The standard contactor symbol electrical representation consists of a coil (drawn as a rectangle or circle) mechanically linked via a dashed line to its main and auxiliary contacts (parallel lines intersected by a diagonal actuator slash). Crucially, all standard schematic symbols depict the contactor in its de-energized, resting state. If the coil is unpowered, a Normally Open (NO) contact symbol remains open, and a Normally Closed (NC) contact symbol remains closed.
The Complete Contactor Symbol Reference Table
Before tracing wires on a machine panel, you must identify which drafting standard the original equipment manufacturer (OEM) used. Below is the definitive cross-reference for contactor symbols across the three major global standards.
| Component | IEC 60617 (Global / EU / AU) | NEMA / NFPA 79 (North America) | Legacy BS 3939 (Old UK Pre-1990s) |
|---|---|---|---|
| Contactor Coil | Rectangle labeled 'K' or 'KM' (e.g., K1, KM2) | Circle labeled 'M' (Motor) or 'CR' (Control Relay) | Circle with parallel internal lines, labeled 'C' |
| Main Contact NO | Two parallel lines with a diagonal slash. Terminals: 1-2, 3-4, 5-6 | Two parallel lines with a diagonal slash. Terminals: L1-T1, L2-T2, L3-T3 | Two parallel lines with a diagonal slash. Terminals: 1-2, 3-4, 5-6 |
| Main Contact NC | Two parallel lines with a diagonal slash crossing through them. Terminals: Rare on main poles. | Crossed parallel lines. Terminals: L1-T1 (with NC indicator) | Crossed parallel lines. No standardized terminal numbering. |
| Auxiliary Contact NO | Parallel lines, diagonal slash. Terminals end in 3-4 (e.g., 13-14, 23-24) | Parallel lines, diagonal slash. Terminals labeled with letters (e.g., A-B, C-D) | Parallel lines, diagonal slash. Terminals: a1-a2 |
| Auxiliary Contact NC | Parallel lines, diagonal slash crossing through. Terminals end in 1-2 (e.g., 11-12, 21-22) | Crossed parallel lines. Terminals labeled with letters (e.g., E-F) | Crossed parallel lines. Terminals: b1-b2 |
| Mechanical Interlock | Dashed triangle or inverted 'V' linking two adjacent coil rectangles. | Dashed line linking two circles with a mechanical interlock text note. | Dashed line linking two circles. |
Rows People Get Wrong (And How to Read Them in Practice)
Even experienced technicians misinterpret specific rows in the table above when transitioning between regional standards or troubleshooting legacy panels. Here are the most common pitfalls and how to resolve them on the bench.
1. Confusing Main vs. Auxiliary Terminal Numbering
In the IEC 60617 standard, the symbol itself doesn't explicitly state the current rating; the terminal numbering does. Main power poles always use single-digit pairs (1-2 for Phase A, 3-4 for Phase B, 5-6 for Phase C). Auxiliary contacts always use double digits. If the second digit is a 3 or 4 (e.g., 13-14), it is a Normally Open (NO) auxiliary. If the second digit is a 1 or 2 (e.g., 11-12, 21-22), it is a Normally Closed (NC) auxiliary. NEMA schematics ignore this numbering logic, relying instead on ladder logic rung placement and letter designations (A-B for NO, E-F for NC), which causes massive confusion for EU-trained electricians working in the US.
2. The 'De-Energized State' Assumption
A frequent error is assuming a Normally Open (NO) symbol means the contact is open while the machine is running. The symbol strictly represents the physical state of the contact when the coil has zero voltage applied. When you read a schematic showing a NO auxiliary contact (13-14) wired in parallel with a start pushbutton to create a holding circuit, that contact is physically open on the drawing, but closes to seal the circuit the moment the coil energizes.
3. Safe Interpretation When Markings are Faded or Missing
On older panels (especially those housing legacy Square D or Telemecanique contactors), the physical terminal stamps and schematic labels often fade or are obscured by arc-flash soot. You must rely on physical testing, but this requires strict safety protocols.
Bench Testing Protocol for Faded Contactors:
- Identify the Coil: Set your multimeter to Ohms. Probe the suspected coil terminals (A1 and A2 on IEC contactors). A healthy 120V AC coil on a Schneider TeSys D-line (e.g., LC1D09) typically reads between 15 and 30 ohms. A 24V DC coil reads much lower (2 to 5 ohms). An infinite reading (OL) means a burned-out coil.
- Identify Main Poles: Set the meter to continuity. Manually depress the contactor's moving armature with an insulated screwdriver. The three main pole pairs should read < 0.5 ohms when depressed, and OL when released.
- Identify Auxiliaries: Probe the side-mounted or front-mounted auxiliary blocks. Find the pair that reads OL when released, and < 0.5 ohms when depressed (this is your NO, likely 13-14). Find the pair that reads < 0.5 ohms when released, and OL when depressed (this is your NC, likely 11-12).
Regional Standards: Which One Applies to Your Panel?
Understanding the contactor symbol electrical standard in use dictates how you source replacement parts and read the logic.
- IEC 60617 (Global Standard): Used across Europe, Asia, Australia, and increasingly in modern North American OEM equipment. IEC contactors (like the Siemens SIRIUS 3RT2 or ABB AF series) are sized by exact kW/HP ratings and utilize the strict 13-14 / 11-12 auxiliary numbering. Schematics are highly standardized and modular.
- NEMA / NFPA 79 (North America): Dominant in legacy US and Canadian industrial plants. NEMA contactors (like the Allen-Bradley 100-C series or Eaton Freedom series) are physically larger, built with heavier silver-cadmium oxide contacts for harsh environments, and use NEMA size ratings (Size 00 through Size 9) rather than exact ampacity. Schematics prioritize vertical ladder logic readability over physical layout accuracy.
- Legacy BS 3939 (Old UK): Officially superseded by IEC/BS EN 60617 in the 1990s, but you will still encounter these symbols in British municipal water works, old textile mills, and legacy railway signaling panels. If you see circular coils labeled 'C' and 'a1-a2' auxiliary terminals, you are looking at a BS 3939 schematic. Do not attempt to wire modern IEC blocks without mapping the old terminal designations to the new 13-14 standard.
For deeper study on industrial schematic standards, refer to the NFPA 79 Electrical Standard for Industrial Machinery for North American practices, and the Electrical Engineering Portal's guide on contactor symbols for global IEC interpretations.
Frequently Asked Questions
What does the contactor symbol electrical diagram look like for a 3-phase motor starter?
A 3-phase motor starter schematic combines three distinct symbols. First, the contactor coil (rectangle or circle) controls the logic. Second, three Main NO contacts (parallel lines with diagonal slashes, labeled 1-2, 3-4, 5-6) are drawn in the power circuit to switch the three phases. Third, a thermal overload relay symbol (a square with a bimetallic strip curve inside, or a heater element symbol) is drawn in series with the contactor coil's control circuit. If the motor draws too much current, the overload's NC auxiliary contact (95-96) opens, breaking the circuit to the contactor coil and dropping out the main power poles.
How do I read auxiliary contactor symbols on a PLC wiring diagram?
On a PLC wiring diagram, auxiliary contactor symbols act as the bridge between high-voltage power logic and low-voltage digital logic. You will typically see the IEC 13-14 (NO) or 21-22 (NC) auxiliary contacts drawn as inputs to the PLC's discrete input cards (e.g., wired to I:0/0). The PLC reads a 24V DC signal when the contactor pulls in. When tracing these, always verify that the auxiliary block is physically rated for the PLC's input current; while most modern PLC inputs draw less than 10mA, using a low-current logic-level auxiliary block (like the Schneider LADN01) prevents contact oxidation issues that plague standard power-rated aux blocks switching micro-ampere PLC signals.
What is the difference between a relay and contactor symbol in electrical schematics?
Schematically, the symbols for a control relay and a power contactor are nearly identical—both use a coil and NO/NC contact lines. The difference lies in the context, labeling, and current capacity. In IEC schematics, contactors are typically labeled with 'K' or 'KM' and switch loads >15A (motors, heaters). Control relays are labeled 'CR', 'KA', or 'K' with higher numbers (e.g., K10, K11) and switch loads <10A (PLC inputs, indicator lights, other coils). Physically, contactors feature arc chutes to extinguish the plasma generated when breaking high inductive motor loads, whereas relays do not.
How to safely test a contactor when the schematic symbols and physical markings are faded?
If you are faced with an unmarked, grease-covered contactor in a legacy panel, rely on the physical geometry and multimeter testing. After strictly applying LOTO and verifying dead power: 1) Locate the two isolated terminals at the top or bottom center of the component; these are almost always the coil (A1/A2). Test for resistance (expect 10-50 ohms for AC coils). 2) Locate the three largest screw terminals on the top and bottom; these are the main power poles (L1/L2/L3 and T1/T2/T3). 3) Any smaller terminals on the sides or front are auxiliaries. Use the continuity test while manually pushing the armature to map out which pairs are NO (open at rest) and NC (closed at rest). Document your findings and label the physical device with a permanent marker before re-energizing.
