IEC 60129: Disconnectors — No Current Interruption, But Must Survive Short-Circuit

Disconnectors: Don’t Interrupt Current, But Must Survive Short-Circuit — IEC 60129

IEC 60129 specifies AC disconnectors and earthing switches. A disconnector critical safety function: providing a visible isolation gap in the open position — giving maintenance personnel absolute confidence the circuit is dead.

Core ratings: (1) Rated short-time withstand current (I_k) — the disconnector must endure fault current thermal effects (typically 1–3 s) in the closed position without welding or mechanical deformation. Typical 220 kV rating: 40–50 kA / 3 s. (2) Rated peak withstand current (i_p) — withstand peak electromagnetic force (i_p = 2.5× I_k) without the moving contact being forced open. (3) Bus-transfer current switching — not a circuit-breaker, but must handle very small capacitive/inductive currents (typically 0.5–2 A) during busbar transfers.

Operational safety: Disconnectors typically feature mechanical interlocks — operation is blocked unless the associated circuit-breaker is open. Violating the interlock (opening a disconnector under load) produces a violent arc, risking phase-to-phase faults and personnel injury.

TN Lab — A disconnector does not interrupt current, but must withstand short-circuit. These two contradictory requirements define its entire design.