Key Switch Symbol
Definition: The Key Switch symbol represents a mechanically-operated switch that requires a physical key to actuate in circuit diagrams, drawn as a standard switch symbol (two terminals connected by an actuating arm) with a key icon or 'K' annotation, standardised under IEC 60617-07 and ANSI Y32.2 / IEEE 315, used to restrict circuit control access to authorised personnel, with designator S or SW.
Also known as: key-operated switch, keyed switch, lock switch, key lock switch, keyswitch.
What the Key Switch symbol means
The Key Switch symbol denotes a single-pole switch whose actuation mechanism requires inserting and turning a specific physical key, preventing unauthorised operation of the controlled circuit. In wiring and control diagrams it marks an access-controlled ON/OFF point for security-critical or safety-critical circuits such as machinery start, alarm arming, or equipment enable.
The symbol's In and Out pins represent the two switch contacts; inserting and turning the key closes (or opens, in NC variants) the contact path. Key switches are inherently SPST (single-pole single-throw), and their symbol is the same as a general switch but annotated with a key symbol or the word 'KEY' to communicate the mechanical access restriction. They are widely used in industrial machinery, vehicle ignition, security panels, and electrical distribution lockout-tagout systems.
How to identify the Key Switch symbol
The Key Switch symbol is drawn as a standard SPST switch: two terminal points (In and Out) connected by a line representing the actuating arm, which is shown either open (NO — normally open) or closed (NC — normally closed). A small key icon, an inverted triangle or diamond representing the key insertion slot, or the text 'K' or 'KEY' is placed adjacent to the switch symbol to distinguish it from a plain push-button or toggle switch.
Function in a circuit
A key switch mechanically engages its electrical contacts only when the correct key is fully inserted and rotated to the operated position. The locking cylinder retains the key in the operated position until deliberately rotated back and removed, providing a physical security barrier against unauthorised circuit control. Some key switches are momentary (spring-return when key is released); others are maintained (stay in switched position until key is turned back).
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 (switches and relays): the key switch uses the general single-pole switch symbol with a key-indicator annotation. IEC 60947-5-1 covers low-voltage control circuit devices including key-operated control switches. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2-1975 / IEEE 315-1975: uses the standard SPST switch symbol with a key-operator indicator. NEMA ICS 1 and ICS 5 cover industrial control equipment including key switches. |
| Key difference | IEC and ANSI switch symbols are essentially the same for a key switch: a standard contact symbol with a key-operator annotation. IEC symbols use a stylised key glyph per IEC 60617-07; ANSI practice uses a 'K' label or key icon. Functionally and graphically near-identical. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Contact configuration: SPST (most common), DPST, DPDT available. Rated voltage: 24 V DC to 250 V AC. Rated current: 2–10 A. IP rating: IP40 (panel-mount) to IP67 (outdoor key switches). Key retention: key removable in one or both positions depending on cylinder type. Operating temperature: −25 °C to +70 °C.
Where the Key Switch symbol is used
- Industrial machinery start/stop panels where only authorised operators may energise equipment
- Security alarm system arming and disarming to prevent unauthorised bypass
- Vehicle ignition circuits (automotive key switches rated for starter motor cranking current)
- Electrical panel lockout-tagout (LOTO) switches to positively isolate circuit sections during maintenance
- Elevator emergency power and fireman's service mode switches required by lift codes
- Data centre cabinet and network equipment power enable switches requiring physical key access
Example
In a CNC machine tool safety interlock schematic, the Key Switch symbol shows its In pin connected to the 24 V DC control bus and its Out pin connected to the enable input of the PLC safety module. The machine cannot be started without inserting and turning the operator key, which closes the switch and enables the PLC's motion-control outputs per ISO 13849 safety category requirements.
Key facts
- The Key Switch symbol is a standard SPST switch symbol (two terminals with an actuating arm) annotated with a key icon or 'KEY' label per IEC 60617-07, indicating that a physical key is required to operate the switch.
- Key switches have two pins: In (the circuit input) and Out (the circuit output); closing the switch completes the circuit between them, controlled by key insertion and rotation.
- Key switches are used where physical access control over circuit operation is required — industrial machinery, security panels, vehicle ignitions, and lockout-tagout isolation points.
- The reference designator for a key switch is S or SW per IEC 60617 and ANSI Y32.2 / IEEE 315, e.g., S1 or SW-KEY.
- Key switches may be normally-open (NO, default open circuit, key closes it) or normally-closed (NC, default closed circuit, key opens it); the symbol convention is the same as for general switches.
- Maintained key switches hold their position after the key is removed; momentary key switches spring-return to their default position when the key is released or removed.
- IEC 60947-5-1 governs low-voltage control circuit devices including key-operated switches for industrial applications; vehicle ignition key switches fall under automotive standards such as ISO 7637.
Frequently asked questions
What does the Key Switch symbol mean in a circuit diagram?
The Key Switch symbol means that the circuit can only be controlled — turned on or off — by a person with the correct physical key. It marks an access-controlled switching point used in machinery, security systems, and isolation circuits where unauthorised operation must be physically prevented.
What does the Key Switch symbol look like?
The Key Switch symbol looks like a standard SPST single-pole switch — two terminal points (In and Out) joined by a line representing the movable contact arm — with a small key icon, inverted triangle, or 'KEY' label beside it to indicate key-operated actuation. The switch arm is shown open for a normally-open (NO) type and closed for a normally-closed (NC) type.
What is the difference between a key switch and a normal switch?
A normal switch can be operated by anyone who can reach it. A key switch requires insertion of a specific physical key to actuate, restricting operation to authorised personnel. The electrical function is identical — connecting or disconnecting two circuit points — but the mechanical access control is the defining distinction. The schematic symbol is the same as a standard switch with a key-operator annotation.
What standard defines the key switch symbol?
The key switch symbol is governed by IEC 60617-07 (switches and relays) for IEC schematics, and ANSI Y32.2-1975 / IEEE 315-1975 for North American schematics. IEC 60947-5-1 covers low-voltage control circuit devices including key-operated switches in industrial applications.
What is the designator for a key switch in a schematic?
The reference designator for a key switch is S or SW per IEC 60617 and ANSI Y32.2 / IEEE 315, the same as for any switch, numbered sequentially — S1, S2, etc. Some designers add a descriptive suffix such as S-KEY or SW_KEY to distinguish key switches from other switch types in the schematic notes.
What is a maintained vs momentary key switch?
A maintained key switch stays in its operated (switched) position after the key is turned, even if the key is removed; it returns to its default position only when the key is reinserted and turned back. A momentary key switch spring-returns to its default position when the key is released or removed. The schematic symbol is the same for both; the type is specified in the component annotation or BOM.
Can a key switch be normally closed?
Yes. Key switches are available in both normally-open (NO, circuit open without key) and normally-closed (NC, circuit closed without key) contact configurations. On the schematic, the NO type shows an open switch arm between In and Out; the NC type shows a closed connection. Inserting and turning the key changes the contact state in either case.
Place the Key Switch symbol on a wiring diagram or schematic in the free online circuit diagram maker — no download required.