SPDT Switch

Switch SPDT Symbol: Meaning, Diagram & How to Read It

Thursday, May 28th, 2026

The switch SPDT symbol appears in almost every schematic that routes a signal between two destinations. It looks simple—one input arm pointing at two contacts—but reading it correctly tells you exactly how current flows through the circuit. This guide explains what the symbol means, how to read it, the IEC vs ANSI versions, and how to draw it yourself.

Figure 1: The basic Switch SPDT Symbol— one common (pole) terminal and two throw terminals.

What Is a SPDT Switch? (Single Pole, Double Throw Explained)

SPDT stands for Single Pole, Double Throw. In plain terms:

Single pole → one common input terminal (the “pole”).
Double throw → that input can connect to one of two output terminals.

So a SPDT switch has three terminals and acts as a changeover (or selector): it sends the common line to output A or output B, but never both at once. In British English, a SPDT used to control one light from two locations is called a two-way switch.

What Is the Symbol for a SPDT Switch?

The SPDT switch symbol is a single line (the moving contact) that pivots from one common terminal toward two separate contacts. The common terminal is the pole; the two contacts are the throws. The position of the arm shows which throw the pole is connected to.

You’ll also see it called a SPDT schematic symbol or SPDT circuit symbol—they all refer to the same thing.

Figure 2: A labeled SPDT switch symbol showing the COM, NC, and NO terminals.

How to Read a SPDT Switch Symbol: The Pole and Two Throw Terminals

Read it in three steps:

Find the common terminal (pole). It’s the single terminal the moving arm starts from—usually drawn as a dot or short line. This is your input.
Find the two throw terminals. These are the two contacts the arm can reach. They’re your two possible outputs.
Look at where the arm points. The arm shows the switch’s current state—connected to the upper throw or the lower throw.

The three terminals are commonly labeled:

COM — the common (pole)
NC — normally closed (connected to COM when the switch is at rest)
NO — normally open (connects to COM when the switch is actuated)

A SPDT is break-before-make by default: the arm leaves one throw before touching the other, so the two outputs are never shorted together.

SPDT Switch Schematic Symbol: IEC vs ANSI/IEEE Standards

The SPDT symbol is nearly universal, but two standards govern the fine details:

IEC 60617 (international / European): draws the contacts as short bars or open marks, with a clean pivoting line for the moving contact.
ANSI/IEEE 315 (North American): very similar, typically with small open circles or dots for the contact points.

In both standards the logic is identical—one pole, two throws—so a schematic is readable either way. The differences are cosmetic: the shape of the contact points and the angle of the actuator line.

Figure 3: The SPDT switch symbol in IEC 60617 and ANSI/IEEE 315 styles.

SPDT Symbol for Different Switch Types: Toggle, Slide, Limit & Pressure

The same SPDT symbol is used no matter the physical switch, because the symbol describes the function (one pole, two throws), not the actuator. The actuator may be shown with an extra mark:

SPDT toggle switch symbol — the standard pivoting-arm symbol; the most common form.
SPDT slide switch symbol — same contacts, sometimes drawn with a small slider mark.
SPDT limit switch symbol — the arm gains a small roller/lever or actuator flag to show it’s mechanically tripped.
SPDT pressure switch symbol — the actuator is drawn as a diaphragm or pressure symbol that moves the contact.

The takeaway: find the pole and the two throws first—the extra marks just tell you how the switch is operated.

Figure 4: SPDT symbol variations for toggle, slide, limit, and pressure switches.

SPDT vs SPST vs DPDT: Switch Symbols Compared

These three are easy to tell apart once you count poles and throws:

SPST (Single Pole, Single Throw) — 1 pole, 1 throw, 2 terminals. A simple on/off switch; the symbol is one arm and one contact.
SPDT (Single Pole, Double Throw) — 1 pole, 2 throws, 3 terminals. One arm selecting between two contacts.
DPDT (Double Pole, Double Throw) — two SPDT switches operated together, 6 terminals. Drawn as two SPDT symbols joined by a dashed line showing the poles move as one.

Figure 5: SPST vs SPDT vs DPDT switch symbols compared, with terminal counts.

Comparison table:

Switch Type	Poles	Throws	Terminals	Symbol Cue
SPST	1	1	2	One arm, one contact
SPDT	1	2	3	One arm, two contacts
DPDT	2	2	6	Two SPDT symbols + dashed link

SPDT Relay Symbol vs SPDT Switch Symbol

A SPDT relay uses the same contact set as a manual SPDT switch—often called a Form C contact—but adds a coil. The symbol shows:

A coil (rectangle or loop) that, when energized, moves the contact.
The SPDT contact set: COM, NC, and NO.

When the coil is de-energized, COM connects to NC. Energize the coil, and COM switches to NO. So the difference between the switch and relay symbols is simply the added coil and the dashed line linking it to the moving contact.

Figure 6: The SPDT relay symbol — a coil plus Form C (COM/NC/NO) contacts.

How to Draw a SPDT Switch Symbol Step by Step

You can sketch a correct SPDT symbol in four steps:

Draw the common terminal (pole) as a dot or short line on the left—this is the input.

Draw two throw contacts on the right, one above the other—these are the two outputs.

Add the moving arm from the pole, angled so it touches one of the two throws.

Label the terminals: COM on the pole, NC and NO on the two throws.

For a DPDT, draw two of these stacked and connect the poles with a dashed line. For a relay, add a coil and link it to the arm with a dashed line.

FAQs About Switch SPDT Symbol

How many terminals does a SPDT switch have?
Three: one common (COM/pole) and two throws (often labeled NC and NO).

What do COM, NO, and NC mean on a SPDT symbol?
COM is the common pole (input). NC (normally closed) is connected to COM at rest. NO (normally open) connects to COM when the switch is actuated.

Is a SPDT switch the same as a changeover switch?
Yes. “Changeover” and “two-way switch” (British English) both describe a SPDT switch.

What’s the difference between the SPST and SPDT symbols?
SPST has one arm and one contact (2 terminals, on/off). SPDT has one arm and two contacts (3 terminals, selects between two outputs).

Can a SPDT switch be used as a SPST switch?
Yes—just use the common terminal and one throw, and leave the other throw unconnected.

Is a 2-way switch the same as SPDT?
Yes. “2-way switch” is the common British term for a SPDT switch, often used in staircase lighting.

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