A relay contact symbol represents the physical switch state inside an electromechanical or solid-state relay when the coil is de-energized (the normal, resting state). Misinterpreting these symbols is one of the most common causes of control circuit failure, especially when transitioning between international equipment and legacy North American panels. Below is the definitive reference for identifying, applying, and troubleshooting relay contacts across global standards.
Master Reference Table: Relay Contact Symbols
The following table maps the most common relay contact configurations to their respective IEC and ANSI/IEEE schematic representations, along with standard industrial applications. Use this as your primary bench and jobsite reference.
| Contact Type (Form) | IEC 60617 Symbol Description | ANSI/IEEE 315 Symbol Description | Standard Pinout (8-Pin Octal) | Practical Application |
|---|---|---|---|---|
| SPST-NO (Form A) | Gap with a diagonal line (knife switch) pointing away from the pivot. | Gap with a straight bridging line, no overlap. | Common: 8 / NO: 6 | Start buttons, momentary triggers, enabling a motor starter coil. |
| SPST-NC (Form B) | Gap with a diagonal line overlapping the stationary contact point. | Gap with a bridging line that crosses over the stationary contact. | Common: 8 / NC: 5 | Stop buttons, emergency stops, thermal overload interlocks. |
| SPDT (Form C) | Single wiper (moving contact) positioned between an NO and NC gap. | Wiper connecting to a central point, branching to NO and NC. | Common: 8 / NO: 6 / NC: 5 | Reversing motor direction, switching between primary and backup power. |
| DPDT (Form C x2) | Two SPDT symbols mechanically linked by a dashed line. | Two SPDT symbols linked by a dashed line or common actuator bar. | Pole 1: 8,6,5 / Pole 2: 1,3,4 | Simultaneous polarity reversal, dual-circuit isolation. |
| On-Delay NO (Timed) | Form A symbol with an arrow pointing away from the contact gap. | Form A symbol with a standard timer arrow pointing up/away. | Varies by timer module | Soft-start sequences, HVAC fan delays after compressor shutoff. |
| Off-Delay NC (Timed) | Form B symbol with an arrow pointing toward the contact gap. | Form B symbol with a timer arrow pointing down/toward. | Varies by timer module | Run-on cooling fans, delayed alarm sirens after fault clearance. |
Regional Standards: Which Symbol Set Applies to You?
The symbol for relay contact you see on a schematic depends entirely on the region of manufacture and the governing standard of the facility. Applying the wrong standard can lead to catastrophic miswiring, particularly with timed or safety interlock contacts.
| Standard | Primary Region | Visual Style & Key Traits | When You Will Encounter It |
|---|---|---|---|
| IEC 60617 | EU, UK, AU, Global | Stylized 'knife switch' aesthetic. Relies heavily on alphanumeric designators (e.g., K1 for relay, 13/14 for NO contacts). | Modern PLC panels, imported European machinery, global IEC motor control centers. |
| ANSI/IEEE 315 | US, Canada | More literal, geometric representations of contact gaps. Uses device function numbers (e.g., 86 for lockout relay). | North American power generation, utility substations, legacy US manufacturing plants. |
| NEMA ICS 19 | US (Heavy Industrial) | Ladder-logic optimized. Contacts are drawn as simple limit-switch style brackets. Focuses on readability for electricians tracing vertical power rails. | NEMA-rated motor starters, heavy industrial conveyor systems, US commercial HVAC. |
The 'Rows People Get Wrong' Field Notes
Even experienced technicians misinterpret specific relay symbols under pressure. Here are the most common pitfalls and how to avoid them.
1. The Definition of 'Normal' State
The most critical concept in relay schematics is that 'Normal' means the de-energized state of the coil, not the normal operating state of the machine. For example, in a supervised fire alarm circuit, the relay coil is continuously energized to monitor line integrity. The circuit relies on the 'Normally Closed' (NC) contact to pass current during standard operation. If you wire a 'Normally Open' (NO) contact because the machine is 'normally running', the safety loop will fail. Always trace the symbol back to the coil's resting, unpowered state.
2. Form A, B, and C Nomenclature
Datasheets from manufacturers like Omron, Schneider, and Finder often use Form letters instead of SPST/SPDT. Memorize this translation: Form A is Always NO. Form B is Always NC. Form C is the Changeover (SPDT). If a datasheet specifies a 'DPST-NO' relay, it will be listed as '2 Form A'.
3. Timed Contact Arrow Direction
In IEC schematics, the arrow on a timed contact indicates the direction of the delayed movement, not the physical shape of the contact. An arrow pointing away from the gap means the contact is delayed in closing (On-Delay NO). An arrow pointing toward the gap means the contact is delayed in opening (Off-Delay NC). Confusing these two will completely invert your timing sequence.
Safe Interpretation When Markings Are Faded or Missing
On the jobsite, you will frequently encounter opaque, unmarked, or heavily faded octal-base relays (like the classic 8-pin or 11-pin plug-in types) with no schematic diagram available. Here is the exact procedure to safely identify the coil and contact pins using a digital multimeter (DMM).
Step 1: Isolate and Extract
Shut off the control circuit breaker. Verify zero voltage at the relay base terminals using a non-contact voltage tester and a DMM. Pull the relay straight out of the socket.
Step 2: Identify the Coil Pins
Set your DMM to resistance (Ohms). Probe the pins. On a standard 8-pin octal relay, pins 2 and 7 are almost universally the coil. You should read a resistance value typically between 100 Ω (for a 12VDC coil) and 10,000 Ω (for a 240VAC coil). If you read 0.0 Ω (short) or OL (open), the coil is burned out and the relay is scrap.
Step 3: Map the Contacts (The Continuity Test)
Switch your DMM to continuity mode (the diode/beep setting). With the relay completely de-energized (held in your hand):
- Probe the remaining pins. Any pair that beeps (reads less than 1.0 Ω) is a Normally Closed (NC) contact and its common.
- Any pair that reads OL (Open Loop) is a Normally Open (NO) contact and its common.
- The pin that shares continuity with one pin, and reads OL with another adjacent pin, is your Common (C) terminal.
Step 4: The Bench Test (Optional Verification)
If you need to verify the mechanical throw, apply the rated coil voltage (e.g., 24VDC) directly to the coil pins using a bench power supply. You will hear an audible click. Re-test the contact pins with your DMM; the continuity states will perfectly invert. The previously beeping NC pair will now read OL, and the previously open NO pair will now beep. For a comprehensive database of specific manufacturer pinouts, consult the Macromatic relay contact forms guide or the All About Circuits relay switch tutorial.
By mastering the symbol for relay contact across IEC and ANSI standards, and knowing how to empirically verify unmarked components, you eliminate the guesswork that leads to short circuits, blown fuses, and unsafe machine restarts.
