Auxiliary contacts are important components used in electrical power systems and are commonly employed in automation and control circuits. Although they are small and compact in size, they provide accurate operation, enhance safety, and improve the efficiency of electrical devices.
In this post, we will explore the key features of auxiliary contacts and discuss their importance in electrical systems. Let’s get started.
What is an Auxiliary Contact?
An auxiliary contact is an additional contact connected with primary switching devices, such as a relay, contactor, or circuit breaker.
Its main function is to send feedback about the operating condition of the main device or provide additional control for circuit operation.
Auxiliary contacts are important components in automation and control systems, providing functions such as sequential control and electrical interlocking.
Common Forms of Auxiliary Contacts
Auxiliary contacts are commonly in the form of:
Attached Auxiliary Contact Block
This is a separate add-on unit mounted directly onto the main device (ACB, MCB, or MCCB) to provide status feedback or additional control functions.
It features a snap-on or screw-mount design and can be added or removed without modifying the main device, allowing easy customization for feedback or control purposes.
Integral Type Auxiliary Contact
This type is built directly into the main switching device (such as a contactor), forming part of its mechanical structure and operating simultaneously with the main contacts.
It has a fixed design that is not removable or replaceable separately, ensuring faster and more reliable operation due to direct mechanical coupling.
What’s the Difference between Main Contact and Auxiliary Contact?
Compared with main contacts that handle load current in power circuits, auxiliary contacts are designed to manage low-level control voltages and signals used for system monitoring and automation.
| Feature | Main Contactor Contact | Auxiliary Contact |
| Primary function | Switches high-power loads | Sends control or feedback signals |
| Current capacity | High current (power circuits) | Low current (control circuits) |
| Typical use | Motors, heaters, power distribution | PLC signals, interlocks, status indication |
| Circuit type | Power circuit | Control or monitoring circuit |
Auxiliary Contact Design and Construction
Auxiliary contacts are designed to operate together with main switching devices such as circuit breakers, disconnect switches, relays, and contactors. Here are the key components of auxiliary contacts:
1. Contact Points
Contact points are metallic surfaces that open or close to enable circuit operation. Typically, silver alloys such as silver–nickel or silver–cadmium oxide are used to manufacture these contacts.
Different factors are considered when selecting materials for contacts:
- Low Resistance: Silver alloys have low resistance to current flow, which helps ensure proper signal transmission in control circuits.
- High Conductivity: This feature prevents excessive heating, even during switching operations.
- Resistance to Oxidation and Corrosion: Control panels often operate in dusty and humid environments. Silver alloys have good resistance to corrosion, which helps maintain reliable electrical connections.
- Arc Resistance: Although auxiliary contacts carry small currents, they can still generate small arcs during contact opening or closing. Silver alloys are a suitable option for handling these arcs without causing serious damage.
2. Actuating Mechanism
The actuating mechanism is the mechanical connection that links the auxiliary contact with the primary switching device.
When the main device, such as a contactor coil, is energized:
- The coil produces a magnetic field
- The plunger or armature moves toward the coil
- The main contacts open or close
- The auxiliary contacts change state simultaneously
The mechanism provides the following features:
- Synchronized Operation: The auxiliary contacts operate simultaneously with the main contacts, ensuring that their status accurately reflects the main device’s switching action.
- Automatic State Switching: When the primary device is activated, the auxiliary contacts automatically switch between open and closed positions.
- Accurate Position Feedback: Because the auxiliary contacts move in direct coordination with the main device, they provide reliable feedback about the device’s operating condition to the control system.
3. Housing
Auxiliary contact housing provides proper electrical insulation and protection from the environment.
Key functions of housing include:
- Electrical Insulation: Some housings are made of high-strength thermoset plastics, which provide excellent electrical insulation and prevent short circuits between terminals.
- Thermal Stability: It helps maintain structural integrity in control panels that may face high temperatures.
- Mechanical Protection: The housing provides mechanical strength to protect the internal structure from mechanical stress and vibrations.
- Environmental Protection: It protects contacts from moisture, dust, and other particles that could affect electrical performance or shorten the component’s lifespan.
4. Terminals
Terminals are external connection points used to connect auxiliary contacts with control circuits.
They are commonly designed in the form of:
- Screw terminals – secure and adjustable wiring
- Spring clamp terminals – tool-free wire insertion
- Plug-in connectors – fast installation and replacement
Terminals help create strong and reliable wire connections.
Design Considerations for Auxiliary Contacts
Auxiliary contacts are carefully designed based on electrical and mechanical characteristics.
Electrical Ratings
Auxiliary contacts are used in control circuits, not for handling large load currents.
Some typical electrical rating values are:
- Current rating: Usually 1 to 10 amperes
- Voltage rating: Ranges from 24 V to 240 V AC/DC
These ratings make them suitable for:
- PLC input signals
- Interlocking circuits
- Alarm circuits
- Control relays
Mechanical Life
Auxiliary contacts operate whenever the main device functions, so they must be capable of handling frequent mechanical operations.
Many industrial auxiliary contact designs are rated for hundreds of thousands to millions of switching cycles, allowing them to operate reliably during continuous use.
Modularity Design
Modularity is also an important design feature of auxiliary contacts. It allows auxiliary contactors to be easily integrated with various electrical devices while providing flexibility for system configuration and maintenance.
In many modern electrical systems, auxiliary contact blocks can be installed in multiple ways:
- Side-mounted
- Snap-mounted
- Front-mounted
This modular structure allows auxiliary contacts to be easily installed, removed, or replaced when needed.
The main features of modular design include:
- Enhanced functionality by expanding the capabilities of existing devices
- Flexible system customization to meet different control requirements
- Simplified maintenance with faster component replacement
- Reduced downtime during servicing or upgrades
The modular design and compact structure of auxiliary contacts make them easy to configure with:
- Motor control centers (MCCs)
- Industrial control panels
- Switchgear assemblies
- Building management systems
- Automation systems
They require little space and provide the necessary feedback and control functions.
In complex automation systems, auxiliary contacts operate like communication links between mechanical switching devices and electronic control systems, ensuring safe operation of electrical equipment.
How Do Auxiliary Contacts Work?
Auxiliary contacts operate with main switching devices such as relays, contactors, or circuit breakers. Their main function is to be physically linked with the position of the main contacts, so they change state when the main device turns on or off. While they do not carry the main load current, auxiliary contacts are used for control logic, status indication, and system automation.
The operation of auxiliary contacts involves three main phases, explained below:
1. Mechanical Coupling with the Main Device
Most auxiliary contacts are mechanically connected to the moving components of the main switching devices. Devices such as contactors or relays have moving armatures that shift when the device is energized.
When the main device coil receives voltage:
- The electromagnetic coil becomes energized
- The magnetic field attracts the armature toward the coil
- As a result of this movement, the main power contacts open or close
- Similarly, the position of the mechanically connected auxiliary contacts also changes
Because of this mechanical linkage, auxiliary contacts operate at the same time as the main contacts. This ensures that the control system accurately knows the device’s operating condition.
For example, a normally open auxiliary contact closes when the main device operates, while a normally closed auxiliary contact opens when the main device operates.
2. Signal Transmission in Control Circuits
When the state of an auxiliary contact changes, it opens or closes a low-current electrical circuit. Unlike main contacts that carry high load currents, auxiliary contacts switch small control signals used in monitoring and automation systems.
When the auxiliary contact closes, it can:
- Complete the circuit for an indicator lamp
- Send a signal to a PLC input module
- Activate another relay, timer, or control circuit to enable or disable certain functions
When the contact opens, it can:
- Interrupt the control signal
- Disable devices
- Activate a safety interlock
Since these signals use low current, auxiliary contacts are well-suited for logic operations and communication within control systems.
3. Feedback Loop for Monitoring and Automation
A primary function of auxiliary contacts is to provide a feedback loop that ensures electrical devices operate correctly and safely.
They can indicate important operating conditions, such as:
- Contactor: Energized or de-energized state
- Circuit breaker: Tripped status
- Disconnect switch: Open or closed position
This status information is transmitted to automation and monitoring platforms, including:
- Programmable logic controllers (PLCs)
- SCADA systems
- Building management systems (BMS)
Types of Auxiliary Contacts
Auxiliary contacts come in different types according to operation, installation, and functionality.
Based on Operation
Normally Open (NO) Auxiliary Contact
The contacts close when the main device operates. They are used for starting sequences, signaling, or interlocking.
Normally Closed (NC) Auxiliary Contact
The contacts open during the operation of the main device. They are commonly used for safety circuits, interlocking, and shutdown systems
Changeover (1NO 1NC) Auxiliary Contact
This type combines one NO and one NC contact with a common terminal, allowing switching between two circuits.
Based on Installation
Front-Mounted Auxiliary Contact Block
Connected to the front side of the primary device for easy access and maintenance.
Side-Mounted Auxiliary Contact Block
Connected to the sides of the device, providing additional contact options.
Mechanical Snap-On Auxiliary Contact Block
Provides a quick connection for modular designs.
Integral Auxiliary Contact
Built directly into the main device, it operates simultaneously with the main contacts and does not require separate installation.
Based on Electrical Function
Signaling Auxiliary Contact
Used for visual indicators or digital inputs to a PLC.
Interlocking Auxiliary Contact
Prevent operational conflicts, such as ensuring one breaker opens before another closes.
Time-Delay Auxiliary Contact
Configured with timers to provide delayed switching operations.
Instantaneous Auxiliary Contact
Changes state immediately with the operation of the main device.
What Are Auxiliary Contacts Used For?
Auxiliary contacts are used in many electrical system applications.
1. Control Feedback:
They provide the operating status of the main device to control systems, panels, or SCADA systems.
2. Sequential Control:
They enable staged operations, such as starting multiple motors in sequence to control inrush current and avoid overload.
3. Safety Interlocks:
They prevent hazardous operations, such as starting a motor while a safety guard is open.
4. Signaling and Alarms:
They trigger indicator lights, buzzers, or remote alarms.
5. Automation:
They send signals to PLCs for advanced process automation.
Auxiliary Contacts in Electrical Devices
Auxiliary contacts are used in different electrical devices to ensure safe and efficient operation.
Contactors
Contactors are electromechanical switches used to control power to motors or other high-current loads. A contactor may or may not contain auxiliary contacts, but when present, contactor auxiliary contacts provide essential functionality in control circuits.
Figure: Position and wiring diagram of contactor aux contacts
Auxiliary contacts on contactors perform functions such as:
- Sending motor status signals to control panels
- Interlocking with other contactors managing the same operation
- Supporting start/stop sequences in automated processes
ELEHUB provides high-quality contactors equipped with auxiliary contacts, ensuring reliable status feedback and seamless integration into control circuits.
Relays
Relays use a small control signal to switch a larger load.
Auxiliary contacts in relays are commonly used to:
- Provide feedback about the relay’s status
- Send alarm signals when the relay does not operate
Circuit Breakers
Circuit breakers are protective switching devices designed to interrupt electrical current under abnormal conditions, such as short circuits and overloads, thereby preventing damage to equipment and wiring.
In addition to their primary protection function, circuit breakers can be equipped with auxiliary contact modules to provide status feedback and control signals.
In most circuit breakers, including ACB, MCB, and MCCB, the auxiliary contact is provided as a separate accessory module rather than an integral part of the main current path. This module must be installed in a dedicated accessory compartment or slot within the breaker housing.
Figure: Installation position of the auxiliary contact inside an MCCB
When the circuit breaker changes position—such as ON, OFF, or TRIPPED:
- The internal mechanical linkage simultaneously actuates the auxiliary contact.
- This actuation causes the auxiliary contact to switch its electrical state, typically between normally open (NO) and normally closed (NC).
- By switching states, the auxiliary contact transmits a status signal to external control circuits.
Auxiliary contacts in circuit breakers are commonly used to:
- Indicate the breaker tripping condition
- Send signals to SCADA or the building management system
- Provide remote control and monitoring capabilities
For ELEHUB’s circuit breakers, an auxiliary contact for MCCB or ACB can be installed based on client requirements, offering flexible status feedback and enhanced control capabilities.
Disconnect Switches
Disconnect switches isolate parts of an electrical system for maintenance purposes.
Auxiliary contacts in disconnect switches are commonly used to:
- Ensure accurate switch position indication (open or closed)
- Connect to alarms and interlocking systems
- Enable remote status monitoring
By integrating auxiliary contacts, these devices provide real-time feedback, automated control, and improved operational safety across electrical systems.
Advantages of Auxiliary Contacts
- Simplified Wiring: Auxiliary contacts reduce the number of wires needed for different control points, minimizing wiring complexity, errors, and simplifying maintenance.
- Galvanic Isolation: They provide electrical isolation between input and output circuits, protecting sensitive components. A fault in one circuit does not affect other circuits.
- Signal Multiplication: A single control signal can operate multiple devices simultaneously, such as turning on lights and ventilation at the same time, improving installation ergonomics.
- Electrical Level Adaptation: Auxiliary contacts allow low-voltage signals (e.g., 24V) to control higher-voltage devices (e.g., 230V) without special equipment, facilitating system integration.
- Remote Monitoring: In circuit breakers, auxiliary contacts can transmit status information, such as open/closed state, to the main control system for real-time monitoring.
- Long Service Life: Auxiliary contactors typically operate up to 10 million cycles, providing reliable performance over extended periods.
- Improved Safety: Properly installed auxiliary contacts reduce the risk of circuit faults caused by wiring errors or damaged contacts.
Limitations of Auxiliary Contacts
- Limited Current Capacity: They are not designed for high-power loads; overloading can damage the contacts.
- Wear and Corrosion: Frequent operation or long-term use can lead to mechanical wear or corrosion, affecting reliability.
- Dependence on Main Device: Auxiliary contacts operate only when the main device functions; a failure in the primary device can impact their operation.
- Environmental Sensitivity: Factors like moisture, vibration, and dust can reduce performance and working life.
Auxiliary Contact Wiring
Accurate wiring and proper understanding of symbols are important for auxiliary contact connections.
Common Auxiliary Contact Symbols
- NO Contact: Represented by parallel open lines.
- NC Contact: Represented by parallel closed lines with a diagonal line crossing them.
- Combined Contacts: In some circuits, NC and NO contacts are shown together in multifunctional devices.
Connection Methods
Auxiliary contacts are connected in series or parallel combinations with control devices, depending on the required operation.
- Series wiring: Used when multiple contacts must all be closed or opened to complete a control circuit.
- Parallel wiring: Used to allow any one of multiple contacts to complete the circuit.
Wire Sizing and Insulation
For controlling voltage, proper wire sizing and insulation must be used.
Labeling
To avoid confusion, labels should be used for contacts on control panels.
Electrical Isolation
Ensure that electrical isolation exists between the main power circuits and the control circuits to prevent overloading.
Maintenance and Troubleshooting
Over time, auxiliary contacts may experience wear or degradation due to repeated mechanical operation, environmental factors, or contamination. Regular maintenance and timely troubleshooting are necessary to ensure optimal performance and prevent failures in control and monitoring circuits.
Common Issues
- Contact pitting – Small burns or erosion on contact surfaces due to repeated switching.
- Misalignment – Improper mechanical linkage causes contacts not to close or open correctly.
- Mechanical wear – Springs, levers, or linkage components may loosen or fatigue, affecting contact performance.
- Oxidation or corrosion – Environmental conditions may cause contacts to lose conductivity.
- Dust or debris accumulation – Can prevent proper contact engagement.
Troubleshooting Steps
Step 1: Verify Contact Function
- Check if the auxiliary contact responds to main device operation.
- Manually actuate the main device if possible and monitor the NO/NC response.
Step 2: Inspect Wiring
- Confirm that wiring follows the schematic.
- Ensure no loose or shorted connections.
- Check for broken wires or incorrect terminals.
Step 3: Measure Electrical Parameters
- Use a multimeter to check resistance across contacts.
- NO contacts should read open circuit when inactive; NC contacts should read closed.
- Excessive resistance indicates contact wear or contamination.
Step 4: Mechanical Inspection
- Examine springs, levers, and actuators for proper alignment.
- Replace worn or damaged auxiliary contact blocks if operation is inconsistent.
Step 5: Replace if Necessary
- If testing and inspection reveal damage or persistent operational issues, replace the auxiliary contact or contact block according to the manufacturer’s guidelines.
- Using the correct replacement ensures accurate NO/NC operation and restores reliable feedback to the control system.
Preventive Maintenance
Follow these steps for proper maintenance:
- Visual inspection helps identify corrosion or wear.
- Perform a mechanical operation test by manually operating the main device and observing the contact response.
- Conduct electrical testing to measure continuity for NO and NC contacts to verify correct operation.
- Remove dust and oxidation using a suitable contact cleaner.
- If any contact is damaged, replace it with a new one.
Conclusion
Auxiliary contacts are small but important components in electrical and automation systems. Their main functions include providing feedback, interlocking, signaling, and control. They help improve system functionality, safety, and reliability in devices such as contactors, circuit breakers, relays, and disconnect switches.
A proper understanding of their operation, design types, and applications helps in designing reliable control systems and performing effective circuit troubleshooting.
In practical projects, auxiliary contacts are often integrated with advanced switching devices such as the ELEHUB HVC600 High Voltage DC Contactor, which is compatible with SPST-NO auxiliary contacts and provides monitoring and control in high-power DC systems.
These contactors are used in applications with a rated current of 600 A and switching capability up to 1000 VDC, with coil voltages of 12 V and 24 VDC. ELEHUB integrates auxiliary contacts to provide system feedback and operational control in high-voltage DC power devices.
Although auxiliary contacts have some limitations, such as wear and low current capacity for signaling circuits, they are very useful for monitoring and control functions. Proper installation, wiring, and maintenance help ensure reliable operation and improved performance in industrial and power-control applications.
🔍 FAQs
Can auxiliary contacts handle high currents?
No, they are designed for control circuits and low-current signaling. High current flows through the main contacts.
What is the difference between NO and NC auxiliary contacts?
NO (Normally Open) contacts remain open when the device is in the off state and close when it operates. NC (Normally Closed) contacts remain closed when the device is off and open when the device operates.
Can auxiliary contacts be added to any contactor?
Modern contactors are often modular and allow additional auxiliary contacts to be connected according to the design and manufacturer’s specifications.
How often should auxiliary contacts be inspected?
Visual and electrical inspections should be performed annually or according to the manufacturer’s maintenance schedule.
Why are auxiliary contacts important in automation?
They provide feedback to control systems such as PLCs and help operate automated processes, sequential control, and safety interlocks.
Can auxiliary contacts fail independently of the main device?
Yes, they can experience mechanical or electrical failure even if the main device is operating correctly, so proper testing should be performed.
