The double pole double throw relay symbol represents an electromechanical switch with two independent circuits (poles), each capable of connecting to two different paths (throws). In schematic form, it depicts two separate switch blades mechanically linked to a single electromagnetic coil. When the coil energizes, both poles simultaneously shift from their Normally Closed (NC) contacts to their Normally Open (NO) contacts. Because global drafting standards differ significantly, correctly interpreting this symbol—and mapping it to the physical 8 pins on the relay base—is the first step in avoiding a dead-short or a malfunctioning control circuit.

The DPDT Relay Symbol & Pinout Reference Table

Physical DPDT relays typically use an 8-pin configuration, but the pin numbering depends entirely on whether the manufacturer follows the IEC DIN standard or the legacy US Octal standard. Use this table to map the schematic symbol to the physical terminals.

Function IEC 60617 / DIN Pin Number US Octal Base Pin Number Schematic Symbol Representation Practical Bench Note
Coil (+) A1 2 Rectangle (IEC) or Circle (ANSI) On DC relays with a built-in flyback diode, polarity matters here. Reverse it and the relay may not pull in.
Coil (-) A2 7 Rectangle (IEC) or Circle (ANSI) AC coils are non-polarized. Expect 100-1000 ohms resistance on DC coils, much lower on AC coils.
Pole 1 Common (COM1) 11 3 Center blade of switch 1 The moving contact. This is your line/load feed point.
Pole 1 NC 12 1 Top fixed contact of switch 1 Connected to COM1 when the coil is de-energized (resting state).
Pole 1 NO 14 4 Bottom fixed contact of switch 1 Connected to COM1 only when the coil is actively energized.
Pole 2 Common (COM2) 21 6 Center blade of switch 2 Electrically isolated from Pole 1. Can switch a completely different voltage.
Pole 2 NC 22 8 Top fixed contact of switch 2 Connected to COM2 at rest.
Pole 2 NO 24 5 Bottom fixed contact of switch 2 Connected to COM2 when energized.

Regional Standard Variants: IEC vs. ANSI/NEMA vs. Old UK

While the physical relay doesn't change, the way the double pole double throw relay symbol is drawn on a schematic changes based on your region's governing standard. Misreading the standard can lead to confusing a mechanical linkage for a physical wire.

  • IEC 60617 (Europe, International, Modern Global): The coil is drawn as a simple rectangle. The switch contacts are drawn as distinct symbols separated by a dashed line. Crucially, IEC mandates the two-digit terminal numbering (11, 12, 14) directly on the symbol. The dashed line strictly represents the mechanical armature linkage, not an electrical connection.
  • ANSI/IEEE 315 & NEMA (North America): The coil is often drawn as a circle or a rectangle with diagonal hash marks. The switch throws are drawn as literal 'knife-blade' switches. ANSI schematics rarely include terminal numbers on the symbol itself, relying instead on wire numbers and a separate bill of materials. The mechanical link is shown as a dashed line connecting the pivot points of the knife blades.
  • Old UK (BS 3939 - Superseded): You will still see this on legacy plant drawings in the UK. It looks similar to IEC but uses specific hash marks inside the coil rectangle to denote AC vs. DC, and the mechanical linkage is sometimes drawn as a solid line with a small 'break' in the middle to indicate it is non-conductive.
Bench Tip: If you are reading an ANSI drawing and see a dashed line connecting two switch blades, do not trace it with your multimeter expecting continuity. It is purely a graphical indicator that both switches are actuated by the same coil.

Rows and Pins People Get Wrong

When translating the schematic to the physical relay socket, these specific misinterpretations cause the most field failures:

  1. Confusing the Octal and DIN Pinouts: This is the most common error when retrofitting old US panels with modern European relays. If you wire a harness for a US Octal base (Pins 1 and 8 as NC) but plug in an IEC DIN relay (Pins 12 and 22 as NC), your circuit will fail to operate, or worse, you will short the coil voltage directly into your load.
  2. Ignoring Coil Polarity on DC Relays: On a bare DC relay, A1/A2 (or 2/7) are interchangeable. However, many modern industrial DPDT relays (like the Omron MY2N DC24S) include a built-in flyback diode or LED indicator. If you wire A2 to positive and A1 to negative, the diode will forward-bias, effectively shorting your 24VDC power supply and blowing the control fuse.
  3. Overloading the 'Poles': A DPDT relay is rated for a specific current per pole (e.g., 10A). A common mistake is assuming that because there are two poles, you can parallel COM1 and COM2 to switch a 20A load. Due to microscopic differences in contact resistance and mechanical timing, one pole will always make/break before the other, taking the full inductive inrush and welding the contacts shut.

Decision Path: Selecting and Wiring the Right DPDT Relay

Stop guessing which relay to order. Follow this decision tree to terminate on the exact part number for your application.

Application Parameter If This... Then Choose This...
Control Voltage 24V AC or 120V AC AC Coil variant (e.g., MY2N AC24). No flyback diode needed.
Control Voltage 12V DC or 24V DC from a PLC DC Coil variant with built-in surge suppression (e.g., MY2N DC24S).
Mounting Style Industrial Panel / DIN Rail Plug-in style with a socket base (e.g., Omron PYF14A-E).
Mounting Style Custom PCB / Protoboard Through-hole PCB mount (e.g., Finder 55.34 series).
Load Type High Inrush (Motors, Transformers) AgSnO2 (Silver Tin Oxide) contacts to resist welding.
The Concrete Default Pick: For 90% of general-purpose industrial and heavy-DIY control panels, standardize on the Omron MY2N DC24S (24VDC coil, built-in surge diode, 10A contacts) paired with the Omron PYF14A-E DIN socket. It provides the best balance of global availability, clear IEC pin markings on the socket, and reliable 10A switching. If you are soldering directly to a PCB, use the Finder 55.34.9.024.0040.

Safe Interpretation When Markings Are Faded or Missing

On older equipment, the schematic diagram printed on the side of the relay or socket often flakes off or is obscured by grime. Never guess the pinout by looking at the physical shape of the pins. Use a digital multimeter (DMM) to safely map the double pole double throw relay symbol back to the physical terminals.

Step 1: Isolate and De-energize. Remove the relay from the circuit entirely. Never perform resistance or continuity tests on a live relay. Lock out the panel and verify zero voltage.

Step 2: Locate the Coil. Set your DMM to the Ohms (Ω) range. Probe the pins in pairs. The coil pins will show a distinct resistance (typically 400-800 Ω for a 24VDC coil, or 10-50 Ω for a 12VDC coil). AC coils will read much lower, sometimes just a few ohms. The two pins that show this resistance are your coil terminals. All other pins should read infinite resistance (OL) to the coil pins.

Step 3: Map the Poles (Resting State). Set your DMM to Continuity (the diode/beep symbol). With the relay de-energized (at rest), probe the remaining 6 pins. You will find two sets of three pins. In each set, one pin (the Common) will beep when touched to a second pin (the NC). The Common will read OL when touched to the third pin (the NO). Mark these down.

Step 4: Verify the Throws (Energized State). To confirm the NO pins, apply the rated coil voltage (e.g., 24VDC from a bench supply) to the coil pins you identified in Step 2. You should hear an audible click. While holding the voltage, re-test continuity. The Common should now beep with the NO pin, and read OL with the NC pin. If it behaves this way, you have successfully mapped the physical relay to the standard symbol.

For deeper reference on global schematic standards, consult the IEC standard symbol database or review the IEEE 315 graphic symbols standard. For practical relay wiring and contact material behaviors, Electronics Tutorials provides an excellent breakdown of contact bounce and arcing.