What Is a Load Switch?
A load switch is a device designed to either open or close an electric circuit carrying a load current. Typically, it refers to a product used for controlling high-voltage AC load currents.
Moreover, Load Break Switch (LBS) is often included in this category. Devices responsible for opening and closing circuits in high-voltage electrical lines encompass disconnectors, circuit breakers, and load switches.
A disconnector is a device that cannot interrupt the load current and is primarily used for safety control during maintenance tasks. On the other hand, a circuit breaker is capable of interrupting both load currents and fault currents during short circuits, and it finds extensive use in main power receiving equipment and power distribution systems.
A load switch can control load currents but cannot interrupt high-current short-circuit events. Generally, load switches are combined with current-limiting fuses to safeguard upper circuits against accidental overcurrents.
Applications of Load Switches
Load switches are widely deployed in high-voltage transmission and distribution circuits. Here are some examples of load switch applications:
- Connecting lines for commercial facilities supplied with high-voltage power
- Located upstream of transformers that step down high voltage to low voltage
- Located upstream of high-voltage phase advance capacitors
- Located upstream of high-voltage motors
The first type is often installed on utility poles, and it utilizes air-load switches. The second and third types are frequently situated inside cubicles, where LBS is employed. The fourth type is typically found in factories and infrastructure facilities and utilizes vacuum electromagnetic switches. The fifth type is generally installed in factories and employs vacuum electromagnetic switches.
Principle of Load Switches
A typical load switch, such as LBS, comprises a main contact, an operating mechanism, and a current-limiting fuse.
1. Main Contact
The main contact is a metal component that allows the flow of electric current. In LBS, the main contact often consists of a blade and a blade holder. It becomes energized when the blade is inserted into the holder.
Interrupting the load current in a high-voltage circuit can lead to an arc discharge phenomenon. This involves electrical discharge through tiny airborne metal particles, which can break down air insulation. To prevent extended arc discharge, an arc chute is frequently installed in the blade receptacle of LBS.
2. Operating Mechanism
The operating mechanism is a structural component responsible for opening and closing operations. The simplest design consists of an operating handle attached to the blade, which is opened and closed using an operating rod made of insulating material. Electrically operated LBSs are also available for those requiring interlocks and other features.
3. Current-Limiting Fuse
Current-limiting fuses are components installed to interrupt a circuit in the event of a short circuit in the downstream wiring. Inside the fuse, there is a fuse element and silica sand. In the event of a short circuit, the fuse element breaks, and the silica sand extinguishes the arc, providing protection.
Strictly speaking, current-limiting fuses are not part of the load switches, as they can interrupt fault currents, but they are typically installed together.
Types of Load Switches
While load switches commonly refer to LBSs, there are other types, including air load switches and vacuum load switches.
1. Air Load Switches
Air-load switches are employed to switch the load on and off in high-voltage circuits and are also known as PAS (Pole Air Switches). They are often installed at the point where transmission and distribution companies connect with customers.
Since PAS itself does not provide overload or ground fault protection, it is frequently combined with an SOG (Storage Over Current Ground).
2. Vacuum Load Switches
Vacuum load switches utilize a vacuum to extinguish arcs around the contacts. They are often equipped with a current-limiting fuse and are also referred to as high-voltage combination units or VCS (Vacuum Circuit Switches).
The principle is similar to that of vacuum circuit breakers, and many products are operated by electromagnetic coils. Even with frequent load current switching, arcing is unlikely due to this principle, and the fuse melts faster than typical circuit breakers.