NO and NC Auxiliary Contacts on a Contactor: Functions, Wiring, and Uses

What are NO and NC auxiliary contacts on a contactor? NO (normally open) and NC (normally closed) auxiliary contacts are low-current switching elements integrated into or mounted alongside a contactor, typically rated 6–10 A under IEC 60947-5-1 utilization category AC-15, used for control circuit signaling, interlocking, and feedback rather than load switching. Applying main-circuit ratings to auxiliary contacts — or confusing AC-15 with AC-3 — leads to accelerated contact erosion, failed interlock logic, and non-compliant control circuit protection. This guide covers auxiliary contact definitions and construction, signaling versus interlocking functions, IEC utilization categories and rating distinctions, schematic reading conventions, common field wiring errors, and application-specific selection criteria.

What NO and NC Auxiliary Contacts Actually Are

Walk into any motor control center built in the last forty years and you will find contactors with little blocks clipped to the top or side. Those are the auxiliary contacts. The main poles below them switch the motor. The auxiliaries do everything else.

An auxiliary contact is a secondary set of switching contacts driven by the same electromagnet that pulls the main contactor closed. When the coil energizes, the armature moves, and every contact attached to that armature changes state simultaneously. Mechanically linked. Electrically isolated from the main poles.

The two-digit terminal scheme is not arbitrary. Per IEC 60947-5-1 Annex L, the first digit indicates the contact's sequence number on the device, and the second digit indicates the function. Once you know this, reading a German or Italian schematic becomes trivial regardless of language. A terminal labeled 53–54 is the fifth auxiliary contact, and it is normally open. A terminal labeled 21–22 is the second auxiliary, and it is normally closed.

Why mechanical linkage matters

Engineers often overlook one detail: the NO and NC contacts on a single contactor are not independent. They are mechanically forced. When one closes, the other opens, and they cannot ever be closed at the same time. This is the basis of "mirror contacts" — a specific subclass of NC auxiliary defined under IEC 60947-4-1 Annex F, where the NC contact is guaranteed to be open whenever any main pole is welded closed. Mirror contacts are used in safety circuits to detect contact welding before a restart.

The Two Jobs Auxiliary Contacts Do in Practice

In the field, auxiliary contacts do two things. They feed back state to a control system, and they participate in the control logic that energizes the coil. Sometimes both at once.

Self-holding (latching) circuits

The most common use of an NO auxiliary is the seal-in contact. You press a momentary START button, current flows to the coil, the contactor pulls in, and one of its NO auxiliaries closes in parallel with the START button. Now you can release the button — the contactor holds itself closed through its own auxiliary. Press STOP (a normally closed pushbutton in series with the coil), and the whole chain breaks. The contactor drops out, the seal-in NO opens, and the circuit is back to its rest state.

This is the classic three-wire control circuit. It has been used since the 1930s. It still works because it fails safely: any wire break, any loss of control voltage, drops the motor. There is no software involved, no firmware bug, no PLC scan time to worry about.

Interlocking

NC auxiliaries are the workhorse of interlocking. In a reversing starter, the NC contact of the FORWARD contactor is wired in series with the REVERSE contactor's coil, and vice versa. If FORWARD is energized, its NC opens, making it electrically impossible to energize REVERSE. This prevents a phase-to-phase short across the motor terminals if both contactors closed simultaneously — a fault that would typically draw 20–40 kA into your bus and trip the upstream breaker, assuming the breaker is rated for it.

In our experience, the electrical interlock alone is not enough for high-power reversing applications. Above roughly 30 kW, we always specify mechanical interlocks in addition — a physical lever that prevents both armatures from closing. ABB's AF and EH series accept clip-on mechanical interlock kits, and we treat them as non-negotiable on cranes, hoists, and any reversing duty above AC-3 30 kW at 400 V.

Status feedback to PLCs

Modern installations rarely run pure relay logic. The contactor still has the auxiliaries, but now one NO feeds a 24 VDC PLC input to confirm the contactor closed when commanded. If the PLC commanded ON and the auxiliary stays open longer than, say, 200 ms, the PLC raises a "contactor failed to close" alarm. This catches stuck armatures, blown coils, and broken control wiring.

Ratings, Utilization Categories, and Why AC-15 Is Not AC-3

Here is where procurement decisions get expensive if you guess wrong. The current rating printed on the side of an auxiliary contact block is meaningless without the utilization category.

Per IEC 60947-5-1, auxiliary contacts are tested and rated under specific load conditions:

AC-15 covers the control of electromagnetic loads — coils of other contactors, relays, solenoid valves. The inrush current of a contactor coil can reach 6–10 times the holding current for the first 30–50 ms while the magnetic gap closes. A typical 230 V AC contactor coil drawing 50 mA holding might pull 400 mA inrush. The contact has to make against that.

DC-13 is harsher still. Inductive DC loads do not have a natural zero crossing, so when the contact opens, the arc has to be physically stretched until it self-extinguishes. A 24 VDC relay coil that looks innocent on paper will pit and weld auxiliary contacts rated only for AC-15 within a few thousand operations.

Real numbers from real catalogs

An ABB CA5-10 auxiliary block has I_th = 10 A but is rated:

• AC-15 at 230 V: 6 A
• AC-15 at 400 V: 3 A
• DC-13 at 24 V: 6 A
• DC-13 at 220 V: 0.27 A

Notice that last one. At 220 VDC, the same physical contact can only switch 270 milliamps reliably for its rated electrical life of 100,000 operations. We have seen plants put 220 VDC station-battery loads on standard auxiliary blocks and wonder why contacts fail every six months. The answer is in the datasheet.

Wiring Conventions and Reading Schematics

The IEC terminal scheme is consistent across ABB, Siemens, Schneider, Eaton, and most other major brands. NEMA conventions differ, and this catches engineers moving between projects in North America and Europe.

Counting contacts on installation contactors

Installation contactors used in distribution boards (also called modular contactors, DIN-rail mounted) have their NO/NC count built into the part number. ABB's ESB series is a clear example. The ABB 1SBE111111R0611 ESB16-11N-06 has the suffix "11" — meaning 1 NO + 1 NC. The ESB16-02N-06 has "02" — zero NO and two NC, suitable for staircase lighting that should fail to ON if the timer fails. The ESB25-22N-06 gives you 2 NO + 2 NC at 25 A. The ESB25-31N-06 gives 3 NO + 1 NC at the same frame.

For higher current installation duty, the ESB63-40N-06 at 63 A gives 4 NO and no NC, while the ESB63-31N-06 gives 3 NO + 1 NC. The point: on installation contactors, the contact configuration is fixed at the factory. You cannot clip on extra auxiliaries the way you can with industrial contactors. Specify carefully.

Common Wiring Mistakes from the Field

Some patterns repeat on every commissioning job. They are worth naming.

Putting the seal-in on the wrong side of the STOP button

The seal-in NO must be in parallel with the START pushbutton, not with the entire start chain. If you wire the seal-in around the STOP button, pressing STOP does nothing because the contactor holds itself through the seal-in. We have seen this on a refurbished conveyor in a cement plant in 2019 — the previous contractor had moved the seal-in jumper, and the only way to stop the conveyor was to trip the upstream breaker. It took three minutes to find. Three minutes is a long time when a 75 kW belt is running material into a stockpile.

Using AC-15 contacts to switch contactor coils above 230 V

A 400 V control circuit is unusual but not rare in older European installations. The auxiliary contact rating drops sharply with voltage. At 400 V AC-15, the same block that handled 6 A at 230 V is now rated 3 A or less. If the coil being switched draws 0.5 A holding but 4 A inrush, you are right at the edge. Contact welding follows.

Forgetting that NC contacts close on power loss

A common mistake is using an NC auxiliary as an "alarm on contactor open" indicator without realizing that the alarm will also activate every time the control supply blinks. If the upstream control transformer drops out for 100 ms, every NC auxiliary in the panel closes, every alarm input on the PLC goes active, and the operator sees a wall of red. Use NO auxiliaries with proper "running" logic for status, not NC for "stopped" logic, unless your PLC code explicitly debounces.

Mixing AC and DC on the same auxiliary block

Auxiliary blocks have a single insulation rating between contacts within the block. Mixing 230 VAC on one pair and 24 VDC on the adjacent pair is permitted electrically but creates trouble during fault diagnosis and during EMC testing. Capacitive coupling between adjacent contacts can inject noise into the 24 V signal. We separate AC and DC circuits across different auxiliary blocks as standard practice.

Selecting Auxiliary Contacts for Specific Applications

Different industries have different priorities. The same contactor frame might need very different auxiliary configurations depending on what it controls.

Motor starters in a process plant

Standard configuration: 1 NO for seal-in, 1 NO for "running" feedback to the DCS, 1 NC for "stopped" interlock to upstream permissives. That is 2 NO + 1 NC minimum. A side-mount 4-pole auxiliary block (2 NO + 2 NC) gives you a spare and is what we typically specify.

Reversing drives on cranes and hoists

Each contactor needs at least 1 NC for cross-interlocking the opposite direction's coil, plus 1 NO for seal-in, plus feedback. We add a mirror contact (per IEC 60947-4-1 Annex F) for safety category 3 circuits where contact welding detection is required. The NC mirror contact stays open whenever any main pole is welded closed, allowing the safety relay to detect the fault.

Star-delta starters

Three contactors: main, star, and delta. The star contactor's NC is wired in series with the delta coil and vice versa. The main contactor needs an NO for seal-in plus a "running" feedback. The timer that switches from star to delta needs the main contactor's NO as its enabling input. This is a classic application where every auxiliary contact is doing real work — there are no spares left, and the engineer who designed it knew it.

Distribution board contactors for HVAC and lighting

The ESB16-11N series is typical for switching lighting circuits up to 16 A AC-1 per pole. The single NO auxiliary feeds a contactor-on lamp at the panel door. The single NC drives a "lighting off" alarm to the BMS. For 400 Hz applications — aviation ground power, naval auxiliary buses — the ESB63-40N-06 rated for 400 Hz operation is the correct choice. Standard 50/60 Hz contactors can chatter at 400 Hz because the magnetic shading rings are tuned for the wrong frequency.

Coordination with residual current protection

In modern panels, the contactor is downstream of an RCCB (residual current circuit breaker). The ABB F202 AC-100/0.03 at 100 A 30 mA Type AC and the ABB FH204 A-25/0.03 at 25 A Type A are typical upstream devices. The auxiliary contact on the contactor reports state to the BMS, but you also need to consider what happens when the RCCB trips: the contactor coil loses supply, all NO auxiliaries open, all NC auxiliaries close. Make sure your alarm logic distinguishes "operator stopped the load" from "RCCB tripped" — usually by monitoring an auxiliary contact on the RCCB itself.

Maintenance, Diagnostics, and When to Replace

Auxiliary contacts fail in predictable ways. Recognizing the failure mode tells you whether to replace the block, the whole contactor, or rethink the application.

Welded contacts

If an NOauxiliary refuses to open after the coil de-energizes, the contacts have welded. This usually means the contact was switching a load above its rated breaking capacity, or the load had a much higher inrush than predicted. Replace the block. If multiple auxiliary blocks on the same contactor weld within months, the application is wrong — move to an interposing relay or a higher-rated block.

Pitting and erosion

Open the auxiliary block after a few hundred thousand operations and you will see small craters on the contact surfaces. This is normal arc erosion. Silver-nickel and silver-cadmium-oxide contact materials are designed to erode predictably. The contact resistance creeps up over time. When it exceeds a few hundred milliohms, the voltage drop across the contact becomes significant for low-current PLC inputs, and the input may not see a clean ON state. Replace the block when intermittent feedback alarms start appearing.

Mechanical wear

The plastic carrier that holds the auxiliary contact bridges to the armature wears over millions of operations. You will hear it before you see it — the contactor sounds different, slightly louder, slightly less crisp. Mechanical life of a typical IEC auxiliary block is around 10 million operations versus electrical life of 100,000 to 1 million depending on load. In high-cycling applications such as resistance welders or pump-down compressors, mechanical wear becomes the limit, not electrical.

Diagnostic procedure

When a contactor circuit misbehaves, work through the auxiliary contacts systematically. Disconnect control power. Measure resistance across each NC contact pair — should be under 50 mΩ. Measure across each NO contact pair — should read open. Manually depress the armature (most contactors allow this with the cover off). The readings should swap. If they do not, or if the resistance values are wrong, replace the block before chasing PLC code or wiring faults.

Standards That Govern Auxiliary Contacts

Several standards apply, and each addresses a different aspect. Knowing which clause governs which property avoids long debates with auditors and helps procurement teams write defensible specifications.

IEC 60947-1 establishes the general definitions and terminology. Clauses 3.1.18 and 3.1.19 define NO and NC. Annex L gives the terminal numbering convention.

IEC 60947-4-1 covers contactors and motor starters. Clause 8.2.1.2 specifies the requirements for auxiliary contacts integrated with contactors. Annex F defines mirror contacts and their testing.

IEC 60947-5-1 covers control circuit devices and switching elements specifically. This is the standard that defines AC-15 and DC-13 utilization categories. Clause 7.2.1.4 specifies the conventional thermal current. Clause 8.3.3.5 specifies the verification of operational performance — the endurance test that determines electrical life ratings.

IEEE C37.90 covers relay contact requirements for protective relaying applications, more relevant in switchgear than motor control, but the surge withstand test (IEEE C37.90.1) is sometimes referenced for auxiliary contacts in utility substation auxiliaries.

NEMA ICS 5 covers North American conventions for control circuit and pilot devices. The A600 designation under NEMA ICS 5 corresponds roughly to AC-15 under IEC, but the test conditions differ slightly. A contact rated A600 NEMA can switch 720 VA make and 72 VA break at up to 600 V AC. When sourcing globally, do not assume NEMA A600 and IEC AC-15 are identical — they are similar, not equivalent.

UL 60947-4-1 is the harmonized North American version of IEC 60947-4-1. Most modern contactors carry both IEC and UL certifications, with the auxiliary blocks rated under both schemes on the same datasheet. Read both columns. The values can differ.

Conclusion

NO and NC auxiliary contacts are not exotic components. They are the quiet workhorses inside every motor control panel, every distribution board, every reversing starter on every conveyor in every industrial plant. The fact that they are simple does not mean specifying them is trivial.

The key technical points: NO contacts close when the coil energizes and use terminal suffix 3-4. NC contacts open when the coil energizes and use terminal suffix 1-2. Their state is mechanically forced by the same armature, which is why they are useful for both seal-in latching and interlocking. Their current ratings depend strongly on utilization category — AC-15 for AC inductive loads, DC-13 for DC inductive loads — and voltage. The same contact rated 6 A at 230 V AC-15 may only handle 0.27 A at 220 V DC-13.

The procurement points: on industrial contactors, auxiliary blocks are field-installable, so you can add them later but at the cost of downtime. On installation contactors such as the ABB ESB series, the NO/NC count is baked into the part number, so specify carefully at order time. Always order at least one spare NO and one spare NC beyond the strict logical requirement. Spare contacts cost cents at order time and save hours during commissioning when the control engineer asks for one more feedback signal.

The standards points: IEC 60947-5-1 governs the auxiliary contact ratings and test methods. IEC 60947-4-1 Annex F defines mirror contacts for safety applications. NEMA ICS 5 defines parallel North American conventions. Read the actual clauses when writing specifications — vague references to "IEC standard" do not survive procurement audits.

And finally, the field point. The cleanest control panel in the world is the one where every auxiliary contact has a labeled purpose, where spares are documented, and where the schematic matches the wiring. That kind of discipline starts at the bill of materials. Choose the right contactor configuration up front, specify auxiliary blocks that exceed your immediate needs by a margin, and the panel will still be serviceable in twenty years when whoever inherits it opens the door for the first time.