the rating of the bayonet fuse, backup partial range current limiting fuses (or simply “CLFs”) are added to the scheme to provide a full range of protection.

CLFs interrupt dangerous fault currents by forcing the fault current through several sand-filled compartments that quickly stop the current flow.

It should be noted that CLFs are not intended to protect the transformer itself but rather to prevent electrical fires and explosions that could be caused by a failing transformer. Once a CLF operates, it is almost certain that the transformer has failed.

As its name suggests, the CLF is a "backup" fuse that only covers a "partial range" of overcurrent, specifically the very high short circuit type of current. Its job is to handle what the bayonet cannot. You can think of the CLF as the "pinch hitter" on the team, as the bayonet fuse interrupts lower-level overload currents at the transformer, and the CLF is only there for very high short circuit currents. Together, the bayonet fuse and CLF fuses provide a full range of protection at the transformer.

What is transformer full range protection?

Full range is a protection coordination concept made with one or more protection devices (like fuses or breakers) to cover the 3 over-current areas of the transformer (overloads, external secondary short circuits, and internal short circuits).

Transformer backup current limiting fuse connected to bayonet fuses

Isolation Links

In cases where the transformer is relatively small and low voltage, and the available short circuit current is also relatively low, bayonet fuses by themselves may be able to provide a sufficient range of overcurrent protection without the addition of CLFs. In these instances, isolation links must be used in conjunction with bayonet fuses to create a physical break in the circuit in the event of a transformer failure to prevent electrical workers from mistakenly re-energizing a failed transformer by replacing the bayonet fuse.

Transformer isolation links comparing a new isolation link with a melted isolation link

Isolation links are not fuses. Like fuses, they create a break in the circuit, but they have a key difference. Fuses have the ability to safely interrupt a circuit during a fault to prevent excessive current flow from damaging components in the system, which is known as interrupting capacity. On the other hand, isolation links do not have this ability, so they cannot function as an additional overcurrent protection device like a bayonet fuse. The isolation link's sole purpose is to melt during a transformer failure, effectively isolating the transformer from the rest of the system and preventing re-energization. Therefore, isolation links cannot replace fuses as a means of overcurrent protection. For this reason, when isolation links are used in place of backup CLFs, the rated fault current must be less than or equal to the expulsion fuse's interrupting capacity (AIC rating).

Transformer design engineers carefully coordinate protection devices to ensure that the system is protected from a full range of overcurrent conditions while ensuring the reliable operation of the transformer. In summary, a well coordinated fusing scheme in padmount transformers provides comprehensive protection, with each protection device playing a unique role in protecting the transformer and the electrical system.

Transformer bayonet fuses with isolation links diagram


Transformer fuses play a critical role in protecting electrical systems. Contact us if you have any questions about transformer fusing or if your project requires a specific fusing configuration.

Maddox padmount transformer loaded on truck

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