When troubleshooting a control panel or designing a PCB, correctly interpreting the relay symbol electrical schematics is the difference between a quick fix and a dead short. The standard schematic representation of a relay depends entirely on the governing standard of the region or the legacy of the machine. Modern global and European schematics use the IEC 60617 standard (rectangular coils), while North American industrial prints frequently rely on ANSI/IEEE 315 (circular coils). Regardless of the standard, the golden rule of reading any relay symbol is that the switch contacts are always drawn in their de-energized, resting state.
The Complete Relay Symbol Reference Table
The following table maps the most common relay components to their respective standard symbols. Use this as a bench reference when tracing control logic on schematics from different manufacturers like Schneider, Omron, or Finder.
| Component | IEC 60617 Symbol Description | ANSI/IEEE 315 Symbol Description | Practical Bench Meaning |
|---|---|---|---|
| Standard Relay Coil | Empty rectangle (often labeled K or CR) | Empty circle or semicircle | The electromagnetic actuator. Energizing it changes the state of all mechanically linked contacts. |
| Polarized / Latching Coil | Rectangle with a solid black fill or diagonal line | Circle with a diagonal line or dual opposing arrows | Requires specific DC polarity to set or reset, or a pulse to latch. Will not drop out on power loss. |
| SPST-NO Contact | Two parallel lines with a hinged diagonal line resting open | Two dots with a hinged line resting open | Normally Open. Circuit is broken until the coil energizes. Used for start/enable logic. |
| SPST-NC Contact | Two parallel lines with a hinged diagonal line resting closed | Two dots with a hinged line resting closed and intersecting | Normally Closed. Circuit is complete until the coil energizes. Used for stop/interlock logic. |
| SPDT Contact | Three parallel lines; hinged line connects Common to NC, leaves NO open | Three dots; hinged line connects Common to NC | Single Pole Double Throw. Provides both a NO and NC circuit from one common feed. |
| Time-Delay (On-Delay) | Standard coil with a small arrow pointing away from the coil | Circle with an arrow pointing right or an 'X' inside | Contacts change state only after a set time after the coil is energized. |
| Time-Delay (Off-Delay) | Standard coil with a small arrow pointing toward the coil | Circle with an arrow pointing left | Contacts change state immediately on energize, but delay returning to rest when power is removed. |
| Mechanical Linkage | Dashed line connecting the coil rectangle to the contact symbols | Often omitted, or shown as a dashed line linking contact groups | Indicates that the contacts are physically moved by that specific coil. Crucial for tracing multi-page schematics. |
Regional Variants and the Rows People Get Wrong
While the physics of electromagnetism do not change across borders, the drafting standards do. If you are working on imported machinery or legacy systems, you must know which rulebook the original drafters used.
- IEC 60617 (Global/EU/Modern): Uses rectangular boxes for coils. Contacts are drawn with straight lines and hinges. This is the standard for almost all modern PLC wiring diagrams and European machinery (e.g., Siemens, ABB).
- ANSI/IEEE 315 (North America): Historically uses circles for coils and dots for contact terminals. You will still see this on older US-manufactured motor control centers (MCCs) and legacy Allen-Bradley prints.
- BS 3939 (Old UK Legacy): Largely superseded by IEC, but you may encounter it on British machinery built before the 1990s. It used unique graphical symbols for relays that often look like a mix of vacuum tube symbols and modern IEC lines.
The Rows People Get Wrong
Even experienced technicians misread specific symbol variations on the bench. Watch out for these common traps:
A standard rectangular coil symbol tells you nothing about whether the contacts are NO or NC. The coil just identifies the actuator. You must look at the individual contact symbols linked by the dashed mechanical line to determine the resting state.
A standard SPDT symbol implies a break-before-make transition (the common disconnects from NC before touching NO). If the schematic shows the NO and NC lines overlapping or bridged by a small cross, it indicates a make-before-break (shorting) contact, used to prevent open-circuit voltage spikes during transition.
Because ANSI/IEEE often omits the dashed mechanical linkage line to reduce clutter, technicians frequently fail to realize that a contact on page 4 is driven by a coil on page 2. Always cross-reference the component tag (e.g., "CR104") rather than relying on visual proximity.
Safe Interpretation When Markings Are Faded or Missing
On the bench, you will frequently pull a relay like an Omron MY4N or a Finder 55.34 from a dirty, oily, or UV-exposed panel, only to find the printed schematic symbol on the casing is completely illegible. Never guess the pinout based on physical appearance; guessing a pinout on a 14-pin relay can send 24VDC directly into a 5V PLC input, frying the optocoupler.
Follow this strict multimeter mapping procedure when the relay symbol is missing:
- Identify the Coil: Set your DMM to resistance (Ω). Probe the pins. For a 24VDC relay, you are looking for a resistance typically between 600Ω and 1.2kΩ. For a 120VAC relay, the coil resistance will be much higher (often 3kΩ to 10kΩ). The two pins that show this resistance are your coil (A1 and A2, or pins 2 and 7 on an 8-pin octal).
- Identify the Poles (Commons): Switch the DMM to continuity/diode mode. Find the pins that show continuity to multiple other pins when the relay is manually actuated (use a small flathead screwdriver to press the plastic test button on the relay armature). These are your Common (C) terminals.
- Map NO and NC: With the relay at rest (unactuated), the pin showing continuity to the Common is your Normally Closed (NC). The pin showing an open loop (OL) is your Normally Open (NO). Actuate the relay; the states will swap.
Safety Caveat: Never apply mains voltage to a relay coil to 'test' it if you are unsure of the coil rating. A 12VDC coil subjected to 120VAC will explode or catch fire instantly. Always verify the coil resistance and physical part number against a manufacturer datasheet first.
Frequently Asked Questions
What does the rectangle with a diagonal line mean on an electrical relay symbol?
In IEC 60617 drafting standards, a relay coil represented by a rectangle with a diagonal line (or a solid black filled rectangle) indicates a polarized or magnetic latching relay. Unlike a standard monostable relay that drops out when power is removed, a latching relay maintains its last contact state. The diagonal line warns the technician that removing power will not reset the circuit; a specific reverse-polarity pulse or a dedicated reset coil must be energized to return the contacts to their normal state.
How do I read the pinout on a standard 8-pin octal relay symbol?
The 8-pin octal relay (such as the ubiquitous Omron LY2 or Schneider Harmony RXM) follows a highly standardized physical and schematic layout. On the schematic symbol, the coil is almost universally mapped to pins 2 and 7. The two sets of SPDT contacts are mapped as follows: Pole 1 uses pins 1 (NC), 3 (Common), and 4 (NO). Pole 2 uses pins 8 (NC), 6 (Common), and 5 (NO). Physically, the socket will have a keyway notch between pins 1 and 8 to prevent the relay from being plugged in 180 degrees out of phase, which would short the coil power to the contact bus.
Why does my relay symbol show a contact with a small triangle or curved line?
If you see a standard NO or NC contact symbol augmented with a small triangle, an arrow, or a curved line intersecting the hinge, you are looking at a time-delay relay contact. The direction of the marker dictates the behavior. An arrow or triangle pointing away from the contact hinge indicates an "On-Delay" (the contact delays changing state when the coil energizes). An arrow pointing toward the hinge indicates an "Off-Delay" (the contact changes state immediately on energize, but delays returning to rest when the coil is de-energized). For authoritative drafting rules on these specific timing markers, refer to the IEEE 315 standard documentation or standard relay logic tutorials.
