A Small Component With an Outsized Job
An auxiliary switch (often called an 'aux switch' or contact block) is a multi-contact device mechanically ganged to the breaker's main operating shaft. As the breaker opens or closes, the aux switch's contacts change state in step with it. It carries no load current itself — its entire job is signalling: telling relays, indicator lamps, SCADA/RTU systems, interlocks, and the trip/close circuits themselves whether the breaker is actually open or closed.
Why "Actually" Matters
The word actually is doing real work in that last sentence. A breaker's physical contact position and its aux switch position are supposed to always agree, but they are mechanically separate contact sets — a worn or misaligned linkage can let them drift out of sync. This matters more than it sounds: control schemes routinely use aux switch contacts to permit or block operations (e.g., "don't allow a close command unless the breaker is confirmed open," anti-pumping circuits, or racking interlocks). If the aux switch reports the wrong state, you can get a spurious block on a legitimate operation, or worse, a permit for an operation that shouldn't be allowed yet.
Typical Contact Arrangement
Most VCB aux switches provide a mix of normally-open (NO) and normally-closed (NC) contact pairs — commonly referenced as "a" contacts (follow breaker position, closed when breaker is closed) and "b" contacts (follow inverted, closed when breaker is open). A typical medium-voltage breaker aux switch block might offer anywhere from 4 to 12 contact pairs, split across position indication, trip/close circuit supervision, SCADA feedback, and spare contacts for future use. Spec sheets list this as something like "6 NO + 6 NC" or "4a + 4b" — always check how many of each your control scheme actually needs before ordering a replacement, since a lower-contact-count unit is a cheaper mistake to make than it looks.
Common Failure Modes
- Contact wear/pitting from repeated switching, especially on circuits with any inductive load (indicator lamps, relay coils) that aren't properly suppressed.
- Mechanical linkage wear between the main shaft and the aux switch cam — this is what causes position-indication drift rather than a clean open/closed mismatch.
- Loose or corroded wiring terminals on the aux switch block itself, particularly in humid or dusty substation environments — often misdiagnosed as a "faulty aux switch" when the switch itself tests fine.
- Cam/actuator breakage from age or an over-travel condition in the breaker mechanism, which can shear the physical link entirely.
Diagnosing and Replacing
A continuity/contact-resistance check across each contact pair, done with the breaker manually cycled through open and closed, is the standard diagnostic — you're checking both that contacts change state at the right point in the travel and that resistance across a closed contact is low and stable. If a contact pair fails this test or the unit shows physical damage to the cam or housing, replacement is straightforward as long as the contact configuration (count of NO/NC pairs) and the mechanical mounting/actuator interface match your breaker model exactly — these are not universal parts across manufacturers or even across model generations from the same manufacturer.
Check current stock for auxiliary switches matching your breaker, or if you're unsure of the exact contact configuration your panel needs, send us your nameplate details via request a quote and we'll confirm compatibility before you order.