What is auxiliary switch used for?
An auxiliary switch is a secondary contact that mirrors the main switch’s position and provides signaling for other circuits without carrying the main load.
What an auxiliary switch is
Auxiliary switches, also known as auxiliary contacts, are small, independent electrical contacts that change state in tandem with the primary contacts of a device. They are designed to carry only part of the circuit’s current—enough to signal a status or trigger a control input—while the main contacts handle the heavier load.
Common configurations and how they appear in equipment:
- Normally Open (NO) auxiliary contact: closes when the main device is energized, signaling an active state.
- Normally Closed (NC) auxiliary contact: opens when the main device is energized, signaling de-energized state or a fault condition.
- Multiple contacts per module: 1, 2, or 4 contacts per auxiliary block, often configured as a mix of NO and NC (for example, 2 NO + 2 NC) to support multiple signaling paths.
These auxiliary contacts do not carry the main load current; instead, they provide feedback or interlock signals that help other components operate safely and automatically.
Typical applications
Auxiliary switches are used to provide reliable feedback to control systems, enhance safety, and enable proper sequencing in automation. They help ensure that downstream equipment only operates when conditions are correct, and they provide status information for monitoring and alarms.
Typical uses include:
- Remote status indication to PLCs, building management systems, or automation controllers
- Interlocking in safety circuits to prevent machine startup when guards are open or doors are not secured
- Feedback signals for motor-control circuits to confirm that a contactor or relay has energized
- Sequencing and interlocking in lighting, HVAC, manufacturing lines, and processing equipment
By separating the signaling function from the main load path, auxiliary switches contribute to safer and more controllable electrical systems.
Technical considerations
When selecting an auxiliary switch, engineers look at several key specifications to ensure compatibility and reliability in the intended application.
Common specifications and considerations:
- Contact type and count: NO, NC, or both; 1–4 poles per module depending on the device
- Electrical rating for the auxiliary contacts: typically lower than main contacts, often in the range of 0.5–5 A at 24–240 VAC or equivalent DC ratings
- Mechanical life and operating torque: designed to endure many life cycles with consistent performance
- Mounting compatibility: must fit the main device (e.g., a specific contactor or relay) and match terminal styles
- Standards and compliance: IEC 60947-5-1 (or local equivalents), UL 508 (US), and other regional standards where applicable
These factors help ensure that the auxiliary switch provides reliable signaling without impacting the main circuit’s performance.
Industry examples
Auxiliary switches appear across a range of sectors, from manufacturing floors to building automation and power distribution. They enable safer automation, better visibility into system status, and smoother interlocking between devices.
- Industrial automation panels and control cabinets
- HVAC systems for sequence control and fault indication
- Motor control centers and starter assemblies
- Safety interlocks and guard-door monitoring in machinery
In each case, the auxiliary switch provides a lightweight, reliable signal path that complements the main switching function.
Summary
Auxiliary switches deliver essential signaling and interlock capabilities without handling heavy loads. They mirror the main device’s state, provide inputs to controllers and safety circuits, and support safer, more automated operations across diverse industries.
