In high-voltage power systems, disconnect switches are critical equipment, but their function is quite specific. They cannot interrupt normal load currents or short-circuit currents. Their core function is to establish a visible air-insulated break in the circuit, and this break must have sufficient safety distance to ensure the safety of personnel during electrical equipment maintenance. Therefore, their installation and use in the system almost always require coordination with other electrical appliances with breaking capabilities to form a complete functional unit.
Core Partner: Circuit Breaker
This is the most common and important pairing. The circuit breaker's responsibility is clear: it is responsible for interrupting all currents in the circuit, whether normal load currents or short-circuit currents during faults. We install disconnect switches on both sides of the circuit breaker, that is, the power supply side and the load side. Its main function is to completely isolate the circuit breaker electrically after it trips, thus allowing safe maintenance of the circuit breaker itself or adjacent lines and equipment.
This combination of "circuit breaker + disconnect switches on both sides" is the basic module of substation main wiring (such as single busbar wiring and double busbar wiring). During operation, the electrical switching sequence must be strictly followed: open the circuit breaker first, then open the disconnector; close the disconnector first, then close the circuit breaker. Never operate a disconnector under load.
Combination with Fuses
In some 35kV and below power distribution systems, especially 10kV systems, we often use an economical and simple combination-a high-voltage fuse paired with a disconnector. In this configuration, the high-voltage fuse (taking drop-out fuses or fixed current-limiting fuses as examples) is responsible for overload and short-circuit protection, and also for interrupting fault current.
The disconnector's function is to form a safe electrical isolation point when the fuse blows or needs replacement. In addition, the disconnector can also open and close lines under no-load conditions, enabling switching operations on equipment such as distribution transformers.
Combination with Grounding Switches
To ensure absolute safety during maintenance, disconnectors are often used in pairs with grounding switches, or directly integrated together. When a line or equipment requires maintenance, we first disconnect the isolating switch to create an isolation point, and then immediately close the corresponding grounding switch. This reliably grounds the equipment or line on the maintenance side, releasing any residual charge and preventing accidental energization from endangering maintenance personnel.
Modern high-voltage disconnecting switches often integrate the grounding switch (i.e., the grounding switch) as an integral part, forming a "three-position" design, including closed, open, and grounded states. They are also equipped with reliable mechanical or electrical interlocks to prevent malfunctions between the disconnecting switch and the grounding switch.
Cooperation with Current Transformers and Other Equipment
In incoming and outgoing circuits, disconnecting switches are often arranged in conjunction with measuring devices such as current transformers (CTs). Regarding the arrangement between the bus-side disconnecting switch and the circuit breaker, we connect the CT in series between them to facilitate current measurement and protection of that circuit. The role of the disconnecting switch here is also to provide a clear isolation point, allowing us to maintain or replace the CT without affecting bus operation.
Other Specific Applications
In double busbar configurations, disconnect switches, acting as "busbar-side disconnect switches," play a special role. They can switch a line or transformer from one busbar system to another without interrupting power. However, this operation requires the circuit breaker to be closed, with the circuit breaker handling the load current switching.
In capacitor banks or motor circuits requiring frequent switching, vacuum contactors handle the frequent operations, while disconnect switches provide final isolation during maintenance. In transmission lines, disconnect switches installed on both sides of the line can connect or disconnect the line in conjunction with dispatch commands when the line is unloaded.