Contactor and relay terminals are numbered with a code: letters A1 and A2 for the coil, numbers 1-2, 3-4 and 5-6 for the power poles, and two digits for auxiliary contacts, where the units digit gives the function (1-2 normally closed, 3-4 normally open) and the tens digit the sequence number. The overload relay uses 95-96 and 97-98.
Why terminals are numbered
A motor control panel holds dozens of contactors and relays. If each terminal had an arbitrary label, nobody could wire, check or troubleshoot without the equipment manual. Standardized numbering lets you read a diagram and know which terminal is which regardless of brand. This reading follows the European practice for marking the terminals of control devices, which most manufacturers use.
Contactor: power, coil and auxiliaries
A three-pole contactor with an auxiliary contact block shows three groups of terminals:
| Group | Terminals | What they are |
|---|---|---|
| Coil | A1, A2 | Control coil supply |
| Power poles | 1-2, 3-4, 5-6 | Three poles; input at the top (1, 3, 5) and output at the bottom (2, 4, 6) |
| Auxiliary contacts | 13-14, 21-22, 33-34, 43-44, etc. | Signal and seal-in contacts |
On the power side it is usual for terminals 1, 3, 5 to receive L1, L2, L3, and for 2, 4, 6 to connect to T1, T2, T3 toward the load. The poles of a four-pole contactor add 7-8 or the pair the manufacturer indicates.
Auxiliary contacts: how to read two digits
On an auxiliary contact the code has two digits:
- The units digit gives the function: 1-2 for a normally closed (NC) contact and 3-4 for a normally open (NO) contact.
- The tens digit gives the sequence number of the contact in the device.
| Marking | Reading |
|---|---|
| 13-14 | First auxiliary contact, NO |
| 21-22 | Second auxiliary contact, NC |
| 33-34 | Third contact, NO |
| 41-42 | Fourth contact, NC |
| 53-54, 61-62, 71-72, 83-84 | Fifth and following, with the function given by the units digit |
A four-contact block can be 13-14, 23-24, 33-34 and 43-44 (four NO), or 13-14, 21-22, 33-34 and 41-42 (two NO and two NC). The type of each contact is read from the units digit, and its symbol is usually printed on the device.
There are special-function contacts, with units 5-6 (NC timed on opening or on closing) and 7-8 (timed NO), in timers and delay relays. Auxiliary relays with changeover contacts use three terminals numbered 11-12-14: 11 is the common, 12 the NC and 14 the NO.
Overload relay (thermal or electronic)
| Terminals | Function |
|---|---|
| 1-2, 3-4, 5-6 (with L1, L2, L3 and T1, T2, T3) | Measuring elements in series with the phases |
| 95-96 | NC trip contact; goes in series with the contactor coil |
| 97-98 | NO trip signaling contact |
When the relay trips, contact 95-96 opens and 97-98 closes. Some manufacturers use their own layout, and on some equipment the auxiliary contact numbering follows a different sequence; the diagram printed on the device prevails.
When the overload relay is mounted directly on the contactor, its terminals 1, 3, 5 line up with terminals 2, 4, 6 of the contactor. This makes the connection possible without an intermediate cable.
Other devices
- Circuit breaker and motor circuit breaker: the power poles are marked 1-2, 3-4, 5-6 as on the contactor.
- Push buttons and selectors: the NC contact is marked 1-2 and the NO 3-4, or with two-digit numbering in multi-contact blocks.
- Auxiliary relay coils: A1 and A2.
- Timers: A1/A2 for the supply and 15-16-18 for the changeover contact, depending on the model.
Example (illustrative)
A direct-on-line starter with a contactor and an overload relay is to be wired:
| Connection | From | To |
|---|---|---|
| Power input | L1, L2, L3 | Contactor terminals 1, 3, 5 |
| Power output | Contactor terminals 2, 4, 6 | Overload relay terminals 1, 3, 5 |
| Power to motor | Relay terminals 2, 4, 6 | Motor U, V, W |
| Control: overload | Control phase | Relay terminal 95 |
| Control: stop | Relay terminal 96 | Stop push button (NC) |
| Control: seal-in | Start in parallel with contactor contact 13-14 | Coil A1 |
| Control neutral | Coil A2 | Neutral |
With this table, any plant technician can check the wiring against the drawing without opening the manual.
Use in troubleshooting
- The contactor does not close: measure voltage between A1 and A2. If voltage is present and it does not close, suspect the coil or the mechanism; if there is no voltage, work back up the control circuit toward the source. The numbering tells you which contact to test at each step.
- The contactor closes and drops out when start is released: the 13-14 seal-in contact does not close or is wired wrongly.
- The motor does not start but the contactor does close: measure voltage at 2, 4, 6 and then at the relay. A missing phase points to a pole, a fuse or a cable.
- Immediate relay trip: check 95-96 and the setting, against the measured current.
Conductor and terminal block marking
Device numbering is complemented by the marking of conductors and terminal strips. Each cable carries the same identification at both ends, matching the terminal or signal number on the drawing, and the strips are grouped by function: power, control, field signals and auxiliary supply. With this approach, whoever opens the panel can follow each signal to the field without lifting wireways.
When the panel is modified, the new cable is marked in the same way and recorded on the drawing. An unmarked cable, or one marked differently from the drawing, is one of the most common causes of troubleshooting delays, because it forces the circuit to be verified point by point with a multimeter.
Contact blocks and spare parts
When replacing a contactor or an auxiliary contact block, confirm that the spare has the same contact configuration (how many NO and how many NC), the same coil voltage and the same rating. A block with two NO instead of one NO and one NC changes the logic without anything looking out of place. Keep a list of part numbers in the panel with the numbering of each block, and check with a multimeter before energizing that each contact closes or opens as its number indicates.
Common mistakes
- Swapping power input and output on the contactor (1-3-5 with 2-4-6) or on the overload relay.
- Using an NC contact (21-22) as seal-in instead of an NO (13-14).
- Connecting 97-98 in series with the coil instead of 95-96.
- Trusting the wire color and not the terminal.
- Assuming that all relays from other brands follow the same numbering.
- Not updating the drawing when an auxiliary contact block is changed.
- Poorly tightened terminals, which cause heating and false contacts.
Checklist
- Equipment diagram consulted when the brand is not a usual one.
- Coil connected on A1-A2 with the voltage and frequency on the label.
- Power input on 1, 3, 5 and output on 2, 4, 6.
- Contact 95-96 in series with the coil and 97-98 for signaling.
- Seal-in contacts and interlocks as NO or NC according to the drawing.
- Terminal tightening torque per the manufacturer.
- Conductor markings equal to those on the drawing.
- Each contact tested with a multimeter at rest and actuated.
Frequently asked technical questions
- Can an NC contact become NO? No; on removable blocks the block is replaced. Changeover contacts have both functions.
- Can a contactor be expanded with more auxiliary contacts? Many allow front or side blocks; the numbering of the additional blocks continues the device series.
- How are contacts identified on a solid-state relay? With the numbering supplied with the device; see the catalog.
How we can help
A panel with well-identified terminals and up-to-date drawings is diagnosed in minutes. At Tiza Group we review and document control panels remotely, worldwide, and test their operation before the installation is commissioned, with field work in Colombia, or elsewhere on request. See engineering design and studies, or write to us through contact.
Related guides
- Interlocking: mechanical and electrical
- Contactors: utilization categories AC-1, AC-3 and AC-4
- Industrial electrical diagrams: IEC and NEMA symbols
- LV Motor Protection: TeSys T, SIMOCODE pro and E300
- Intelligent MCC: CENTERLINE 2100, SIVACON S8, MNS, Okken
- LV Switchboard and MCC: IEC 61439, Forms and Testing
- Substation and Plant Single-Line Diagram: How to Read It
- Induction motor protection: 49, 46, 50/51, 37, 66, 87M
References
- Schneider Electric, TeSys D contactor documentation: contact numbering.
- EN 50005 and EN 50012, terminal marking of low-voltage control devices.
- IEC 60947-4-1, electromechanical contactors and motor-starters.
Frequently asked questions
What do 13-14 and 21-22 mean on a contactor?
13-14 is a normally open auxiliary contact and 21-22 a normally closed one. The first digit is the sequence number of the contact within the device; the second indicates its function.
What are terminals A1 and A2?
They are the coil terminals of the contactor or relay. Applying control voltage between A1 and A2 energizes the coil. On DC coils there may be a terminal with marked polarity.
What is the 97-98 contact of the thermal relay for?
It is a normally open contact that closes when the relay trips, and is used for signaling or alarm. The normally closed 95-96 opens on trip and is placed in series with the contactor coil.
Is the numbering the same across brands?
The numbering of auxiliary and power contacts generally follows European practice. There are exceptions on special terminals and on equipment built to other standards; when in doubt, confirm on the manufacturer's own diagram.