What does an auxiliary switch do?
An auxiliary switch provides a secondary control signal or status indication for a primary device, enabling additional control options and feedback without changing the main power path.
In practical terms, auxiliary switches are small, secondary contacts that move in tandem with a larger component such as a relay, contactor, motor starter, or other equipment. They help send signals to controllers, interlock systems, or monitoring panels, and they can be used to improve safety, sequencing, and automation without redesigning the primary circuit.
Core concept
Auxiliary switches are designed to ride with or be mounted next to a main device. Their purpose is to provide a separate set of contacts that change state when the main device operates. This separation allows a control system to learn the status of the device or to trigger a different circuit without carrying the main load.
Common configurations
The following list outlines typical contact arrangements you’ll encounter on auxiliary switches. It helps when selecting replacements or planning control circuits.
- Form A — SPST normally open (NO): closes when the actuator is energized.
- Form B — SPST normally closed (NC): opens when the actuator is energized.
- Form C — SPDT: one common contact that can connect to either NO or NC (often labeled Form C or Form AB/AC depending on standards).
- DPDT or higher — multiple poles: contains two or more Form C contacts in a single device for signaling across multiple circuits.
Auxiliary switches can provide both NO and NC signaling, and may be configured as single-pole or multi-pole devices. The choice depends on whether you need a signal when a device is energised, de-energised, or both.
Forms A, B and C explained
Understanding Form A, B and C helps in predicting how the auxiliary contacts will behave in a given control circuit. Form A (NO) makes a connection when active, Form B (NC) breaks or makes a connection when active depending on the wiring, and Form C (SPDT) offers a choice between a connected NO path or an NC path with a common terminal.
Applications and examples
Auxiliary switches appear across industrial settings and consumer equipment to provide signaling, interlocks, and sequencing. Here are common applications.
- Motor and power equipment: Provide status feedback to PLCs or control circuits about coil or contactor state.
- Industrial automation: Send machine-state signals to sequencing and safety interlocks.
- Safety and interlocks: Ensure guards, doors, or safety devices are in the correct position before operation.
- Building automation: Signaling damper positions, valve states, or door status for HVAC and control systems.
- Automotive and consumer devices: Indicate position or status without carrying the main load (for example, door switches or trunk switches).
These signaling and interlocking roles help improve safety, reliability, and automation efficiency by providing real-time feedback to control systems without modifying the primary power path.
Choosing and installing
Choosing the right auxiliary switch and installing it correctly are essential for reliability and safety. Consider these factors when selecting and mounting an auxiliary switch.
- Electrical rating: Match current and voltage ratings to the circuit you’re signaling or controlling.
- Contact form and poles: Select Form A, Form B, or Form C as needed; decide on SPST, SPDT, or DPDT based on how many signals you need.
- Mechanical fit: Ensure the contactor or device has space and mounting provisions for the auxiliary contacts.
- Environmental rating: Check for temperature, humidity, dust, and ingress protection suitable for the environment.
- Operation type: Decide between maintained (latches) and momentary (returns to a baseline) behavior based on control logic.
Choosing the right configuration and ensuring proper installation reduces wiring errors and helps the control system read accurate, timely signals from the auxiliary switch.
Summary
An auxiliary switch adds a secondary set of contacts to a primary device, delivering signaling and control opportunities without altering the main power circuit. It supports safer interlocks, clearer status Feedback to controllers, and more flexible automation across industries. When selecting one, pay attention to contact form (NO/NC/NO+NC), number of poles, ratings, mounting, and environmental conditions to ensure compatibility with your system.
