Door Interlock Switch Symbol
Definition: The Door Interlock Switch symbol represents a safety switching device in electrical and control system diagrams that monitors the open/closed state of an enclosure door, guard, or access panel—automatically interrupting the control circuit or de-energising dangerous machinery when the door is opened—depicted as a normally-closed (NC) switch with In and Out terminals, referenced in IEC 60947-5-1, IEC 60204-1 (machinery safety), and ANSI / NFPA 79.
Also known as: door safety switch, enclosure interlock, guard interlock switch, access panel switch, safety door switch, tongue interlock, trapped-key interlock.
What the Door Interlock Switch symbol means
The Door Interlock Switch symbol marks a mandatory safety point in a machine control circuit where access to a hazardous area is controlled by monitoring the state of a physical barrier (door, guard, or cover). When the door is closed the switch contacts are held in the closed (Run) state, allowing the machine to operate; when the door is opened the switch contacts open, interrupting the control circuit and commanding the machine to stop or de-energise before personnel can enter the hazard zone.
In control system schematics the door interlock switch symbol appears in the safety circuit chain—often in series with other safety devices (emergency stop, light curtains, pressure mats)—with the In pin connected from the upstream safety relay or power supply and the Out pin continuing to the downstream safety chain or contactor coil.
How to identify the Door Interlock Switch symbol
The Door Interlock Switch glyph is drawn as a normally-closed (NC) switch symbol—two short parallel lines (fixed and movable contacts in the closed position) with a diagonal line indicating the actuator that is held in by the door mechanism—on a pair of conductor lines representing the In and Out circuit connections. Many symbols include a small box or rectangle to represent the switch housing mounted on the door frame, and an actuating element (key, tongue, or cam) that engages when the door closes. It is distinguished from a general limit switch by context (door-frame mounting) and from a pushbutton by its maintained (not momentary) operation.
Function in a circuit
A door interlock switch works by mechanically linking a contact mechanism to the door or guard position. In the closed-door (safe) state the actuating key or cam holds the contacts in the normally-closed position, completing the safety circuit. When the door opens, the actuator disengages, the NC contacts spring to the open state, and the safety relay or PLC safety input detects the open circuit, initiating a controlled stop or emergency stop sequence. High-integrity interlocks use dual-channel (redundant) contacts and self-monitoring to detect contact welding, actuator failure, or wiring faults.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-5-1 covers position switches including safety door interlocks used as control circuit devices. IEC 60204-1 (Safety of Machinery — Electrical Equipment) Clause 9.3 requires interlocking guards with position monitoring on all movable guards protecting dangerous machine zones. ISO 14119 specifically governs interlocking devices associated with guards, defining four locking principles and four coding levels. |
|---|---|
| ANSI/IEEE 315 | ANSI / NFPA 79 (Electrical Standard for Industrial Machinery) Section 9.3 requires that every movable guard providing access to a hazard zone be equipped with an interlocking device that stops the hazardous motion before the guard can be opened. ANSI B11 series standards also address machine safeguarding requirements. |
| Key difference | IEC 60204-1 and ISO 14119 are the primary international standards; ANSI / NFPA 79 is the North American equivalent. Both mandate interlocking but differ in detailed requirements for coding levels and locking device categories. The schematic symbol is functionally the same in both standards—an NC position switch mounted at the guard. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Contact ratings: typically 3 A to 10 A at 250 V AC / 24 V DC. Safety category: up to PLe / Category 4 (IEC 13849-1) with redundant contacts. Actuator force: 5–50 N depending on type. IP rating: IP54 to IP67 for washdown environments. Operating temperature: −25°C to +70°C.
Where the Door Interlock Switch symbol is used
- CNC machine tool enclosures preventing spindle and axis motion when the door is open for tool change or workpiece loading
- Robot cell safety fencing detecting access gate opening to initiate robot safe-stop before personnel enter the cell
- Press and stamping machine guarding requiring positive door closure before the ram can cycle
- Electrical panel and switchgear enclosures interlocking the door with the main isolator to prevent opening under live conditions
- Industrial oven and furnace doors stopping heating elements when door is open to prevent burn hazards
- Elevator and lift landing doors verifying door closure before allowing car movement per EN 81-20
- Food and pharmaceutical processing equipment ensuring hygienic enclosures are closed before automated cleaning cycles begin
Example
In a CNC machining centre safety circuit diagram, a Door Interlock Switch symbol (S3) is wired in series in the safety relay input channel between the 24 V DC safety supply and the safety relay module input. The In pin is connected to the +24 V safety channel; the Out pin connects to the safety relay input terminal. When the machining centre door is closed, S3 contacts are closed, the safety relay is energised, and the spindle and axis drives receive their enable signals. Opening the door opens S3, the safety relay detects the loss of input, and initiates a Category 1 stop sequence within 200 ms.
Key facts
- The Door Interlock Switch symbol represents a normally-closed (NC) position switch that opens its contacts when the guarded door is opened, interrupting the safety circuit and initiating a machine stop; it has In and Out terminals wired in series in the safety control circuit.
- ISO 14119 governs interlocking devices associated with guards and defines four interlock principles: form-fit actuation, spring-retained actuation, gravity actuation, and magnetic actuation, as well as four coding levels from low to high tamper resistance.
- IEC 60204-1 Clause 9.3 mandates that movable guards protecting hazardous zones be interlocked so that the hazardous machine function stops before the guard can be opened—the door interlock switch is the primary device satisfying this requirement.
- High-safety-integrity door interlocks use dual-channel (two independent NC contacts) wired to a safety relay or safety PLC monitoring both channels; contact welding in one channel is detected at the next machine start attempt.
- Door interlock switches achieve safety performance levels from PLc to PLe (IEC 13849-1) depending on the number of channels, self-monitoring, and the safety relay category; safety-rated installations require periodic proof-test intervals.
- Tongue (captive-key) interlock switches prevent the door from being opened until the actuating tongue is withdrawn, requiring deliberate two-step action and providing higher tamper resistance than simple snap-action switches.
- Trapped-key interlocks use a key transfer mechanism to physically prevent machine operation unless the access door key is in the machine lock, providing mechanical proof that the operator is inside the hazard zone before the machine is de-energised.
Frequently asked questions
What does the door interlock switch symbol mean in a control diagram?
The Door Interlock Switch symbol means that a safety position switch is installed at a guarded access point; it opens its normally-closed contacts when the door or guard is opened, interrupting the control circuit and triggering a machine stop or de-energisation sequence to protect personnel from hazardous motion or energy.
What does the door interlock switch symbol look like?
The Door Interlock Switch symbol looks like a normally-closed (NC) switch symbol—two short lines representing closed contacts with an actuating diagonal line—drawn on the In and Out circuit lines. A small rectangle or housing outline is often added to represent the switch body mounted on the door frame, with an actuating key or cam element shown engaging when the door is closed.
What is the IEC standard for door interlock switches?
IEC 60947-5-1 covers position switches and control circuit devices including door interlocks. ISO 14119 specifically governs interlocking devices associated with guards. IEC 60204-1 Clause 9.3 mandates the use of interlocking guards on machinery with hazardous zones. Together these three standards define the design, installation, and performance requirements for door interlock switches.
Is a door interlock switch normally open or normally closed?
A door interlock switch is normally closed (NC) in its de-actuated state—meaning when the door is open (actuator disengaged), the switch contacts are open, interrupting the safety circuit. When the door is closed (actuator engaged by the door), the contacts are held closed, allowing the machine to run. This fail-safe configuration ensures a broken cable or missing actuator results in a machine stop, not continued operation.
What is the difference between a door interlock and a limit switch?
A limit switch is a general-purpose position-sensing switch used for machine sequencing and position detection. A door interlock switch is a safety-rated position switch specifically designed for guard monitoring with features such as higher contact reliability, tamper resistance, positive opening (direct actuation ensures contacts separate even if the spring fails), and dual-channel contacts for safety-integrity applications. Standard limit switches do not meet the safety requirements of ISO 14119 or IEC 60204-1.
What safety performance level can a door interlock switch achieve?
A single-channel door interlock switch without self-monitoring can achieve up to Performance Level c (PLc, Category 1) per IEC 13849-1. A dual-channel redundant installation with a safety relay monitoring both channels achieves PLd or PLe (Category 3 or Category 4) depending on the safety relay category and proof-test interval. The required PL is determined by the risk assessment for the specific hazard.
What is a tongue (captive-key) interlock switch?
A tongue (captive-key) interlock switch uses a shaped actuating tongue that must be inserted into the switch body when the guard closes; removing the tongue (opening the guard) immediately opens the safety contacts. The captive-key design prevents the tongue from being removed while the machine is running and provides higher tamper resistance than a simple push-type actuator, making it suitable for PLd/PLe safety applications per ISO 14119.
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