Wiktor Wise
In electric power transmission, a bushing is a hollow electrical insulator that allows a conductor to pass safely through a conducting barrier, such as a transformer or circuit breaker, without making electrical contact. Bushings are typically made from porcelain, although other insulating materials like resin, paper, and polymer are also used. They are necessary to control the shape and strength of the electric field and reduce electrical stress in the insulating material. Providing insulated bushings or smooth, rounded entries at raceway and cable terminations reduces the risk of insulation failure. They are required when raceways contain conductors of 4 AWG or larger to protect the conductors from abrasion.
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What You'll Learn
Required for #4 AWG and larger conductors
Insulated bushings are required for #4 AWG and larger conductors to ensure electrical safety and maintain the integrity of the wiring system. Insulated bushings provide a protective barrier between the conductor and its surroundings, preventing accidental contact and potential electrical hazards. This is especially important for larger conductors, which carry higher electrical currents and pose greater risks in the event of a short circuit or insulation failure.
The purpose of insulated bushings is twofold. Firstly, they offer mechanical protection to the conductor, providing support and strain relief at the point of entry or exit from a piece of electrical equipment, such as a switchgear or transformer. Secondly, and most crucially for larger conductors, they ensure electrical insulation by creating a secure barrier that separates the conductor from its surrounding environment, including any nearby conductive components or enclosures.
In the case of #4 AWG and larger conductors, the use of insulated bushings is mandated by electrical safety standards and codes. These standards recognise the increased risk associated with larger conductors and aim to prevent electrical accidents, fires, or equipment damage. By requiring insulated bushings, the risk of arcing, short circuits, and potential electrical fires is significantly reduced.
The specific requirements for insulated bushings may vary depending on local electrical codes and standards, as well as the specific application and environment in which the conductors are being used. For instance, installations in wet or corrosive environments may necessitate additional sealing or specialised materials to maintain insulation integrity and protect against moisture ingress.
It is important to consult relevant electrical codes and seek guidance from qualified professionals when installing or working with #4 AWG or larger conductors. This ensures compliance with safety regulations and helps maintain a reliable and safe electrical system. Proper selection, installation, and maintenance of insulated bushings are vital to prevent hazards and ensure the long-term performance of electrical systems utilising larger conductors.
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Used with threaded rigid metal conduit
Insulated bushings are required when using threaded rigid metal conduits in electrical installations to provide a secure and insulated passage for conductors. Bushings are essential in these applications for several reasons.
Firstly, they act as a protective barrier between the conductor and the conduit, ensuring electrical insulation. This is particularly important in threaded conduits, where the potential for electrical contact between the conductor and the conduit is higher due to the threaded connections. The bushing's insulation properties prevent electrical leakage, arcing, or short circuits, enhancing the overall safety of the installation.
Secondly, bushings facilitate smooth conductor pulling. When conductors are pulled through a threaded conduit, bushings help reduce friction and provide a smooth surface, minimising the risk of damage to the conductor insulation. This is crucial during the installation process to ensure that conductors are installed safely and efficiently without compromising their integrity.
Additionally, insulated bushings play a vital role in maintaining the integrity of the conduit system. They help seal the conduit, preventing moisture ingress and protecting the conductors from corrosion and other environmental factors. This is especially important in outdoor applications or environments with high humidity, where moisture can be a significant concern.
Moreover, bushings provide a secure and finished appearance to the installation. They offer mechanical protection at conduit entry and exit points, ensuring that conductors are securely held in place and protected from sharp edges. This not only enhances the overall aesthetics of the installation but also ensures compliance with electrical safety standards and regulations.
When selecting insulated bushings for use with threaded rigid metal conduits, it is essential to consider the specific application requirements, including conduit size, conductor size, and environmental conditions. It is also important to follow local electrical codes and manufacturer guidelines to ensure proper selection, installation, and maintenance of insulated bushings, ensuring a safe and reliable electrical installation.
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Control of electric stress fields
Electric stress fields are a critical consideration in the design of high-voltage equipment, such as bushings. The control of these fields is essential to prevent insulation failure and ensure the reliable operation of electrical equipment.
In the context of electrical bushings, electric stress fields arise due to the presence of an energized conductor near a material at earth potential. This can result in very high electric field strengths, particularly where the field lines curve sharply around the earthed material. To address this, bushings are designed to control the shape and strength of the electric field, thereby reducing electrical stresses on the insulating material.
The electric field optimization techniques aim to minimize stresses and prevent issues like corona discharge. One method involves using the charge simulation method (CSM) to create an overall equipotential surface, within which finite element analysis (FEA) is applied to study localized field enhancement effects. This hybrid approach offers benefits over traditional methods.
Additionally, the use of condenser bushings can significantly reduce electric stress fields. These bushings employ metallic foils inserted during the winding process, creating a capacitive effect that evenly distributes electrical energy within the insulated paper. This design stabilizes the generated electrical fields and reduces stress between the energized conductor and any earthed material.
Furthermore, the choice of insulating material is crucial for controlling electric stress fields. While paper insulation was traditionally used, resin-impregnated paper bushings have gained popularity due to their extended application in Extra High Voltage (EHV) and Ultra High Voltage (UHV) systems. Resin insulation offers greater dielectric strength and requires less stress control at voltages below 25 kV. However, at voltages above 12 kV, even resin bushings may require stress control screens to manage the external electrical field and internal energy storage.
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Required for voltages above 12 kV
In electric power applications, bushings are hollow electrical insulators that enable an electrical conductor to pass safely through a conducting barrier, such as a transformer or circuit breaker, without making electrical contact. Bushings are typically made from porcelain, but other insulating materials like resin, paper, and polymer are also used. The choice of material depends on the voltage level and the specific requirements of the application.
When it comes to voltages above 12 kV, insulated bushings are specifically required to control the external electrical field and ensure effective energy distribution. At higher voltages, the dielectric strength of paper insulation, which is commonly used in bushings, becomes marginalised. To address this, metallic foils can be inserted during the winding process to create a condenser/capacitor bushing.
The condenser bushing design involves inserting thin layers of metallic foil into the paper insulation. This addition produces a capacitive effect, which helps dissipate the electrical energy more evenly throughout the insulated paper. Consequently, it reduces the electric field stress between the energised conductor and any earthed material. This stabilisation of electrical fields and homogenisation of internal energy are crucial for safe and reliable performance in high-voltage applications.
It is important to note that the use of resin-insulated bushings in high-voltage applications above 12 kV also has its considerations. While resin insulation offers greater dielectric strength than paper, certain switchgear designs with closer proximity to earthed materials may require stress control screens in resin bushings operating at voltages as low as 12 kV. Therefore, careful design and material selection are essential to ensure the electrical strength of the insulated material can withstand the electrical energy passing through the conductor.
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Used with EMT connectors
Insulated bushings are required when using EMT connectors in a variety of electrical applications to ensure safety and maintain the integrity of the electrical system. EMT, or electrical metallic tubing, is a thin-walled conduit used to enclose and protect electrical conductors and cables. It is commonly used in commercial and industrial buildings due to its durability and ease of installation.
When EMT connectors are used, insulated bushings play a critical role in providing a secure and insulated passage for conductors entering or exiting the tubing. These bushings are typically made of a durable insulating material, such as rubber or plastic, and are designed to fit snugly within the connector. They create a protective barrier between the conductor and the metal surface of the EMT, preventing electrical contact and potential short circuits or electrical hazards.
The use of insulated bushings with EMT connectors is particularly important in applications where moisture or corrosive elements may be present. By creating a tight seal, the bushings help prevent water ingress, condensation, or contaminants from entering the conduit system, which could lead to electrical faults or corrosion of the conductors. This is especially critical in outdoor installations, areas with high humidity, or environments with harsh chemicals or pollutants.
Additionally, insulated bushings provide strain relief and protect conductors from sharp edges during pulling or installation. They ensure that the conductors do not come into direct contact with the connector or conduit, reducing the risk of insulation damage or conductor abrasion. This is crucial for maintaining the integrity of the electrical insulation and preventing potential faults or short circuits over time.
It is important to note that the specific requirements for using insulated bushings with EMT connectors may vary depending on local electrical codes and standards. These codes outline the necessary safety precautions and guidelines for electrical installations, ensuring compliance with industry best practices. It is always recommended to consult these codes and seek guidance from qualified electrical professionals to ensure that installations meet the necessary safety standards and regulations.
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Frequently asked questions
Yes, a bushing is required for feeder wires coming into the panel, regardless of whether the conduit is EMT or PVC.
Yes, an insulating bushing is required when using 4 AWG or larger wires or if you have the raw edge of the EMT.
Yes, where raceways contain conductors 4 AWG or larger, the conductors must be protected by a fitting that provides a smooth, rounded insulating surface, such as an insulating bushing.
Yes, for IMC and RMC, a metal or plastic bushing must be installed on conduit threads at terminations, regardless of conductor size, unless the design of the box, fitting, or enclosure provides equivalent protection.
An electrical bushing is a hollow electrical insulator that allows an electrical conductor to pass safely through a conducting barrier, such as a transformer or circuit breaker, without making electrical contact with it.
