{"id":2747,"date":"2026-01-23T08:20:50","date_gmt":"2026-01-23T08:20:50","guid":{"rendered":"https:\/\/xbrele.com\/?p=2747"},"modified":"2026-04-07T12:32:56","modified_gmt":"2026-04-07T12:32:56","slug":"earthing-switch-basics-making-capacity-interlock-guide","status":"publish","type":"post","link":"https:\/\/xbrele.com\/it\/earthing-switch-basics-making-capacity-interlock-guide\/","title":{"rendered":"Nozioni di base sugli interruttori di messa a terra: Capacit\u00e0, sequenza di sicurezza e note di interblocco"},"content":{"rendered":"\ufeff\n

An earthing switch is a mechanical switching device that connects de-energized circuit conductors directly to earth potential, eliminating induced voltages, trapped capacitive charges, and residual energy that could injure maintenance personnel. Unlike circuit breakers or load-break switches, earthing switches carry no interrupting capability\u2014their sole function is bonding isolated conductors to ground before workers access exposed equipment.<\/p>\n\n\n\n

In medium-voltage systems rated 3.6 kV to 40.5 kV, earthing switches mount on busbars, cable terminations, and feeder compartments within metal-enclosed switchgear. The device appears mechanically simple: a conductive blade pivoting into a fixed contact connected to the station grounding grid. Yet this simplicity masks critical engineering requirements governing making capacity, operating sequence, and interlocking logic.<\/p>\n\n\n\n

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What Is an Earthing Switch and Why Is It Essential?<\/h2>\n\n\n\n

The earthing switch represents the final safety barrier before human hands touch exposed conductors. After upstream circuit breakers open and disconnectors create visible isolation gaps, the earthing switch provides positive grounding\u2014a deliberate short-circuit to earth that guarantees zero voltage regardless of induction from parallel circuits, backfeed through transformer windings, or capacitive coupling from adjacent phases.<\/p>\n\n\n\n

Without proper earthing, technicians working on \u201cisolated\u201d equipment face lethal shock hazards from sources invisible to standard voltage indicators. Induced voltages from parallel transmission lines routinely exceed 1,000 V on ungrounded conductors. Trapped charges on cable capacitance can deliver fatal current even hours after isolation.<\/p>\n\n\n\n

The earthing switch eliminates these hazards by providing a low-impedance path to ground. Contact resistance values typically remain below 200 \u03bc\u03a9, ensuring effective fault current conduction. The grounding path from main contacts through the operating mechanism to the earthed frame must carry prospective fault current without excessive temperature rise\u2014usually limited to 250\u00b0C at contact surfaces during short-time duty.<\/p>\n\n\n\n

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How Making Capacity Protects Against Operator Error<\/h2>\n\n\n\n

Making capacity\u2014the maximum prospective current an earthing switch can safely close onto\u2014represents the most critical performance parameter distinguishing protective devices from standard grounding equipment.<\/p>\n\n\n\n

Under normal conditions, an earthing switch closes onto a fully isolated, voltage-free conductor. The operation is uneventful. But what happens when a technician closes the earthing switch while the circuit remains energized due to procedural error, failed isolation, or incorrect switching sequence?<\/p>\n\n\n\n

The earthing switch must survive this fault scenario without welding shut, fragmenting, or producing an uncontrolled arc. This survival capability defines making capacity.<\/p>\n\n\n\n

IEC Classification System<\/h3>\n\n\n\n

IEC 62271-102 classifies earthing switches by making capacity requirements:<\/p>\n\n\n\n

Class E0<\/strong>: No rated making capacity. Designed exclusively for grounding de-energized circuits where accidental energization risk is negligible.<\/p>\n\n\n\n

Class E1<\/strong>: Induced current making capacity up to 160 A. Handles capacitive and inductive coupling from parallel energized circuits.<\/p>\n\n\n\n

Class E2<\/strong>: High making capacity reaching 25 kA to 63 kA for 1-second duration. Protects against scenarios where circuits are mistakenly believed to be de-energized.<\/p>\n\n\n\n

The making capacity formula considers peak current Ipeak<\/sub>\u00a0= k \u00d7 Irms<\/sub>, where k typically equals 2.5 for 50 Hz systems and 2.6 for 60 Hz systems, accounting for DC offset in asymmetrical fault currents. For a Class E2 switch rated at 40 kArms<\/sub>, the peak making current reaches approximately 100 kA.<\/p>\n\n\n\n

Construction Features Enabling High Making Capacity<\/h3>\n\n\n\n

Contact mechanisms in Class E2 earthing switches must withstand severe electromagnetic forces during fault closure. Key design elements include:<\/p>\n\n\n\n