Changeover Contact (SPDT) Symbol
Definition: The Changeover Contact (SPDT) symbol represents a single-pole double-throw relay or switch contact — standardised in IEC 60617-07 and ANSI/IEEE 315 — with one Common terminal that connects to either a Normally Closed (NC) terminal in the de-energised state or a Normally Open (NO) terminal in the energised state, enabling circuit transfer between two paths from a single common point.
Also known as: changeover contact, SPDT contact, single-pole double-throw contact, transfer contact, C-form contact, Form C contact, break-before-make contact, CO contact.
What the Changeover Contact (SPDT) symbol means
The Changeover Contact (SPDT) symbol represents the output switching element of a relay, contactor, or switch that simultaneously breaks one circuit and makes another as the actuating coil is energised. The Common pin (COM) is the shared terminal that moves between positions; the NC pin is the terminal connected to Common in the rest (de-energised) position; the NO pin is the terminal connected to Common in the actuated (energised) position.
Changeover contacts are used wherever a circuit must transfer from one path to another — for example, transferring a load between a main supply and a standby supply, switching an indicator lamp from 'running' to 'fault' state, or routing a signal between two destinations. The standard break-before-make action means Common briefly disconnects from both NC and NO during the transition, preventing simultaneous connection of both paths.
How to identify the Changeover Contact (SPDT) symbol
The Changeover Contact (SPDT) symbol is drawn as a diagonal line (the movable contact arm) pivoting from the Common pin at the top, swinging between two stationary contacts below: the NC contact (left-bottom, connected in the rest position by the arm's natural position) and the NO contact (right-bottom, connected only when the relay coil is energised). IEC 60617-07 shows the arm angled toward the NC contact; ANSI/IEEE 315 uses a similar representation with the arm touching the NC contact at rest.
Function in a circuit
A changeover contact transfers the Common terminal's connection from the NC path to the NO path (or vice versa) when the controlling relay coil, switch actuator, or microswitch is operated. In a relay circuit, this allows a single relay to simultaneously deactivate one circuit (breaking the NC connection) and activate another (making the NO connection), performing two switching operations with one control signal. This is the most versatile contact form, implementing logic functions such as OR, XOR, and transfer switching in ladder diagrams.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 defines symbols for switchgear and controlgear including changeover (SPDT) contacts. IEC 60947-5-1 covers low-voltage control circuit devices including SPDT relay contacts. The IEC symbol shows the movable arm connecting the Common (top) to NC (lower left) in the rest state, with NO (lower right) open. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315 (IEEE Std 315-1975) defines the SPDT contact symbol as a diagonal arm on the Common terminal with NC and NO terminations. NEMA ICS 1 and NEMA ICS 2 cover industrial control equipment contact designations. The ANSI symbol is nearly identical to IEC; both show the arm touching NC at rest. |
| Key difference | The IEC and ANSI/IEEE symbols for the changeover contact are functionally identical. Minor differences exist in line weights and the angle of the movable arm. IEC documentation labels the pins COM/NC/NO; ANSI/NEMA documentation may use 1/3/4 or 11/12/14 terminal numbers for contactor contacts. |
Terminals / pins
| Pin | Name |
|---|---|
| com | Common |
| nc | NC |
| no | NO |
Typical values
Contact voltage ratings: 24 VDC to 480 VAC typical. Contact current ratings: 1 A to 30 A (signal to power relay). Coil voltages: 5 VDC, 12 VDC, 24 VDC, 24 VAC, 120 VAC, 240 VAC. Electrical life: 100,000 to 10,000,000 operations depending on load type. Contact resistance: < 50 mΩ (new). Mechanical life: 1,000,000 to 30,000,000 operations.
Where the Changeover Contact (SPDT) symbol is used
- Supply changeover circuits — transferring a load between main supply and standby/emergency supply via changeover relay
- Alarm and indicator circuits — NC feeds a 'normal' green lamp; NO feeds a 'fault' red lamp, providing simultaneous lamp switching
- PLC input modules — changeover contact provides both a digital input signal and an interlocking output in a single relay
- Safety relay circuits — changeover contacts used to simultaneously open the safety output and indicate fault status
- Automatic transfer switch (ATS) — changeover contacts in contactors route load between utility and generator supply
- Motor forward/reverse circuits — changeover contacts route phase connections for motor direction reversal
- Control panel interlock circuits — changeover contact prevents simultaneous operation of two mutually exclusive functions
Example
In a pump motor control circuit, the motor protection relay's changeover contact transfers on trip: the Common pin connects to the alarm circuit; in normal operation the NC path illuminates the 'running' pilot lamp; when the relay trips, the Common transfers to the NO path, extinguishing the 'running' lamp and simultaneously energising the 'fault' alarm lamp and buzzer.
Key facts
- A changeover contact (SPDT) is a Form C relay contact with three terminals: Common (COM), Normally Closed (NC), and Normally Open (NO), standardised in IEC 60617-07 and ANSI/IEEE 315.
- The symbol has three pins: Common (top), NC (lower-left), and NO (lower-right); the movable arm rests on NC when the coil is de-energised.
- SPDT stands for Single-Pole Double-Throw: one pole (Common) that throws between two positions (NC and NO).
- The standard action is break-before-make: Common briefly disconnects from NC before connecting to NO, preventing both paths from being simultaneously energised during transition.
- Terminal numbering in NEMA/IEC practice: for relay sockets the pins are often numbered 1(COM)/2(NC)/3(NO) or 11(COM)/12(NC)/14(NO) depending on the relay family.
- A changeover contact enables one relay to replace two separate NO and NC contacts in a ladder diagram, reducing component count and wiring.
- In IEC 60617-07 the contact state depicted is always the de-energised (rest) state; the circuit designer must annotate 'operated state' if the diagram shows energised positions.
Diagrams that use this symbol
- changeover switch wiring diagram
- automatic transfer switch wiring diagram
- 2 pole changeover switch wiring diagram
- changeover switch diagram
- spdt relay diagram
- 3 phase changeover switch connection diagram
- automatic changeover switch wiring diagram
- 3 phase automatic transfer switch wiring diagram
Frequently asked questions
What does the changeover contact symbol mean in a circuit diagram?
The changeover contact (SPDT) symbol represents a relay contact that simultaneously breaks one circuit and makes another when the relay coil is energised. The Common terminal connects to the NC (Normally Closed) terminal when the relay is de-energised, and transfers to the NO (Normally Open) terminal when the relay is energised, enabling circuit transfer from one path to another.
What does the changeover contact (SPDT) symbol look like?
The changeover contact symbol is a diagonal line (movable arm) pivoting from the Common terminal at the top, with the NC terminal at the lower-left (the arm rests here in the de-energised state) and the NO terminal at the lower-right (the arm swings here when energised). Both IEC 60617-07 and ANSI/IEEE 315 use this same diagonal-arm representation.
What is the difference between NC and NO contacts?
A Normally Closed (NC) contact is closed (conducting) when the relay coil is de-energised, and opens when the coil is energised. A Normally Open (NO) contact is open (not conducting) when the relay coil is de-energised, and closes when the coil is energised. In a changeover (SPDT) contact, both NC and NO share a single Common terminal, so energising the relay simultaneously opens the NC path and closes the NO path.
What is the IEC vs ANSI difference for the changeover contact symbol?
The IEC 60617-07 and ANSI/IEEE 315 symbols for the changeover contact are functionally identical — both show a pivoting arm resting on the NC terminal. Minor differences exist in line weight and the exact angle of the arm. IEC documents label the terminals COM/NC/NO; ANSI/NEMA documents may use numerical terminal designations such as 11/12/14.
What does SPDT mean for a relay contact?
SPDT stands for Single-Pole Double-Throw. 'Single-Pole' means there is one Common (switching) terminal; 'Double-Throw' means the Common can connect to either of two output terminals (NC or NO). An SPDT relay replaces both a separate NC relay and a separate NO relay in a control circuit, reducing the number of relay poles needed.
What is break-before-make action in a changeover contact?
Break-before-make means the Common terminal disconnects from the NC terminal before it connects to the NO terminal during switching. This brief open interval prevents momentary short-circuit between the NC and NO circuits during the transition. Most standard relay changeover contacts use break-before-make; make-before-break (bridging) contacts exist for special applications where a momentary connection between both paths is required.
What standard defines the changeover contact symbol?
IEC 60617-07 defines the schematic symbol for changeover contacts (labelled CO or C in IEC notation) as part of its switchgear and controlgear symbol set. ANSI/IEEE 315 (IEEE Std 315-1975) defines the equivalent SPDT contact symbol in North American practice. IEC 60947-5-1 specifies the electrical performance requirements for low-voltage control circuit contacts including changeover types.
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