What is an Earthing Switch?

An earthing switch is a mechanical device used to ground electrical circuits and equipment for maintenance, ensuring the safety of personnel during power outages. It is designed to discharge static electricity from the system to be repaired and can withstand abnormal currents (e.g., short circuits) for a specified duration. However, it does not carry load currents under normal operating conditions and is typically integrated as part of a disconnecting switch.

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Features and Functionality of the Earthing Switch

• Purpose:
  The earthing switch is used to intentionally ground a circuit to ensure safety during maintenance activities. It does not handle load currents in normal operation but must be capable of withstanding short-circuit conditions.

• Withstanding Short Circuits:
  During short circuits, the earthing switch can endure specified rated short-circuit currents and peak currents for a defined period, ensuring dynamic and thermal stability. However, it does not require an arc-extinguishing mechanism as it is not intended to break load or short-circuit currents.

• Design and Operation:
  Earthing switches are often integrated with disconnecting switches. In these configurations:

  • The disconnecting switch features an additional earthing switch to ground one end of the circuit after disconnection.
  • The earthing switch and the main contact are mechanically interlocked to prevent improper operation (e.g., the earthing switch cannot close while the disconnecting switch is closed, and vice versa).

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Types of Earthing Switches

1. Open Type:
   The conductive system is exposed to the atmosphere, resembling an isolation switch.

2. Closed Type:
   The conductive system is enclosed in an insulating medium such as SF₆ gas or oil for enhanced safety and durability.

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Classification by Polarity

1. Unipolar:
   Used in systems with a neutral grounding point.

2. Bipolar and Tripolar:
   Used in ungrounded systems, sharing a common operating mechanism.
   

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Key Functions of the Earthing Switch

1. Routine Maintenance Earthing:
   During circuit breaker maintenance, the breaker is opened, disconnecting switches are opened, and the earthing switch is closed to ground the circuit, ensuring safety for both personnel and equipment.

2. Handling Induced Currents:
   In overhead transmission lines that share towers or run parallel, live adjacent lines can induce voltages and currents in de-energized circuits due to electromagnetic or electrostatic induction. The earthing switch grounds these induced voltages and currents for safety.

3. Short-Circuit Switching:
   For earthing switches rated for short-circuit switching, they must handle switching at any voltage up to their rated voltage and any current up to their rated short-circuit switching current. The rated short-circuit closing current matches the rated peak withstand current.