What SPST, SPDT, and DPDT Actually Mean for Your Panel Design

Poles, Throws, and the Two Numbers That Define Every Switch

Every switch contact configuration comes down to two numbers. Poles and throws.

A pole is a set of contacts that makes or breaks one circuit. An SP switch has one pole. A DP switch has two. A 3P switch has three, and you’ll see 4P and higher in multi-circuit designs too. The throw is how many positions each pole can connect to. An ST switch has one throw. Each pole connects to exactly one output terminal. A DT switch has two throws, so each pole can toggle between two possible output terminals.

The abbreviation just combines these. SPST: single pole, single throw. DPDT: double pole, double throw. Once you internalize that pattern, every switch abbreviation in every catalog stops looking like random letters and starts making sense immediately.

COM, NO, and NC: What the Terminal Labels Actually Do

Three terminal labels show up on practically every switch body, and they work the same way across every manufacturer worldwide.

COM stands for Common. It’s always connected to the moving contact inside the switch. Think of it as the input: the terminal that feeds the switch.

NO means Normally Open. When the switch sits in its rest position, COM is NOT connected to NO. Press or flip the switch, and COM connects to NO. Release it, and they separate again. A doorbell button is a classic NO contact: you press it, the circuit completes, the bell rings.

NC means Normally Closed. When the switch is at rest, COM IS connected to NC. Actuate the switch and that connection breaks. An emergency stop button uses NC contacts. The circuit stays complete until you hit the button, then it deliberately breaks.

A SPDT switch has all three: COM, NO, and NC. A DPDT switch has six terminals: two full sets of COM, NO, and NC, one set per pole. Both poles move together inside the switch body. Once you learn these three labels on one datasheet, they translate directly to every other one you’ll ever read.

Two Terminals, One Job

An SPST switch has two terminals and controls one circuit. That’s it. Switch open, circuit broken. Switch closed, circuit complete. No alternate path. No normally closed contact. No second circuit anywhere.

This is the configuration behind a simple power switch on a piece of test equipment, a light switch on a panel, or a door interlock that cuts power when the door opens. In a toggle switch, SPST just says ON-OFF. In a pushbutton, it’s usually momentary. The contacts close only while you hold the button down. A maintained SPST pushbutton stays in the pressed position until you press it again. The datasheet will tell you which behavior you’re buying.

When SPST Makes Sense

Reach for SPST when you need to turn one circuit on and off, and there’s no reason to switch to an alternate circuit when the primary one is off.

A panel indicator light that doesn’t need a dim mode: SPST. A cooling fan that runs or stops with no speed selection: SPST. A safety interlock that breaks a control circuit: SPST. Fewer terminals to wire. Fewer things to go wrong. Lower cost per switch compared to the same model in SPDT or DPDT configuration. If the application only needs on and off, don’t pay for contacts you won’t use. SPST is the most common contact configuration in production equipment for exactly that reason.

The Three-Terminal Switch That Routes a Signal

An SPDT switch has three terminals: COM, NO, and NC. COM connects to either NO or NC depending on the switch position. Never to both at the same time. That’s the defining behavior. This makes SPDT a routing switch.

Connect your signal source to COM. Connect one destination to NO and the other to NC. Flip the switch and the signal goes one way or the other. A common use case: a selector switch that toggles between two signal sources or two operating modes.

In a toggle switch, SPDT is ON-ON. Both positions complete a circuit, just to different output terminals. Some SPDT switches are ON-OFF-ON, with a center position that connects COM to nothing at all. You can also use an SPDT as a simple SPST. Wire COM and NO, leave NC unconnected. That gives you flexibility to change the switching logic later without swapping the switch hardware.

SPDT in Control Panels: Mode Selection and Signal Routing

Control panels lean hard on SPDT switches for mode selection. The classic example is a HAND-OFF-AUTO selector for a motor starter. In HAND, the switch routes control voltage directly to the motor contactor coil. In AUTO, it routes to a PLC output or a thermostat contact. In OFF, nothing is connected. Three positions, one switch. That’s SPDT with a center-off position.

Another common pattern: a two-speed fan control. COM goes to the fan motor. NO goes to the high-speed winding. NC goes to the low-speed winding. Flip the switch and the fan changes speed. SPDT is the workhorse of routing and selection in panels everywhere.

Two Poles, One Actuator: What That Gives You

A DPDT switch has two poles, each with its own COM, NO, and NC terminal. Both poles move simultaneously when you flip the switch. You could wire two separate SPDT switches and toggle them at the same time, but the DPDT guarantees both circuits switch at the exact same instant.

That guarantee matters a lot for polarity reversal. Wire a DPDT as a reversing switch: motor leads go to the two COM terminals, power supply goes to the NO and NC terminals in a cross-connected arrangement. Flip the switch and the polarity to the motor reverses. If the two poles didn’t switch simultaneously, the power supply would briefly short through the motor during the transition. The DPDT mechanism prevents that by moving both contacts together inside the switch body.

Where DPDT Shows Up: Reversing, Switching, and Safety

Motor reversing is the headline application, but DPDT shows up in plenty of other places.

A stereo audio selector uses DPDT to switch both left and right channels simultaneously. A differential signal pair in an instrumentation circuit uses DPDT to route the positive and negative signal lines together. Safety circuits use DPDT for redundancy: both poles break the circuit when the switch opens. If one set of contacts welds closed, the other set still breaks the circuit.

That dual-break arrangement is common on emergency stop buttons and safety interlock switches. In those applications, DPDT isn’t about routing at all. It’s about guaranteed disconnection even when a single contact fails.

SPST, SPDT, and DPDT Switches in Standard Form Factors

LANZ Electronics manufactures electromechanical switches in SPST, SPDT, and DPDT contact configurations across toggle, rocker, pushbutton, and tact actuator types. Every switch family ships with a contact diagram in the datasheet, so the panel builder can verify terminal assignments against the schematic before ordering. The switch catalog covers a range of current ratings and panel cutout sizes that match standard enclosure designs for industrial and commercial applications.

Switches and Connectors for Complete Panel Integration

For a full panel build, LANZ also supplies relay sockets, IEC connectors, and wire harnesses that integrate with every switch family in the catalog. Matching the switch contact configuration to the relay socket wiring and the panel connector layout gets a lot easier when all the components come from the same manufacturer with consistent terminal numbering and datasheet conventions.