Earthing Switch

After the supply is disconnected, hidden hazards remain: stored charges in cables, and supply that may return unexpectedly. The earthing switch is what turns the isolated section from "probably disconnected" into "confirmed earthed" — and on this, technicians entrust their lives.

The Core Function

An Earthing Switch is a switch that connects the isolated section of a circuit — busbars, a cable, or a transformer side — directly to earth. Its importance during maintenance operations is significant:

• Discharging any accumulated charge: cables in particular act as capacitors that retain charge after disconnection, and the switch safely discharges them.
• Holding the isolated section at earth potential: if the supply were mistakenly restored from any direction, the fault would turn into a clear earth fault that protection trips immediately, instead of killing whoever is working on the circuit.
• It is closed during maintenance, linking the transformer body and everything connected to it via a single earthing path, so technicians can work safely.

The Correct Sequence of Use

• 1. Disconnect the load using the load break switch or circuit breaker.
• 2. Verify isolation (and absence of voltage per procedure).
• 3. Close the earthing switch — only now has the section become safe to work on.
• When restoring: open the earthing switch first, then restore the supply — the reverse order is strictly forbidden.

This sequence is guarded by the interlocking system: the earthing switch cannot be closed while the load break switch is closed, nor vice versa — built-in protection against the two most dangerous operational errors.

Fault-Making Capacity

Modern earthing switches are designed with fault-making capacity: if closed by mistake onto a live circuit — despite all precautions — their contacts withstand the closing arc and hold until protection trips. This capacity is a final safety valve, not a license for laxity in verification.