Protective devices must be installed in all power distribution systems in order to prevent equipment from being damaged by unfortunate events. Among these protective devices are earthing switches, which are connected to components of the switchgear to keep electrical equipment safe and prevent further harm, such as a fire outbreak or injury to users.
An earthing Switch is a mechanical switching device used to protect circuit components by maintaining current flow for a specified time under abnormal conditions such as short circuits. Under normal circuit conditions, it does not carry any current. It is only called when an exceptional condition occurs. Every electrical installation should have an earthing switch to protect technicians and switchgear from abnormal current conditions.
This article will explain what an earthing switch is, as well as everything you need to know about protective devices.
1. What is an earthing switch
Earthing switch is also known as earthing switch. Earthing switch is used interchangeably with earthing switch. It is a protective device included in switchgear components such as circuit breakers and isolators. A bus bar section adjacent to the breaker is automatically grounded when the breaker is removed and cranked out. The ground switch makes contact with the bus bar when an isolator isolates the circuit, releasing any accumulated charge.
An earthing Switch in switchgear is used to ground the remaining changes in the power line after the line has been removed from its source. After the circuit is cut or opened by circuit breakers and isolators, the residual charge remains in the circuit. Ground switches are often used to discharge charges.
The earthing Switch has a snap-action closure mechanism. They protect technicians and workers when abnormal electrical currents occur. In addition to being designed to withstand short circuits, they can also be motorized. Such as high-voltage earthing switches and high-speed earthing switches. Substation earthing switches create a short circuit to protect other electrical equipment. It is used in conjunction with a variety of high-voltage switchgear and used as a protection device in the maintenance of high-voltage electrical equipment.
Earthing switches, circuit breakers, and isolators are connected in the ring main unit (RMU) substation. If a circuit needs to be opened or disconnected for maintenance or for any other reason, the correct sequence of operation for these three devices (earthing Switch, circuit breaker, and isolator) must be followed. Not only can you damage circuits and equipment, but you can also put yourself at risk if you don’t follow the correct steps.
Currently, the isolating switch also has an earthing switch, which grounds one end after opening in addition to the main contact. In most cases, the main contact and earthing switch are mechanically interlocked, which means that the earthing switch cannot be closed while the isolating switch is closed, and vice versa.
Earthing switches can be divided into two types: open and closed. The conduction system of the former is exposed to the atmosphere like an earthing Switch of a disconnector, and the conduction system of the latter is enclosed in the charging SF. Or insulating medium, such as oil.
Earthing switches in switch cabinets close short-circuit currents and are dynamically and thermally stable. Since there is no need to break the load current and short-circuit current, there is no arc extinguishing device. The lower ends of the blades are usually connected to the ground through current transformers. Current transformers can provide signals for relay protection.
There are several configurations of earthing Switches. We have single-pole, double-pole, and three-pole earthing Switches. The single pole is only used when dealing with neutral grounded systems. The neutral point ungrounded system adopts two-pole and three-pole structures and uses one operating mechanism for operation.
1) Earthing switch in low voltage system
The fundamental purpose of earthing switch design in low-voltage systems that provide electricity to the widest range of end-users is to ensure the safety of customers using electrical products and their protection against electric shock. The grounding system, combined with other switchgear protection components such as load break switches and residual current devices, ensures that persons cannot come into contact with metal objects whose potential relative to the human potential exceeds a safe threshold, usually set at 50 volts.
From a safety point of view, earthing switches are also necessary on low-voltage networks with phase-to-phase voltages in excess of 240 to 690 volts, typically used in industrial, mining equipment, and machinery rather than public access networks.
2) Earthing switch in high voltage system
The focus of the earthing switch in a substation is that it is not easily accessible to the public, and it is not a priority for safety as it is in low-voltage systems. Instead, it focuses more on power reliability, protection reliability, and the impact on equipment in the event of a short circuit. Since the current path is mostly blocked through the earth, only the magnitude of the most prevalent phase-to-ground shorts is significantly affected by the choice of the grounding system.
2. What is the earthing switch for?
- Earthing switches are used to provide earthing for underground or delta-connected systems.
- An earthing Switch allows the connection of phase-to-phase loads.
- The earthing Switch in the substation is grounded to provide a low-impedance ground connection, thereby maintaining system neutrality.
- Earthing switches in switchgear ensure that the magnitude of transient overvoltages is limited during restrike earth faults.
- Earthing Switches provide a source of ground fault current when there is a line-to-ground problem.
- It is used for grounding electrical equipment to meet protection requirements.
3. What is the function of the earthing switch
- The main function of the earthing switch in the switchgear is to protect the technician and the switchgear from accidental operation. It can reliably shut down short-circuit currents.
- It is connected to the circuit breaker. When the circuit breaker is opened and shaken out, the bus adjacent to the circuit breaker is automatically grounded through the earthing Switch. Technicians, maintenance personnel, and users are protected from any unfortunate accidental voltages.
- For electrostatic and electromagnetic induction currents. In two or more overhead transmission lines arranged on the same tower or adjacently in parallel, when one or more lines lose power, due to the electromagnetic induction and electrostatic induction between it and the adjacent live line, the power failure circuit will generate an induced voltage, Induced current. Hence, the ground switch for this line.
- Used to close short-circuit currents. The earthing switch shall be capable of closing at any applied voltage, including its rated voltage, and at any current, including its rated short-circuit-making current. Earthing switches have a short-circuit current rating that makes the current equal to the rated peak withstand current.
- A substation’s earthing switch is important for large or heavy power transformers because it provides the grid with a consistent voltage reference. Alternatively, the voltage may vary from place to place. It not only ensures the safety of the transformer but also ensures the safety of people using electricity.
4. Types of earthing switches
Earthing switches are divided into the following types:
1) Indoor high voltage earthing switch
In conjunction with high-voltage switchgear, it can be used for indoor 3-12kV three-phase AC 50 (60) Hz power systems. It can also be used as the grounding for the maintenance of high-voltage electrical equipment. The main structure of the earthing Switch is composed of a bracket, a grounding knife assembly, a static contact, a sensor, a shaft, an arm, a pressure spring, a conductive sleeve, and a soft connection.
The closing earthing switch acts as a torque to overcome the resistance torque when the operating mechanism operates it. In conjunction with high-voltage switchgear, it can be used for indoor 3-12kV three-phase AC 50 (60) Hz power systems. Make the earthing Switch close quickly at the closed position. The grounding knife on the grounding knife assembly is in firm and reliable contact with the flange part (knife edge) of the static contact through the disc spring.
When the torque is applied, the main shaft overcomes the main torque, and the spring force drives the boom to rotate in the direction of the opening so that the grounding knife compression spring passes.
2) Indoor high voltage electrical earthing switch
The indoor high-voltage electrical earthing Switch is composed of a motor, a driving device, an electric wind control device, and a main shaft of the earthing Switch.
3) FES fast earthing switch
Depending on the installation location, earthing switches are divided into two types: maintenance earthing switches (ES) and fault speed closing earthing switches (FES).
A fast earthing switch (FES) closes short-circuit current and opens and closes induced current, making it an important protection device during maintenance. It is also operated by an electric spring mechanism. Fault earthing switches are suitable for earthing switches on the outside of the line. It operates on the line side of the line outlet isolating switch and is electrically interlocked with the relevant circuit breaker and isolating switch.
And the earthing Switch is used for grounding or disconnecting. The equipment should be overhauled with obvious grounding points on all sides that may generate electricity. This consists of the busbar earthing switch, the outgoing line earthing switch, and the incoming line earthing switch (installed on the busbar side and the outgoing line side of the main transformer circuit breaker).
5. What is the earthing switch symbol
Earthing switches consist mainly of a bracket, a grounding knife assembly, a static contact, a sensor, a shaft, an arm, a compression spring, a conductive sleeve, and a soft connection. When the earthing Switch is manually closed, the acting torque makes the main shaft overcome the torque and drives the pivoting arm to rotate in the closing direction, so that the grounding knife reaches the static contact through the coil spring and the flange.
The parts (knife edge) are in firm contact with each other. During the manual opening operation, the active force makes the main shaft overcome the torque and spring force and drives the swing arm to rotate in the direction of splitting, The grounding knife pressure spring passes through the dead point, completing the egg pressing energy storage for the next closing.
6. What is High-Speed Ground Switch (HSGS)
The high-speed earthing switch is also called a high-speed earthing switch. Also, sometimes referred to as a quick earth switch. It is a fast isolating switch with closing short-circuit current capability, especially used for artificial grounding of power systems. Generally equipped with hydraulic or electric snap action mechanism.
In a power system with multiple main transformer branches connected in parallel, in order to save investment, the high voltage circuit breaker of the main transformer branch is usually replaced with a fast earthing switch. The fast earthing switch on the high-voltage side is activated when the main transformer body fails. This makes the power system manually grounded, starts the power system relay protection device, opens the circuit breaker on the opposite side of the transmission line, and cuts off the main transformer failure.
In addition to gas-insulated metal-enclosed electrical appliances, fast earthing switches are installed on the outside of each outgoing line to cut off induced current generated by parallel circuits.
In a 110kV power system, the short-circuit current is usually 80kA, while in a 220kV power system, it is 125kA. In the gas-insulated metal-enclosed electrical appliance, the fast earthing switch is sealed in the SF6 gas-insulated gas cylinder, which makes the requirement of closed short-circuit current relatively straightforward. When the short-circuit current that needs to be closed is large, a vacuum circuit breaker is often used instead of an open high-speed earthing Switch.
When the ground fault is closed, the short-circuit current flows from 2 to 1, the three-phase moving contacts maintain interphase insulation through the insulating plate 3, and the fault current is introduced into the ground through 4. Ensure that the HSGS, which is always in the closed state of the ground, also has the ability to withstand short-term current. When HSGS shutdown fails, there are two procedures to fix the shutdown failure:
- When the HSGS moving contact is just in contact, the insertion force should be greater than the reaction force of the static contact spring and the electric repulsion force of the static contact. This confirms that the moving contact can be inserted into the static contact.
- When overtraveling, the kinetic energy of the HSGS moving contact system should be sufficient to overcome the friction and electric friction of the contact spring, so as to ensure that the moving contact is in good condition. Close in place.
7. Difference between an earthing switch and a high-speed earthing switch
The earthing Switch is arranged on both sides of the circuit breaker close to the isolating switch and only plays the role of grounding on both sides of the circuit breaker during maintenance. The high-speed earthing Switch is installed on the line side of the outgoing circuit isolating switch, and it has two functions:
- Due to the electromagnetic induction generated by capacitive current and electrostatic induction, the opening and closing of parallel overhead lines will generate induced current.
- The fast earthing Switch quickly grounds the main circuit. When the insulator in the bushing leaks or there is an arc in the bushing, the circuit breaker is used to cut off the fault current.
The function of the quick earthing switch shall be on the outside of the line switch. When a single-phase fault occurs on the line, the switches on both sides of the faulty line are turned off. However, due to the electromagnetic coupling between the normal phase and the faulty receiving phase, the normal phase provides the possibility for the faulty phase. If the power supply current does not use the fault point to dissociate, it will prolong the arc extinguishing time, or even not extinguish the arc.
The automatic device closes the fast earthing switches on both sides immediately when the switches on both sides are disconnected to reduce the submerged power supply current. Perform single-phase reclosing after the arc extinguishing time is set, then disconnect the fast earthing switch. This can improve the closing success rate.
8. Relationship between circuit breaker and high-speed earthing switch
There are two ways to look at the relationship between a circuit breaker and a fast earthing switch (HSGS). One is from the aspect of switch tripping, and the other is from the aspect of reclosing.
- In terms of switch-tripping, it is actually very simple. When a single-phase fault occurs on the line, the switches on both sides of the faulty line are turned off. A submerged power supply current will be generated due to electromagnetic coupling between the normal phase and fault phase, which is not conducive to extinguishing an arc. By immediately closing the HSGS on both sides when the switches on both sides are turned off, the automatic device will reduce the current of the latent power supply.
- In order to achieve reclosing, HSGS is mostly used to connect with the switch. These are the steps to do this:
- An arc is caused by a single-phase-to-ground short circuit fault.
- Switches (primary arcs) at both ends of the fault phase trip are extinguished, and a submerged arc (secondary arc) is formed.
- On the faulty phase, the HSGS is grounded, and the submerged arc is extinguished.
- HSGS has been disconnected.
- The reclosing action switch recloses.
An earthing switch, also known as an earthing switch, is a protective device that keeps technicians and switchgear safe from unintentional operation. They are used with switchgear components such as isolators and circuit breakers to ensure that accidents cannot occur due to abnormal current flow in electrical systems.
In this article, we discuss everything you need to know about the earthing switch in switchgear, starting with what it is, its purpose, function, types, symbols, and more.