Guide to Types and Selection of Transformer Fuses

1.1 How does a fuse work

• When the transformer encounters dangerous overcurrent or high temperature, the fuse operates and cuts off the circuit, isolating the transformer from the system.

1.2 Types of Fuses

1.3 Protection Range

• Transformer overload: A short circuit occurs when a live part of a circuit comes into contact with another live part or a conductive object. When this situation occurs, the current entering the circuit from the power supply is abnormally large. It is precisely this large current that causes the fuse of the transformer to blow. Short circuits can be caused by many reasons, such as equipment failure, rodents biting through wires, damaged wires or aging/degraded insulation between nearby conductors. Most short circuits occur inside or around equipment powered by transformers, such as faulty motors and faulty devices.

• External short circuit: When an internal transformer malfunctions, such as insulation breakdown between windings, it may cause a short circuit or ground fault, thereby allowing current to flow through the transformer windings. If the fuse of the transformer is working properly, it will detect this excessive current and blow immediately, disconnecting the transformer from the power supply and preventing further damage to the transformer and surrounding equipment.

• Internal short circuit: The transformer is designed to operate within a certain range of power loads. If the load exceeds the overload limit allowed by the transformer, excessive current may cause the fuse to blow. Overload may be caused by connecting too many electrical devices to the transformer or a sudden surge in power demand.

1.4 Selection of Fuses

• The size of a transformer fuse is based on several factors, including: rated voltage, inrush current of the transformer, rated current of the transformer and the available short-circuit current in the electrical system.

• The first step in determining the fuse size of a transformer is to determine the rated voltage of the transformer. This is usually specified by the manufacturer to select a fuse with an appropriate rated voltage.

• After choosing a fuse type that is compatible with the rated voltage of the transformer, a special fuse needs to be selected to accommodate the initial inrush current of the transformer (the rapid current spike that the fuse will observe when the transformer is first powered on). This is accomplished by selecting a fuse with a TCC(Time-Current Characteristic) curve, which is located on the right side of the transformer’s surge curve.

• The next step is to determine the rated current of the transformer. This is the maximum current that the transformer can withstand under normal working conditions. The rated current of the fuse must be equal to or greater than the rated current of the transformer.

• Finally, the available short-circuit current in the electrical system must be taken into account. This is the maximum current that can flow through the system when a fault (such as a short circuit) occurs. The fuse must be capable of safely interrupting the current to protect the transformer and other equipment in the system.

• It is important to note that the correct size of the transformer fuse is crucial for ensuring proper protection of the transformer and the electrical system. A fuse that is too small may not fully protect the transformer, while a fuse that is too large may not interrupt the current quickly enough, causing damage or malfunction to the transformer or other equipment in the system.

2.1 Definition

The Bay-o-net fuse is a protective device used for three-phase oil-immersed transformers, mainly to prevent damage to the transformer caused by overcurrent and high temperature. It protects the safety of the equipment by fusing and cutting off the faulty circuit.

2.2 Principle

• Dual-sensitive fuse

(Dual-Sensitive Fuse) is a specially designed fuse that can be sensitive to two different conditions, namely current and temperature. This enables dual-sensitive fuses to offer more comprehensive protection, especially suitable for equipment that needs to deal with various fault conditions such as overload, low-level short circuits and high temperatures

• Current sensitivity

Fuses can respond to current overload or short circuit situations. If the current exceeds the preset rated value, the fuse will melt rapidly to prevent damage to the power system.

• Temperature sensitivity

When the temperature of the equipment or system exceeds the safe range, the fuse will melt due to excessive temperature, providing overheat protection. This is very important when overloaded for a long time.

• Arc extinguished

When the fuse melts, an arc is generated inside. The vented fuse quickly extinguishes the arc through its specially designed lining and gas generation mechanism.

• “Gas emission”

During the melting process, the exhaust fuse releases gas. By rapidly expelling the gas, it reduces the internal pressure and heat, preventing the danger caused by the melting.

2.3 Structure

• End plug

This is the end part of the fuse, which is used to seal the entire fuse assembly. The end plug can ensure that the fuse has good contact with the equipment during connection and provide sealing to prevent external dust and moisture from entering.

• Cartridge

This is the main housing that houses the fuse, usually made of sturdy materials, and is used to protect the internal fuse elements. It can withstand high temperatures and the impact of electric arcs, ensuring that the fuse safely releases energy when it blows.

• Dual Sensitive Expulsion fuse

It has the characteristics of heat sensitivity and current sensitivity. When the fuse melts, an electric arc will be generated. The interior of an exhaust fuse usually contains special linings or materials, which generate gas when interacting with the heat produced by the arc. Gas rapidly fills the fuse housing and effectively extinguishes the arc, preventing its continuous generation and spread.

• Inner holder

This part is used to fix and support the fuse element, enabling it to remain stable inside the fuse. The inner shell also ensures the reliability and safety of the fuse during operation.

3.1 Definition

• The current-limiting fuse of a three-phase pad mounted transformer is an electrical protection device designed to quickly limit the current passing through in the event of a short circuit or overload, thereby protecting the transformer and its connected equipment from damage. Its main function is to melt when the current reaches the preset threshold, preventing excessive current from exerting pressure on the electrical system and posing potential fault risks.

3.2 Principle

• Short-circuit current induction

When a short circuit or overload occurs, the current will rise rapidly. The fuse element (fuse wire) of the backup current-limiting fuse will quickly sense this abnormal situation according to the magnitude and duration of the current.

• The fuse has blown

When the current exceeds the rated value of the fuse, the fuse will rapidly heat up to the melting point and melt under the action of the high current. This process is very rapid to prevent the fault current from continuing to flow through the equipment.

• Current-limiting effect

The design of the backup current-limiting fuse is intended to limit the short-circuit current passing through. The fuse and filling materials (such as quartz sand) inside it help absorb arc energy, extinguish the arc quickly, and prevent the current peak from rising further.

• Extinguish the arc

When the fuse blows, a fault arc will be generated inside the fuse. The filling material in the fuse (usually quartz sand) can help absorb the heat of the arc, quickly extinguish the arc, prevent the arc from continuing to spread, and ensure that the circuit is completely disconnected.

• Protection range

Backup current-limiting fuses are mainly used for high-current short-circuit faults. Their design purpose is to provide the last line of defense for equipment, preventing large-scale faults from causing further damage to the system and equipment. It is usually used in conjunction with other fuses or protective devices, such as plug-in fuses.

4.1 Function

• Safe isolation

The isolation connection piece can safely cut off the power supply when the fuse needs to be replaced, ensuring that maintenance personnel will not face the risk of electric shock when replacing the fuse.

• Quick replacement

When using an insertable fuse, the isolation connection piece can quickly cut off the current, making the fuse replacement process faster and safer.

• Protect electrical equipment

In the event of a fault, the plug-in fuse will melt rapidly, while the isolation connection piece can help cut off the power supply and prevent the current from continuing to flow to the damaged equipment.

• Clear indication

Isolated links usually have clear status indications, showing whether the device is in an isolated state, thereby enhancing operational safety.

• Design

This is an internal “weak link” exhaust fuse, specifically designed for use in transformer oil or similar applications.

• Protection function

It can protect the power distribution system from damage caused by transformer faults and prevent transformers from suffering excessive damage due to overload or fault conditions.

• Dual sensing function

This fuse can not only sense the internal fault current but also the temperature of the transformer oil, thereby limiting the prolonged heating caused by overload and high-temperature environments.

• Two-level protection system

It can be used alone or in combination with a current-limiting fuse to deal with high current faults.

This type of fuse provides an economical and efficient protective measure, and is particularly suitable for high-voltage side applications of distribution transformers.