A DPST (Double Pole, Single Throw) switch symbol represents a single mechanical actuator that simultaneously opens or closes two independent electrical circuits. In schematics, it appears as two parallel switch blades mechanically linked by a dashed or solid line. Whether you are reading an IEC 60617 blueprint for an industrial motor starter or tracing a NEMA schematic for a 240V baseboard heater, correctly identifying the DPST symbol is the first step in safe troubleshooting and accurate wiring.
DPST Switch Symbol Reference Table (IEC, NEMA, & Legacy)
Switch symbols vary significantly depending on the governing standards body and the region of installation. The table below breaks down the exact visual representations you will encounter on electrical drawings.
| Standard / Region | Symbol Description | Schematic Representation | Common Application |
|---|---|---|---|
| IEC 60617 (Global / EU) | Two parallel lines bridged by a diagonal line, connected by a dashed mechanical link. | Diagonal bridge, dashed link | Appliance isolation (Line + Neutral), industrial motor contactors, EU control panels. |
| NEMA / ANSI IEEE 315 (North America) | Two parallel lines bridged by a straight line with a latch/dot, connected by a solid or dashed link. | Straight bridge, latch dot | 240V HVAC disconnects, dual-pole branch circuit switching, US/Canada residential panels. |
| Old UK BS 3939 (Legacy) | Similar to IEC but often drawn with specific terminal numbering (1,2 and 3,4) and a solid mechanical bar. | Straight bridge, solid bar | Found in older UK distribution boards, legacy maritime panels, and pre-1990s machinery. |
For a deeper dive into general switch taxonomy and how single-pole variants compare, the All About Circuits switch reference provides an excellent baseline for schematic literacy.
The 'Rows People Get Wrong' & Faded Marking Protocols
When reading schematics or inspecting physical hardware, misidentifying a DPST switch can lead to dangerous wiring faults. Here are the most common points of confusion and how to resolve them.
Confusing DPST with DPDT (Double Pole, Double Throw)
The most frequent schematic error is misreading a DPDT symbol as a DPST. A DPST has exactly four terminals (two inputs, two outputs) and one stable 'ON' position. A DPDT symbol will show six terminals and a switch blade that can pivot to connect the common terminal to either of two 'throw' terminals. If the schematic shows the bridge line connecting to two separate nodes on the output side, you are looking at a DPDT, not a DPST.
Dual SPST vs. True DPST
A dual SPST (Single Pole, Single Throw) switch, such as a dual-rocker switch on a power strip, controls two circuits but lacks a mechanical linkage symbol in schematics. A true DPST symbol will always feature a dashed or solid line connecting the two switch blades, indicating that a single physical movement operates both poles simultaneously.
Safe Interpretation When Markings are Faded or Missing
On older industrial equipment or heavily used HVAC disconnects, the stamped DPST symbol or terminal silk-screening often wears off. Never guess the pinout based on physical appearance alone. Use a digital multimeter (DMM) in continuity mode to map the switch safely while it is completely de-energized:
- Step 1: Count the terminals. A true DPST will have exactly 4 main power terminals (excluding any ground lugs).
- Step 2: Toggle the switch to the 'ON' position. Place your DMM probes across terminals 1 and 2. You should read less than 1 ohm (continuity). Repeat for terminals 3 and 4.
- Step 3: Toggle the switch to 'OFF'. Probe 1 and 2 again; the meter should read 'OL' (Open Loop). Repeat for 3 and 4.
- Step 4: Cross-probe (e.g., 1 to 3, 2 to 4) in both states. A true DPST will never show continuity between the two separate poles. If you read continuity across poles, the switch is internally shorted or is actually a DPDT wired in a specific configuration.
For proper continuity testing technique and safety thresholds, refer to the Fluke continuity testing guide.
Regional Standards and Safe Interpretation
Identifying the symbol is only half the battle; understanding how regional electrical codes dictate the application of a DPST switch is critical for safety and compliance.
In North America, under NEC Article 404.2(B), you must never use a DPST switch to simultaneously switch the 'Hot' (Line) and the 'Neutral' conductor of a 120V circuit. If the neutral contact fails or develops high resistance, the appliance chassis can become energized. In the US and Canada, DPST switches are almost exclusively reserved for 240V circuits where both poles are ungrounded 'Hot' conductors (L1 and L2).
European and IEC Appliance Isolation
Conversely, under IEC standards and many European national codes, a DPST switch is frequently used to isolate both the Line and Neutral conductors for Class I and Class II appliances. This ensures complete galvanic isolation from the mains supply, protecting technicians working on the equipment downstream. When reading an IEC schematic, if the DPST symbol bridges a Line and a Neutral, it is functioning as a main isolation switch, not a 240V load switch.
For comprehensive details on switch contact ratings and isolation requirements, the Electronics Tutorials switch guide offers excellent baseline theory on contact mechanics and arc suppression.
Frequently Asked Questions
What is the exact difference between a DPST and DPDT switch symbol?
A DPST (Double Pole, Single Throw) symbol shows two switch blades linked together, each connecting to only one output terminal (4 terminals total). It acts as a simple ON/OFF for two separate circuits. A DPDT (Double Pole, Double Throw) symbol shows two linked switch blades that can each pivot to connect to one of two different output terminals (6 terminals total). DPDT switches are used for motor reversing or transfer switching, while DPST is strictly for simultaneous disconnection.
How do I wire a DPST switch for a 240V baseboard heater?
For a standard 240V, 2-wire baseboard heater in North America, you will use a 2-pole (DPST) line-voltage thermostat or wall switch. Connect the incoming L1 (usually black) to Pole 1 input, and incoming L2 (usually red or white taped black) to Pole 2 input. Connect the heater's two load wires to the corresponding Pole 1 and Pole 2 outputs. Ensure the switch is rated for the specific amperage of the heater (typically 20A or 30A) and that the circuit breaker matches the wire gauge (e.g., 10 AWG on a 30A breaker).
Can a DPST switch control two completely separate 120V circuits?
Physically, yes; a DPST switch can switch two independent 120V hot conductors simultaneously. However, you must verify the switch's voltage isolation rating between poles. If the two circuits are on different phases or have different grounding references, the internal dielectric strength between the two poles must be rated for the potential difference. Furthermore, if the two 120V circuits share a neutral (a Multi-Wire Branch Circuit), local codes often require a handle-tie or a common-trip breaker to ensure both hots are de-energized simultaneously for safety, which a standard wall-mounted DPST switch may not satisfy without specific listing.
