What is the function of an auxiliary switch?
An auxiliary switch provides a secondary set of contacts that mirrors the main device’s position to signal control circuits, alarms, or safety interlocks, without carrying the main load.
How auxiliary switches function in control systems
In electrical controls, an auxiliary switch (often called an auxiliary contact) is a supplementary contact that changes state in tandem with the device it’s mounted on—from a switch, relay, or contactor. It enables status signaling, interlock logic, and remote monitoring while the main contacts handle the primary power.
Contact configurations
The following configurations cover the standard signaling options you’ll encounter:
- Form A (Normally Open, NO): the auxiliary contact closes when the device is actuated, providing a signal that the device is ON.
- Form B (Normally Closed, NC): the contact opens when actuated, signaling a change in state or enabling a safety interlock when the device is active.
- Form C (Combination, NO + NC): a single unit with both a normally open and a normally closed contact for versatile signaling and interlocking.
In practice, Form C is the most flexible because it can deliver a signal and control path in a single unit, simplifying wiring and reducing panel space.
Where auxiliary switches are used
Auxiliary contacts appear in a wide range of equipment to provide feedback without switching main power. They are a cornerstone of automation and safety interlocks.
Typical applications include the following devices and circuits:
- Contactors and motor starters, to feed run feedback to control logic or to control auxiliary circuits (lighting, alarms, or fans).
- Safety switches and door interlocks, ensuring guards or access doors are closed before equipment can operate.
- Programmable logic controllers (PLC) and distributed control systems, where a signal input confirms device status without loading the main line.
- Indication and alarm circuits, to warn operators when a machine is running or when a safety gate is open.
Note that auxiliary contacts carry only a small control current and do not switch heavy loads; their reliability depends on the main device’s actuation and proper wiring.
Design considerations and maintenance
When selecting and wiring auxiliary switches, engineers consider contact form, electrical rating, coil voltage (for devices with coils), and expected mechanical life. Regular checks ensure the auxiliary contacts reflect actual device status, which is crucial for safe operation and proper interlocking.
Auxiliary switches share the same wear and environmental exposure as the primary device, so routine maintenance should include testing the auxiliary contacts and ensuring correct enclosure sealing and vibration resistance in industrial settings.
Summary
Auxiliary switches provide a reliable, low-current feedback channel from a primary device to control systems, enabling status indication, interlocks, and safe automation without carrying the main load. With Form A, B, or C configurations, they offer versatile signaling options across industrial and building automation contexts.
How does an auxiliary switch work on a circuit breaker?
An auxiliary switch is a small but essential supplementary switch that works in conjunction with a larger primary switching device, most commonly a circuit breaker. It's mechanically linked to the circuit breaker's operating mechanism, meaning its state (open or closed) directly reflects the breaker's position.
What does an auxiliary switch do in HVAC?
An auxiliary switch is a secondary switch that works in tandem with a primary device, such as a circuit breaker, motor starter, or relay. Its primary role is to indicate the position of the main device—whether it's open or closed—allowing it to signal the status of the circuit or equipment it's connected to.
What to use truck auxiliary switches for?
I would say the vast majority of the time AUX switches are used for lights. But air compressors, in-vehicle refrigerator/freezers, radios, winches, etc. are all common. Personally I intend on using two of them for lights, one for an air compressor, and a forth for a winch someday in the distant future.
What is the purpose of an auxiliary switch?
Auxiliary switches are used to control and monitor accessories, providing a way to power add-on devices or get information about the status of electrical systems like circuit breakers and motor starters. In vehicles, they allow users to easily power accessories like lights or winches, while in industrial settings, they monitor the position of equipment or trigger alarms based on a circuit's status.
In vehicles
- Powering accessories: They provide pre-wired and fused power for add-on devices like winches, fog lights, and other electrical equipment, so you don't have to add your own aftermarket wiring.
- Control: They give you direct control over these accessories, and some systems, like the "upfitter" switches in a Ford truck, are designed to be easily wired to various devices.
In industrial and electrical applications
- Remote monitoring: In circuit breakers, auxiliary switches provide remote feedback on the breaker's status (open or closed), which helps operators monitor the system from a distance and identify any issues.
- Status indication: They can activate indicator lights or alarms based on the state of a main circuit or piece of equipment.
- Control of secondary functions: In motor control, they can be used to operate other devices, such as a cooling fan, when the motor is running.
How they work
- Vehicle examples: The switches are typically linked to a power distribution box, which then routes power to various locations in the vehicle. When a switch is activated, it completes a circuit and provides power to whatever is wired to it. For safety, it's often recommended to leave the engine running to maintain the battery charge when using high-draw accessories.
- Circuit breaker examples: An auxiliary switch's contacts are mechanically linked to the main contacts of the circuit breaker. When the breaker is opened or closed, the auxiliary switch changes state, which can be used to signal the change to another system.
