A double pole single throw (DPST) switch is an electromechanical device that simultaneously connects or disconnects two separate electrical circuits using a single actuator, offering only one 'on' position. In practical terms, it acts as a single master gate that completely isolates two independent conductors at the exact same time. DPST = 2 Poles (circuits interrupted), 1 Throw (one closed position).
How a DPST Switch Changes a Real Circuit
When you introduce a DPST switch into a circuit, you change the interruption profile from single-leg to full-pole isolation. In a standard 120V single-phase circuit, a single pole switch only breaks the 'hot' leg, leaving the neutral bonded to the appliance. A DPST switch breaks both the hot and the neutral (or both hot legs in a 240V system), ensuring that no voltage can reach the load from either direction, even if a wiring fault exists upstream.
Let's look at a worked numeric example from the field. Suppose you are wiring a 240V, single-phase air compressor with a Full Load Amp (FLA) rating of 18A. The NFPA 70 National Electrical Code (specifically Article 430) requires a dedicated disconnecting means within sight of the motor.
- Switch Selection: You select a 30A DPST safety switch (like a heavy-duty Schneider Electric or Eaton non-fused disconnect).
- Wire Sizing: Because standard switch terminals are often rated for 60°C or 75°C, you size the wire using the 60°C column of NEC Table 310.16. This dictates 10 AWG THHN copper wire, which has an ampacity of 30A.
- The Circuit Change: When you pull the DPST handle to the OFF position, the internal mechanical linkage forces both the L1 and L2 hot contacts to open simultaneously. The voltage across the compressor terminals drops from 240V to 0V. Neither leg can backfeed the motor, and a technician can safely lock out the switch to work on the compressor without risk of shock from a crossed neutral or backfed line.
Where You Meet DPST Switches in Practice
You will rarely find a raw DPST toggle switch on a standard residential 120V wall plate, but they are ubiquitous in industrial, commercial, and specialized environments. Here is where you will actually encounter them:
HVAC and Industrial Motor Disconnects
The red-handled safety boxes mounted next to commercial air handlers, table saws, and large compressors are almost always DPST (for 240V single-phase) or 3-pole (for 208V/480V three-phase). They provide the physical air gap required by lockout/tagout (LOTO) safety procedures.
Marine and Off-Grid DC Battery Isolators
In 12V, 24V, or 48V DC battery banks, a DPST switch is often used as the main battery cutoff. By breaking both the positive and negative main cables, the DPST switch completely isolates the battery bank from the vessel or cabin. This prevents stray current corrosion and ensures that a short to ground on the positive side cannot drain the batteries if the negative side is also disconnected.
Medical and Laboratory Equipment
Under standards like IEC 60601, certain medical devices require double-pole switching. If a device is plugged into a standard 120V receptacle where the hot and neutral might accidentally be reversed by a faulty wall outlet, a single-pole switch might only disconnect the neutral, leaving the internal circuitry energized at 120V. A DPST switch on the equipment's power supply breaks both conductors, guaranteeing the internal chassis is dead regardless of wall polarity.
What People Confuse DPST With: Switches vs. Breakers
The most common mistake hobbyists and junior technicians make is confusing a DPST switch with a Double Pole Double Throw (DPDT) switch, or assuming a 2-pole circuit breaker is just a DPST switch in disguise. Here is how they differ in function and internal mechanics.
| Device Type | Poles / Throws | Primary Function | Overcurrent Protection? |
|---|---|---|---|
| SPST Switch | 1 Pole / 1 Throw | Interrupts one conductor (standard light switch). | No |
| DPST Switch | 2 Poles / 1 Throw | Interrupts two conductors simultaneously (isolation). | No |
| DPDT Switch | 2 Poles / 2 Throws | Routes two circuits between two different destinations (e.g., motor reversal). | No |
| 2-Pole Breaker | 2 Poles / 1 Throw | Interrupts two conductors AND trips automatically on overload/short circuit. | Yes (Thermal/Magnetic) |
While a 2-pole breaker contains a DPST switching mechanism inside its housing, its primary engineering purpose is fault clearing. A standard DPST safety switch has no thermal bimetallic strip or magnetic trip coil. If you pull 100A through a 30A DPST switch, it will not trip; the contacts will overheat, melt, and potentially weld themselves in the closed position, creating a severe fire hazard. Always use a DPST switch for isolation and a breaker or fuse for protection.
For a deeper dive into how switch throws dictate circuit routing, the All About Circuits switch guide provides excellent schematic breakdowns of how the internal wafers and contacts align in single versus double throw configurations.
Frequently Asked Questions About DPST Switches
What is the difference between double pole single throw and double pole double throw?
The difference lies in the 'throw'—the number of closed positions available. A DPST switch has one 'on' position; it simply connects or disconnects two lines. A DPDT (Double Pole Double Throw) switch has two 'on' positions. It routes two input lines to one of two different output destinations. You use DPST for simple on/off isolation, and DPDT for applications like reversing the polarity of a DC motor or switching between two different power sources.
Can I use a DPST switch to isolate a standard 120V circuit?
Yes, but it is usually unnecessary for standard residential lighting or outlets, where a single pole (SPST) switch breaking the hot leg is code-compliant and standard practice. However, if you are building a custom workbench, a lab power supply, or marine equipment, using a DPST switch to break both the 120V hot and neutral provides superior safety. Just ensure the switch is rated for the specific AC voltage and current, and be aware that if the upstream receptacle is wired with reversed polarity, the DPST will still safely kill the circuit, whereas an SPST might leave the appliance chassis energized.
Does a double pole single throw switch provide overcurrent protection?
No. A DPST switch is purely a manual disconnecting means. It lacks the internal thermal or magnetic trip mechanisms found in circuit breakers. If a dead short occurs downstream of a DPST switch, the switch will attempt to carry the massive fault current until the upstream breaker trips or the switch contacts physically vaporize. Always install fuses or a breaker in series before the DPST switch.
How do I wire a DPST switch for a 240V load?
For a standard 240V single-phase load (like a baseboard heater or compressor), you wire the two incoming hot legs (L1 and L2, typically black and red or two black wires with phase tape) to the 'LINE' terminals on the top of the DPST switch. You then wire the two load-side hot legs from the 'LOAD' terminals on the bottom of the switch directly to the appliance. The equipment grounding conductor (bare or green) bypasses the switch entirely and is bonded directly to the switch enclosure's ground lug and the appliance ground terminal. Never switch the ground wire.
