Safety Interlock Switch Symbol
Definition: The Safety Interlock Switch symbol represents a mechanically actuated safety switching device that monitors the position of a physical guard, door, or cover and de-energises hazardous machinery when the guard is opened, depicted in circuit diagrams as a switch block with dual-channel normally-open (NO1, NO2) and normally-closed (NC1, NC2) contact pairs, conforming to IEC 60947-5-1 (low-voltage control circuit devices) and IEC/EN ISO 14119 (interlocking devices associated with guards).
Also known as: guard door switch, machine guard interlock, door safety switch, trapped-key interlock, SIL interlock switch.
What the Safety Interlock Switch symbol means
The Safety Interlock Switch symbol in a circuit diagram represents a device designed to ensure that hazardous machine motion or energy cannot occur while a protective guard is open or a safety barrier is defeated. When the guard door or cover is closed, the interlock switch contacts close the safety circuit, permitting operation. When the guard is opened, the contacts open, removing power from the hazardous elements.
In safety circuit schematics, the interlock switch symbol is a critical element in a machine safety function. Dual-channel contact configurations (two independent NO contact pairs) are shown in the symbol to indicate that a safety relay or safety PLC monitors both channels simultaneously; if one channel fails, the safety relay detects the discrepancy and locks out the machine. This dual-channel architecture achieves Performance Level d (PLd) or Safety Integrity Level 2 (SIL 2) in functional safety systems.
How to identify the Safety Interlock Switch symbol
The Safety Interlock Switch symbol is drawn as a switch block rectangle with four contact pins: NO1 and NO2 on the upper edge (normally-open contacts), and NC1 and NC2 on the lower edge (normally-closed contacts for position feedback or diagnostic purposes). A diagonal actuator arrow or key symbol may be added to indicate mechanical actuation. Some symbols show a padlock or key icon to denote trapped-key interlock variants. The dual-channel contact configuration (two separate NO contacts) distinguishes the safety interlock from a standard single-channel limit switch.
Function in a circuit
A safety interlock switch physically monitors guard-door or barrier position through a direct mechanical link (actuator and switch housing). When the guard closes, the actuator engages the switch, closing the NO contact pairs and completing the safety circuit. When the guard opens, spring-return contacts open the circuit. Dual-channel designs connect each NO contact to a separate channel of a safety relay or safety PLC input module; the safety controller reads both channels and compares them — if one channel fails to open when expected (stuck contact), the safety relay detects the discrepancy within milliseconds and inhibits restart. Additional NC contacts provide door-position feedback for diagnostics and HMI status display.
Standards: IEC vs ANSI
| IEC 60617 | IEC/EN ISO 14119:2013 defines the requirements for interlocking devices associated with guards, including actuator force, manipulation resistance, and diagnostic coverage. IEC 60947-5-1 governs low-voltage switch contact ratings. IEC 62061 and ISO 13849-1 define the SIL/PLd requirements that guide dual-channel circuit architecture. |
|---|---|
| ANSI/IEEE 315 | ANSI/RIA R15.06 (industrial robots) and ANSI/NFPA 79 (electrical standard for industrial machinery) govern safety interlock applications in North America. Symbol conventions follow IEC 60617 for contact representation and ISA-5.1 for instrument loop diagrams. |
| Key difference | IEC and ANSI/NFPA both use the same contact symbols (NO = open, NC = closed at rest) for representing safety interlock contacts. IEC ISO 14119 provides the most detailed requirements; NFPA 79 references IEC 60947-5-1 for contact ratings. |
Terminals / pins
| Pin | Name |
|---|---|
| no1 | NO1 |
| nc1 | NC1 |
| no2 | NO2 |
| nc2 | NC2 |
Typical values
Contact rating: typically 3 A at 240 V AC, 2 A at 24 V DC (positive-guided contacts per IEC 60947-5-1). Actuating force: 2–30 N depending on design. Mechanical life: ≥10⁶ operations. IP rating: IP65–IP67 typical for industrial environment. Diagnostic coverage: up to 99% for dual-channel with cross-monitoring.
Where the Safety Interlock Switch symbol is used
- Machine tool guard doors — preventing spindle or blade operation while the work area door is open
- Robot cell access gates — disabling robot motion when a technician enters the work envelope
- Press and stamping machine guards — ensuring tooling area is guarded before the press cycle starts
- Conveyor belt maintenance access — isolating belt drive motors when inspection covers are removed
- Laser cutter and plasma cutter enclosures — preventing beam activation with the enclosure door open
- Chemical process plant safety gates — isolating pump or agitator drives when maintenance access panels are opened
Example
In a robotic welding cell safety circuit, the Safety Interlock Switch symbol shows NO1 connected to Safety Relay Channel 1 and NO2 connected to Safety Relay Channel 2; with the access gate closed, both channels are closed and the safety relay output energises the robot enable circuit; opening the gate opens both NO contacts, the safety relay detects the dual-channel de-energisation and drops its output contact within 10 ms, cutting power to the robot drive.
Key facts
- Safety interlock switches use positive-guided (force-guided) contacts per IEC 60947-5-1 Annex L, meaning NO and NC contacts are mechanically linked so both cannot close simultaneously — a welded contact is detectable by the opposing circuit.
- Dual-channel connection of the two NO contacts (NO1 to CH1, NO2 to CH2 of a safety relay) enables automatic discrepancy detection, achieving up to SIL 2 (IEC 62061) or Performance Level d (ISO 13849-1).
- IEC/EN ISO 14119:2013 classifies interlocking devices into four types: Type 1 (non-contact, magnetically actuated), Type 2 (actuator operates switch), Type 3 (trapped-key), and Type 4 (coded actuator); the symbol shown corresponds to Type 2.
- NC contacts on safety interlock switches provide door-position status feedback to a PLC or HMI and are used for diagnostic purposes only — they are not part of the safety stop circuit.
- Manipulation resistance is a key safety parameter per ISO 14119; higher manipulation resistance ratings reduce the risk of a deliberate bypass (e.g., taping the actuator closed).
- The reference designator for safety interlock switches in industrial schematics is S (switch) followed by a number, e.g., S1, S2; in safety circuit documentation they are often cross-referenced to the Safety Function document.
- Reset after a safety stop must be manual (deliberate operator action) per ISO 13849-1 to prevent automatic restart, which is a separate circuit function controlled by the safety relay.
Frequently asked questions
What does the safety interlock switch symbol mean in a circuit diagram?
The safety interlock switch symbol represents a mechanically actuated switch that monitors whether a guard door or safety barrier is closed. When the guard is open, the switch opens its NO contacts, de-energising the safety circuit and preventing hazardous machine operation. Dual-channel contacts (NO1, NO2) allow a safety relay to detect single-channel failures.
What does the safety interlock switch symbol look like?
The safety interlock switch symbol is drawn as a switch block with four pins: NO1 and NO2 (normally-open contacts) on one edge and NC1 and NC2 (normally-closed contacts) on the opposite edge. A mechanical actuator arrow or key icon indicates it is physically actuated by guard closure. The dual-contact configuration distinguishes it from a standard single-channel limit switch.
What standard defines safety interlock switches?
IEC/EN ISO 14119:2013 defines the requirements for interlocking devices associated with guards. IEC 60947-5-1 governs the contact ratings for low-voltage control switches. Safety circuit design using interlock switches follows ISO 13849-1 (Performance Level) or IEC 62061 (SIL) depending on the required risk reduction.
What is the difference between NO1/NO2 and NC1/NC2 on a safety interlock switch?
NO1 and NO2 are normally-open contacts that are closed only when the guard door is shut and the actuator is engaged. These contacts form the safety circuit that enables machine operation. NC1 and NC2 are normally-closed contacts that open when the guard closes; they provide position feedback and diagnostic signals to a PLC or HMI but are not part of the safety stop function.
What SIL or Performance Level can a safety interlock switch achieve?
A dual-channel safety interlock switch connected to a safety relay with automatic monitoring can achieve up to SIL 2 (IEC 62061) or Performance Level d, Category 3 (ISO 13849-1). The exact level depends on the switch's diagnostic coverage, common-cause failure data (PFHD/B10d), and the architecture of the overall safety function.
What are positive-guided contacts and why do safety interlock switches use them?
Positive-guided (force-guided) contacts per IEC 60947-5-1 Annex L are mechanically coupled so NO and NC contacts cannot both close simultaneously. If an NO contact welds shut in a closed position, the linked NC contact is held open by the mechanical coupling, allowing the safety relay to detect the fault on its next diagnostic cycle.
Can a standard limit switch replace a safety interlock switch?
No. Standard limit switches do not have positive-guided contacts, dual-channel outputs, or the manipulation resistance required by ISO 14119. Using a standard limit switch in a safety interlock function does not achieve the required Performance Level or SIL and may violate machinery safety regulations (Machinery Directive 2006/42/EC in the EU, ANSI/NFPA 79 in North America).
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