What is an Earthing Switch?

An earthing switch is a mechanical device used to ground circuits on the maintenance side when the power is off. Its primary purpose is to ensure the safety of maintenance personnel. Even if the circuit is unexpectedly energized during maintenance, the switch can divert the electrical energy to the ground, forcing the breaker to trip. Additionally, it can be used to intentionally create a system ground fault for control and protection purposes. Earthing switches are commonly used on the high side of step-down transformers.

Earthing Withstand and Earthing Making

Earthing Withstand

The earthing switch must endure short-term withstand current and peak withstand current. When the system is under maintenance and the earthing switch is closed, it must maintain its grounding state in the event of accidental energization. The switch should not rupture under the mechanical forces generated, allowing protection devices to trip the circuit breaker instead. The earthing switch's withstand current must reach at least 87% of the switch's rated short-term withstand current.

Earthing Making

Earthing refers to closing the earthing switch when the switchgear is live. The switch must reliably close and achieve grounding protection, ensuring that the circuit breaker trips.

Making Capacity Levels:

E0: Not capable.

E1: Capable of 2 operations.

E2: Capable of 5 operations (commonly used).

Service Life Grades:

M1: 1,000 operations.

M2: 2,000 operations.

For 10kV air-insulated earthing switches, a making capacity of 80kA typically requires a closing speed of >5 m/s. For 35kV air-insulated earthing switches, the closing speed must be >6-7 m/s. The operating force for the earthing switch should be no less than 250N.

Earthing Switch Operation Procedures

Power-On Procedure

1. Close all rear doors and covers and lock them (the earthing switch must be in the closed position to lock the doors).

2. Insert the earthing switch operation handle into the hexagonal socket at the lower right side of the middle door and rotate counterclockwise to place the earthing switch in the open position. The interlock plate will automatically block the operation hole, and the lower cabinet door will lock.

3. Push the transfer trolley into position, move the breaker trolley into the cabinet to the isolated position, manually connect the secondary plug, and close the breaker compartment door.

4. Insert the circuit breaker operating handle into the crank hole and turn clockwise for approximately 20 turns. When the handle becomes noticeably resistant and makes a ""click"" sound, remove the handle. At this point, the trolley is in the working position, the secondary plug is locked, and the main circuit of the breaker trolley is connected. Verify the related signals.

5. Operate the on/off selector switch on the instrument panel to close the breaker. Verify that the green indicator turns off and the red indicator lights up, indicating successful closing.

Power-Off Procedure

1. Operate the on/off selector switch on the instrument panel to open the breaker. Verify that the green indicator lights up and the red indicator turns off, indicating successful opening.

2. Insert the circuit breaker operating handle into the crank hole and turn counterclockwise for approximately 20 turns. When the handle becomes noticeably resistant and makes a "click" sound, remove it. At this point, the trolley is in the test position, the secondary plug is unlocked, and the breaker compartment door can be opened. Manually disconnect the secondary plug (the trolley's main circuit is disconnected).

3. Push the transfer trolley into position and lock it. Grasp the locking handles of the trolley with both hands, unlock it by applying force inward, and pull the trolley outward to place it on the service trolley. Lock the trolley and remove the service trolley.

4. Check the voltage indicator (or perform voltage testing). If no voltage is present, proceed to the next step.

5. Insert the earthing switch operation handle into the hexagonal socket at the lower right side of the middle door and rotate clockwise to place the earthing switch in the closed position. Ensure the earthing switch is fully closed, then open the lower cabinet door, allowing maintenance personnel to enter for servicing or repairs.