An auxiliary contact is a low-current control contact used for signaling, interlocking, feedback, self-holding circuits, and PLC input status. A main contact is a power contact used to switch the main load current, such as a motor, heater, lighting circuit, or feeder.
Kısa versiyonu basittir: main contacts carry the load; auxiliary contacts tell the control circuit what happened. They are mechanically linked in many contactors and breakers, but they are not interchangeable.
In a contactor, main contacts are usually marked as L1/T1, L2/T2, and L3/T3. Auxiliary contacts are commonly marked as 13-14 for normally open (NO) and 21-22 for normally closed (NC). In circuit breakers, auxiliary contacts are used to indicate open/closed/tripped status to a control circuit, alarm circuit, building management system, or PLC.
Quick Comparison: Auxiliary Contact vs Main Contact
| Öğe | Main Contact | Auxiliary Contact |
|---|---|---|
| Ana işlev | Switches the power circuit | Signals or controls the control circuit |
| Current level | Higher load current | Lower control current |
| Typical contactor terminals | L1/T1, L2/T2, L3/T3 | 13-14 NO, 21-22 NC, 31-32 NC, 43-44 NO |
| Tipik yükler | Motor, heater, lighting, feeder, transformer | PLC input, indicator lamp, interlock, self-holding circuit |
| Arc duty | Designed for load switching within its utilization category | Not designed to break main load current |
| Contact material and size | Larger, heavier contact system | Smaller signal/control contact system |
| Birbirinin yerine kullanılabilir mi? | Hayır | Hayır |
| Hatalı kullanım durumunda ana risk | Wrong utilization category, contact welding, overheating | Burned auxiliary contact, false feedback, control failure |
An auxiliary contact does not increase the amount of current a contactor can switch. It also does not provide overload or short-circuit protection. Those functions belong to devices such as circuit breakers, fuses, overload relays, motor protection circuit breakers, and protection relays.
What Is a Main Contact?
A main contact is the power-switching contact inside a contactor, relay, circuit breaker, or switch. In a contactor, the main contacts connect and disconnect the load circuit when the coil is energized or de-energized.
For a three-phase contactor, the main power terminals are commonly labeled:
- L1 to T1
- L2 to T2
- L3 to T3
The L side is normally the line or supply side. The T side is normally the load side. For more detail on contactor terminal markings, see VIOX’s guide on what L1, L2, T1, and T2 mean on a contactor.
Main contacts are selected by load type and utilization category. For example, an AC-3 motor load is much harder on a contactor than an AC-1 resistive load because motor starting and interruption involve inductive current. If the main contacts are selected incorrectly, the result can be contact overheating, welding, excessive arcing, or short service life.
For load category details, see VIOX’s guide to AC-1, AC-2, AC-3, and AC-4 contactor utilization categories.
What Is an Auxiliary Contact?
An auxiliary contact is an extra mechanically operated contact used in the control circuit. It changes state when the contactor, relay, or circuit breaker changes state.
In a contactor, an auxiliary contact is often mounted on the front or side of the contactor body. When the contactor coil pulls in, the auxiliary contact changes from open to closed or from closed to open depending on its type.
Auxiliary contacts are used for:
- Self-holding or seal-in circuits
- Electrical interlocking between contactors
- PLC or controller feedback
- Gösterge ışıkları
- Alarm signals
- Remote status indication
- Sequence control
- Safety or permissive logic, when used within the correct control design
The important boundary is this: an auxiliary contact is a control contact, not a power contact. It should not be used to switch a motor, heater, compressor, pump, or other main load.
AC-15 and DC-13 Ratings for Auxiliary Contacts
Main contacts are selected by power-load utilization categories such as AC-1, AC-3, or AC-4. Auxiliary contacts are usually evaluated as control-circuit devices, so the more relevant categories are often AC-15 ve DC-13 under IEC 60947-5-1.
| Kullanım Kategorisi | Tipik Yük Tipi | Why It Matters for Auxiliary Contacts |
|---|---|---|
| AC-15 | AC electromagnetic control loads, such as contactor coils, relay coils, and solenoids | Defines how well the contact handles inductive AC control loads |
| DC-13 | DC electromagnetic control loads, such as DC relay coils, solenoids, or PLC-controlled coils | More demanding because DC arcs do not naturally cross zero |
This is why a small auxiliary contact may have different current ratings for resistive loads, AC-15 control loads, and DC-13 control loads. A 24 V DC coil circuit is not automatically easier than a 230 V AC indicator lamp circuit; the coil inductance and arc energy matter.
For B2B procurement, do not specify only “1NO auxiliary contact.” Ask for the contact configuration and the AC-15/DC-13 ratings at the actual control voltage used in the panel.
NO and NC Auxiliary Contacts Explained
Auxiliary contacts are usually described as NO or NC.
| İletişim Türü | Tam Adı | State When Device Is Not Operated | State When Device Operates | Ortak Kullanım |
|---|---|---|---|---|
| HAYIR | Normalde Açık | Open | Closes | Self-holding, run indication, PLC “contactor ON” feedback |
| NC | Normalde Kapalı | Closed | Opens | Interlock, stop permissive, fault logic, opposite-state indication |
“Normally” means the normal rest state of the contactor or breaker mechanism, not the normal condition of the machine. For a contactor, normal usually means the coil is not energized.
Common Auxiliary Contact Terminal Numbers
IEC-style terminal numbering helps technicians identify contact function without guessing.
| Terminal Number | İletişim Türü | Anlamı |
|---|---|---|
| 13-14 | NO auxiliary contact | Closes when the device operates |
| 21-22 | NC auxiliary contact | Opens when the device operates |
| 31-32 | NC auxiliary contact | Additional NC contact |
| 43-44 | NO auxiliary contact | Additional NO contact |
| 53-54 | NO auxiliary contact | Additional NO contact in some devices |
| 61-62 | NC auxiliary contact | Additional NC contact in some devices |
Always check the wiring diagram printed on the product or datasheet. Terminal numbering is a strong guide, but the actual device configuration must be verified before wiring.
How Auxiliary Contacts Work in a Contactor
In a contactor, the coil creates a magnetic field that pulls the armature closed. This movement closes the main contacts and changes the auxiliary contact state at nearly the same time.
For a simple motor starter:
- The operator presses the start pushbutton.
- The contactor coil energizes.
- The main contacts close and supply the motor.
- A NO auxiliary contact closes.
- That NO auxiliary contact bypasses the start pushbutton and holds the coil circuit energized.
- When the stop button or overload contact opens, the coil drops out and the auxiliary contact returns to its rest state.
This is called a self-holding, seal-in, or latching control circuit. The auxiliary contact is not powering the motor; it is only maintaining the control circuit.
Common Applications of Auxiliary Contacts
1. Self-Holding Circuit
A NO auxiliary contact keeps a contactor coil energized after the start button is released. This is one of the most common uses in motor control panels.
2. Electrical Interlock
An NC auxiliary contact can prevent two contactors from energizing at the same time. This is common in forward/reverse starters, star-delta starters, and transfer circuits.
In a reversing motor starter, the forward contactor’s NC auxiliary contact is placed in series with the reverse coil, and the reverse contactor’s NC auxiliary contact is placed in series with the forward coil. This helps prevent both contactors from closing together.
3. PLC Feedback
A NO auxiliary contact can tell a PLC that a contactor has pulled in. This is often used for status monitoring, sequence confirmation, or alarm logic.
For higher reliability, engineers should understand that auxiliary feedback confirms the contactor mechanism position, not always the actual load current. If the system needs proof that current is flowing, a current sensor or power monitor may also be required.
4. Indicator Light
A NO auxiliary contact can turn on a “RUN” indicator when the contactor is energized. An NC contact can be used for an opposite-state indication if the circuit is designed correctly.
5. Control Sequence Logic
Auxiliary contacts can create simple hardwired logic between relays, timers, contactors, and overload relays. For example, a fan contactor may be permitted to start only after a damper contactor has reached a confirmed state.
Auxiliary Contacts in Circuit Breakers
Circuit breakers can also use auxiliary contacts, but the purpose is usually status indication rather than motor-control self-holding.
In MCCBs, ACBs, and some industrial breakers, auxiliary contacts may indicate:
- Breaker open
- Breaker closed
- Breaker tripped
- Remote status for PLC or SCADA
- Position feedback in a control panel
Circuit breaker auxiliary contacts should not be confused with protection functions. They do not trip the breaker by themselves. They only change contact state to report breaker status.
In some switchgear and breaker documentation, especially in ANSI/IEEE-style project language, auxiliary contacts may also be described as “a contacts” ve “b contacts.” Bir a contact is typically open when the breaker is open and closed when the breaker is closed. A b contact is typically closed when the breaker is open and open when the breaker is closed. Always confirm the device wiring diagram, because project conventions and accessory modules can vary.
For a breaker-specific discussion, see VIOX’s MCCB auxiliary contacts guide.
Auxiliary Contact vs Alarm Contact vs Shunt Trip
These terms are often mixed together, but they do different jobs.
| Device or Contact | Ana Fonksiyon | Tipik Kullanım |
|---|---|---|
| Yardımcı kontak | Indicates device position or supports control logic | ON/OFF feedback, interlock, self-holding |
| Alarm kontağı | Indicates trip or fault condition | Trip alarm, fault signal, remote indication |
| Şönt açtırma bobini | Electrically trips the breaker when energized | Emergency stop, fire alarm trip, remote trip |
| Düşük gerilim bobini | Trips or prevents closing when voltage is too low | Safety interlock, voltage-loss protection |
| Main contact | Switches load current | Motor, feeder, heater, lighting power circuit |
A shunt trip is an actuator. An auxiliary contact is a signal contact. They are not the same device. For breaker trip accessories, see VIOX’s guides on shunt trip coils in MCCBs ve shunt trip vs trip coil.
Auxiliary Contact vs Auxiliary Contactor
An auxiliary contact is a contact block or contact element. An auxiliary contactor is a small contactor or control relay used mainly for control circuits.
| Dönem | Anlamı | Örnek |
|---|---|---|
| Yardımcı kontak | Additional NO or NC contact mechanically linked to a main device | 1NO side-mounted contact on a contactor |
| Auxiliary contact block | Modular add-on contact unit | Front-mounted 2NO+2NC contact block |
| Auxiliary contactor | Small contactor/control relay with multiple control contacts | Used for signal multiplication or control logic |
This distinction matters in purchasing. If you need one extra feedback contact, you may need an auxiliary contact block. If you need multiple control contacts operated by a coil, you may need an auxiliary contactor or control relay.
How to Choose an Auxiliary Contact
Before ordering an auxiliary contact block, confirm:
| Seçim Öğesi | Ne Kontrol Edilmeli |
|---|---|
| Cihaz uyumluluğu | Contact block must fit the exact contactor, MCCB, ACB, or relay series |
| Kontak konfigürasyonu | 1NO, 1NC, 1NO+1NC, 2NO, 2NC, or other combination |
| Montaj konumu | Front-mounted, side-mounted, plug-in, or internal accessory |
| Control voltage and current | Must match PLC input, relay coil circuit, lamp circuit, or control circuit load |
| AC-15 / DC-13 değerleri | Confirms suitability for AC or DC electromagnetic control loads |
| AC/DC değerleri | DC control circuits can be harder to break than AC control circuits |
| Minimum switching load | Important for low-level PLC signals |
| Terminal tipi | Screw, spring, push-in, or plug-in terminal |
| Elektriksel ömür | Must suit switching frequency and control duty |
| Mechanical linkage | Contact must change state reliably with the device mechanism |
For contactor selection more broadly, see VIOX’s contactor guide ve contactor vs relay comparison.
Common Wiring Mistakes
Field Case: Using an Auxiliary Contact to Switch a Load
In one panel review, a small auxiliary contact had been wired directly to switch a cooling fan instead of driving a relay or contactor coil. The circuit worked during commissioning, but the contact soon showed welding and discoloration because the fan’s inrush and inductive behavior exceeded what the auxiliary contact was intended to break.
That is the practical reason behind the rule: auxiliary contacts are not main power contacts. Do not use a 13-14 contact to switch a motor, heater, compressor, fan, or large lamp load unless the contact rating and utilization category clearly allow that duty.
Mistake 2: Assuming NO Means “Power On”
NO and NC describe the rest state of the contact, not the desired process state. Always check whether the coil or breaker mechanism is energized, de-energized, open, closed, or tripped.
Mistake 3: Confusing Auxiliary Contact and Alarm Contact
An auxiliary contact may indicate open or closed position. An alarm contact usually indicates a trip or fault event. Using the wrong one can send a false signal to a PLC or alarm system.
Mistake 4: Ignoring DC Switching Limits
A small auxiliary contact may handle a certain AC control current but a lower DC current because DC arcs do not naturally pass through zero. Always check AC-15 and DC-13 ratings separately at the actual control voltage.
Mistake 5: Relying on Auxiliary Feedback as Proof of Load Current
An auxiliary contact can confirm mechanism movement, but it does not measure current. If process safety depends on actual motor current or load current, use a current sensor, overload relay, or monitoring device.
Mistake 6: Adding Too Many Contact Blocks
Contactors often have limits on how many front or side auxiliary blocks can be installed. Too many blocks can affect fit, panel spacing, or mechanical operation.
SSS
What does 13-14 mean on an auxiliary contact?
13-14 usually identifies a normally open auxiliary contact. It closes when the contactor or device operates.
What does 21-22 mean on an auxiliary contact?
21-22 usually identifies a normally closed auxiliary contact. It opens when the contactor or device operates.
What is AC-15 on an auxiliary contact?
AC-15 is an IEC utilization category for AC electromagnetic control loads, such as contactor coils, relay coils, and solenoids. It is more relevant to auxiliary contact selection than AC-3 or AC-4, which apply to main power contact duties.
What is DC-13 on an auxiliary contact?
DC-13 is an IEC utilization category for DC electromagnetic control loads. It is important because DC coil circuits can be harder for small contacts to interrupt than similar-looking AC control circuits.
What is an a contact and b contact on a circuit breaker?
In breaker and switchgear language, an a contact usually follows the closed position of the breaker, while a b contact gives the opposite state. Confirm the actual accessory diagram before wiring, especially in ANSI/IEEE-style projects.
Can an auxiliary contact carry motor current?
No. An auxiliary contact is not designed to carry main motor current. Motor current must be switched by the contactor’s main contacts or another properly rated power switching device.
Is an auxiliary contact the same as an auxiliary contactor?
No. An auxiliary contact is an individual contact or contact block. An auxiliary contactor is a small control contactor or relay used to switch control circuits.
Is a circuit breaker auxiliary contact the same as a trip contact?
Not necessarily. An auxiliary contact may show breaker open/closed status. An alarm contact usually indicates trip status. A shunt trip is a coil-operated mechanism that trips the breaker.
Can an auxiliary contact be NO and NC at the same time?
A single contact element is either NO or NC, but one auxiliary contact block can contain both NO and NC contacts, such as 1NO+1NC or 2NO+2NC.
Sonuç
Main contacts and auxiliary contacts are both essential, but they solve different problems. Main contacts switch load current. Auxiliary contacts provide control logic, interlocking, feedback, and status indication.
For contactors, the most common auxiliary contact uses are self-holding, interlock, indicator lights, and PLC feedback. For circuit breakers, auxiliary contacts are mainly used for remote status and control-system feedback. In both cases, the correct selection depends on contact configuration, device compatibility, electrical rating, AC/DC duty, and the control circuit design.
If one rule prevents most wiring mistakes, it is this: never use an auxiliary contact as a power contact, and never assume an auxiliary signal proves the load is actually powered.
