Switchgear Auxiliary and Trip Alarm Contacts Selection Guide
How to choose front-mount and side-mount NO/NC auxiliary contact blocks and fault trip alarm contacts for MCBs, MPCBs, and contactors.
Miniature circuit breakers, residual current devices, motor protection circuit breakers, and power contactors report their switching state and fault trips to control circuits and PLCs through Auxiliary Contacts. Selection requires matching the host device's brand/series, mounting style (front snap-on vs side-mount), and contact configuration (NO / NC).
Status contacts (OF) vs fault trip alarm contacts (SD)
- Standard auxiliary status contact (OF): Follows the main poles whenever the device is switched manually or electrically. Used for self-holding (latching) circuits, electrical interlocking, and status lamps.
- Fault signal / trip alarm contact (SD): Actuates only when the breaker trips automatically due to overload, short circuit, or earth leakage—distinguishing a fault trip from normal manual switching.
Auxiliary contacts comparison table
| Contact Block Type | Typical Arrangement | Mounting Position | Primary Application | Category Link |
|---|---|---|---|---|
| Contactor auxiliary contact block | 1NO+1NC, 2NO+2NC, 4NO | Front clip-on or side-mount | Latching circuits, reversing interlocks, and PLC feedback | Auxiliary Contacts |
| Motor protection switch (MPCB) contact | 1NO+1NC, 2NO | Front under-cover or left/right side | Zero-width (front-mount) motor protector status monitoring | Motor Protection Switches |
| MCB / RCCB auxiliary & alarm contact | 1 CO (OF / SD) | Side-mounted on DIN rail (9 mm width) | Remote monitoring of critical branch circuit trips | Miniature Circuit Breakers |
Frequently asked questions
When is a Normally Open (NO) vs Normally Closed (NC) contact used?
Normally Open (NO, 13-14) contacts are wired in parallel with start pushbuttons for self-holding circuits and run indication; Normally Closed (NC, 21-22) contacts are wired in series with the opposite coil for electrical interlocking.
What is the advantage of front-mounted vs side-mounted auxiliary contacts on MPCBs?
Front-mounted blocks snap directly into the breaker face without adding DIN-rail width, whereas side-mounted blocks add 9 mm of rail width but provide higher contact density and easier wire routing.