Electrical distribution systems and operation conditions require frequent co-ordination between the power source and the loads. This can pose a significant challenge and may become a major safety concern when large motor loads are added or taken away from the system. Such things can be avoided using a switch called a Load break Switch. In this article let us discuss what is a load break, its working applications.
What is a Load Break Switch?
A load break switch is a device that disconnects the electrical current from an electric device or circuit when the current exceeds the nominal value. They are tripped by the magnetic field in the major power system and re-close when the electrical demand decreases, reducing current peaks in low impedance circuits. These switches are very simple to use and assemble and they are applicable in public distribution, industrial sector, distribution panels, motor feeders, and emergency switching.
The load break switch symbol is shown below.
Construction:
The load break switches are mainly made in sheet steel enclosure which provides a simple & solid construction including epoxy insulators, lagging pin, double switch blades, forged contacts with loop less current path & arc quenching chamber. The insulation of this switch will make it lightweight including high mechanical strength & greater electrical safety. The panel of this switch is available in different configurations like standing Indoor/Outdoor & RMU type. So this article discusses an overview of a load break switch – working with applications.
Load Break Switch Working Principle
The working principle of the load break switch is to switch the current by moving their contacts mechanically with a suitable speed to make or break the current supply. These switches are exposed to different stresses like thermal, dielectric, and mechanical throughout the switching operation. These switches give protection to the transformer and it is an alternative to the two-pole structure.
Features
The features of the load break switch include the following.
- Outstanding arc-extinguishing characteristics.
- It has a light & compact design.
- High current-limiting power fuses.
- These are economical.
- Fuse replacement can be done safely.
- A shunt trip mechanism
- Indicator of a blown fuse.
- Auxiliary contacts.
- Available motor-driven-types.
Design of Load Break Switch
The SF6 gas insulated load break switch mainly includes two circuits like main circuit & the earthing circuit which are enclosed in a dielectric casing based on epoxy resin. Here, this dielectric casing is outstanding in contamination resistance as well as insulating performance. The dielectric casing is composed of the lower and upper insulating covers.
The inside chamber of this dielectric is filled with SF6 gas with 0.4bar relative force. The lower insulating cover wall is nearly thinner as a measure taken to avoid explosion. In case of an accident, the over-pressured gas can break with the explosion-proof channel and connect directly to the back of the cabinet, discharge the pressure & ensure personal security.
The design of the SF6 load switch can be done by using different parts. So the parts of the LBS include the closing position, isolated position, earthling position, upper insulating cover, upper stationary contact, spindle, M ring, lower insulating cover, sensor, lower stationary contact, air chute, movable contact, ground coupler, and ground contact.
Closing Position
In the SF6 LBS, the closing refers to the contacts staying surrounded through SF6 gas at 2•8 kg/cm2 pressure. When this switch operates, the moving contact will be pulled separately & an arc is struck in between the contacts.
Movable Contact
Movable contact in this switch is used to switch the current by moving their contacts mechanically with an appropriate speed to open or close the current.
Upper and Lower Stationary Contacts
Stationary contacts in the switch are points where an LBS throw will make or break a circuit. These contacts are always available in pairs. The contact that is fixed with the actuator of the switch is known as stationary contact.
Upper and Lower Insulating Cover
Insulation cover is an insulative material that covers around the LBS to make the switch safer.
Arc Chute
Arc chute includes a set of metal plates that are arranged in parallel and mutually insulated from each other. These plates can cool, split and securely reduce an electric arc. These are also known as arc dividers & arc splitters.
Spindle
A spindle is a rotating shaft including a fixture used for holding a tool or a workpiece. This spindle shaft severs as a positioner, support & rotary drive for the tool or workpiece.
Working
This gas-insulated load break switch operates at 24kV for overhead lines. This switch meets the demand for maintenance-free & oil-less operation through SF6 gas. The required parts which we have mentioned above are installed within its stainless steel tank which is hermetically sealed.
This switch is used to disconnect a circuit in load conditions due to a fault, isolation, or problem. So this switch switches the current by mechanically turning its contacts through an appropriate speed to open or close the current. This switch detects a surge in current flow by measuring the voltage drop across its contacts. When this voltage drops below a certain level, it signals that an overload condition exists. When this happens, the switch opens and breaks the circuit so there can be no further increase in current flow until another circuit breaker closes to restore the circuit.
The SF6 gas-insulated Load Break Switch is very helpful in switching current when an overload fault occurs. When contacts are opened or closed throughout the switching operation, the flow of current can be carried throughout an arc & a transient recovery voltage can be formed across the switch contacts. In the load break switch, SF6 gas has outstanding arc-quenching & dielectric properties.
Types of Load Break Switch
There are different types of load break switches available in the market which is discussed below.
LBS Type Load Break Switch
LBS type air load break switch is available with 3.6kV or 7.2kV up to 600 Amps 3-pole and 3.6kV or 7.2kV with 200 & 400 Amps 3-pole including power fuse. This LBS air load break switch unit is attached with power fuses and is also a striker. This feature is not found in usual LBSs.
The striker in this switch is a trip mechanism that works when the fuse blows. Once the fuse blows, the striker causes 3-pole to open simultaneously. If these types of switches do not include this striker feature then it is feasible for some of the phases to stay alive once the fuse has blown, which results in a hazardous situation. When these switches are available with an inbuilt auto trip mechanism, then R293B & R290B remote gang-operated mechanisms cannot be appropriate.
LB Type Load Break Switch
This type of air LBS size is reduced so that it can be connected in compact cubicles. In this LBS, greater operating safety, and improved interrupting performance can be achieved with a rotary arc contact as well as redesigned arc chute. Different accessories are assembled with a modular system, so that gang-operated mechanisms, power fuse frames & auxiliary switches can be simply included in the main body of the switch when required.
Additionally, the shunt trip mechanism can be integrated into the fundamental frame. Formerly they were considered accessories. The R293B or R290B remote gang-operated mechanism cannot be included in these shunt trip mechanisms. Those 200AF & 400AF type LB air load break switches can be connected with power fuses. But, if this type of LBS has 400AF ratings then we required only up to 200 Amps power fuses.
RF Type Load Break Switch
The RF type load break switch is available with 12V or 24V or 36kV equal to 1200 Amps 3-pole. This type of LBS includes an auxiliary blade, an arc chute & the main blade. The auxiliary blade is arranged at the arc chute and is attached to the main blade. So it will make contact simultaneously like the main blade.
The auxiliary blade will be held in contact temporarily within the arc chute once the main blade opens. Once this main blade has attained the travel limit the auxiliary blade will quickly go back to its place against the main blade in the stored energy influence within the spring. So this quick-break device works efficiently even operated manually, and as the contact shifts at high speed then the arc is quickly stretched out & gas-cooled within the arc chute.
SF6 Load Break Switch
SF6 gas Insulated LBS is mainly used for overhead lines which operate at up to 24kV. These types of switches meet the demand for maintenance-free & oil-less operation through SF6 gas, the associated parts, as well as devices, are connected in its stainless steel and hermetically sealed tank.
This low brake switch is operated manually or motorized for isolation of fault, automation & remote control to suit power line necessities. The advantages of the SF6 LBS mainly include security, high reliability, and easy installation.
Load Break Switch Vs Circuit Breaker
The difference between a load break switch and a circuit breaker is discussed below.
| Load Break Switch | Circuit Breaker |
| A LBS is designed to provide the making or breaking of specified currents within a circuit. | A circuit breaker (CB) is an electrical protection device, used to defend an electrical circuit from a short circuit or overcurrent. |
| It is a disconnect switch. | It is an electrical switch operated automatically. |
| Generally, load break switches have been developed in two types air-blast & SF6. | There are three kinds of CBs available standard single-pole & double pole, ground fault circuit interrupter & arc fault circuit interrupter CBS. |
| This switch mainly includes an arc extinguishing chamber, disconnect blade & operating mechanism. | The circuit breaker includes fixed & moving contacts. |
| It is operated manually. | It is operated manually and electrically. |
| Not necessary to close or open this switch frequently. | It is required to open & close frequently. |
| Its mechanical life average is 1000 times. | Its mechanical life average is 10k times. |
| This switch cannot break short circuit current. | This circuit breaker can break short-circuit current about 20 to 50 times. |
| It carries a small current that ranges from 0 to 125A. | It carries a rated current that ranges from 0~5000A. |
Advantages and Disadvantages
The advantages of load break switch include the following.
- LBS is less costly as compared to a two-pole structure.
- This switch provides a secure power grid operation.
- This switch very easily controls the supply by switching ON & OFF.
- It gives protection to the transformer by using HRC fuses.
- The LBS is simple to design, easy to use, simple operation, and reliable to function.
The disadvantages of load break switch include the following.
- These are expensive.
- Occupies space.
- Less secure.
Applications
The load break switch applications mainly include the following.
- An LBS is mainly designed to provide specified currents making or breaking.
- These switches are applicable in power distribution grids with a recloser to enhance control capabilities.
- This type of switch is normally used to unlock the electrical circuit under the load.
- These LBSs are extensively used in different applications like condensers, transformer switching, and RMG (ring main gears).
- LBSs are mainly used for strengthening or de-energizing a circuit that has some limited amount of line charging or transformer exciting currents.
Thus, this is all about an overview of a load break switch – design, working with applications. These switches are economical, reliable, secure, and available with a simple structure, and easy for mounting & operating. Here is a question for you, what is an air blast circuit breaker?