Safety Light Curtain Symbol
Definition: The Safety Light Curtain symbol represents an electro-sensitive protective device (ESPE) that creates an invisible infrared detection field across a machine access zone and generates a safety stop signal via its Output Signal Switching Device (OSSD) when an object or body part interrupts the beam array, depicted in circuit diagrams as a vertical bar block with +24V supply and OSSD output pins, conforming to IEC 61496-1 (safety of machinery — electro-sensitive protective equipment) and IEC 61496-2 (active opto-electronic protective devices — AOPDs).
Also known as: light guard, AOPD, optoelectronic safety barrier, finger protection light curtain, area guard sensor, ESPE.
What the Safety Light Curtain symbol means
The Safety Light Curtain symbol in a circuit diagram represents a paired transmitter-and-receiver array of infrared beams that detects intrusion into a hazardous machine access zone. The transmitter emits a set of parallel infrared beams; the receiver monitors all beams continuously. When any beam is broken by a hand, finger, or body part, the OSSD outputs transition from HIGH (24 V) to LOW (0 V) within milliseconds, signalling the safety PLC or safety relay to stop the machine.
In machine safety schematics, the safety light curtain symbol indicates a non-contact, area-guarding safety function. Unlike physical guard doors, a light curtain permits frequent operator access (e.g., manual part loading) without mechanical barriers. The OSSD dual-channel output architecture provides automatic fault detection — if either OSSD output disagrees or shorts to 24 V, the safety controller detects the fault and inhibits restart.
How to identify the Safety Light Curtain symbol
The Safety Light Curtain symbol is drawn as a narrow vertical rectangle (representing the curtain body) labelled 'Safety Light Curtain' or 'Light Guard', with a +24V power supply pin at the top and an OSSD (Output Signal Switching Device) pin at the bottom. Some detailed symbols show two OSSD pins (OSSD1 and OSSD2) to represent the dual-channel safety output. Arrowed lines or dashes between two facing bar symbols may represent the beam field between transmitter and receiver.
Function and usage boundary
A safety light curtain operates by having the transmitter emit a rapid sequential scan of infrared beams across the protection field width. The receiver detects each beam; if all beams arrive unobstructed, the OSSD outputs remain at +24 V, permitting machine operation. When any beam is interrupted, the OSSD outputs switch to 0 V within the response time (typically 5–20 ms), triggering the safety circuit to de-energise the hazardous actuator. The two OSSD outputs are independently switched transistors that are monitored by the safety controller for cross-channel consistency — a short to 24 V or a disagreement between channels is detected as a fault.
Standards: IEC vs ANSI
| IEC 60617 | IEC 61496-1:2020 defines general requirements for electro-sensitive protective equipment (ESPE). IEC 61496-2 specifically covers type 2 and type 4 AOPDs (active opto-electronic protective devices). Type 4 AOPDs achieve the highest diagnostic coverage (DC ≥ 99%) and are required for applications up to SIL 3 / PLe. |
|---|---|
| ANSI/IEEE 315 | ANSI/UL 61496-1 is the North American equivalent of IEC 61496-1. OSHA 1910.217 and ANSI B11 standards reference light curtains as presence-sensing device initiation (PSDI) and supplemental guarding. ANSI/RIA R15.06 covers robot cell applications. |
| Key difference | IEC and ANSI standards are technically aligned (IEC 61496 is adopted as UL 61496 in North America). The primary classification difference is Type 2 (single-beam, lower diagnostic coverage, SIL 1/PLc) vs. Type 4 (multi-beam with self-monitoring, highest diagnostic coverage, SIL 3/PLe). |
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Logical terminals
| Logical ID | Editor terminal name |
|---|---|
| top | +24V |
| bottom | OSSD |
Where the Safety Light Curtain symbol is used
- Press brakes and stamping machines — finger and hand protection during the die-closing stroke with muting capability for tooling pass-through
- Robot cell perimeter guarding — detecting operator entry into the robot work envelope and triggering a safe-speed or stop state
- Assembly line part-load stations — permitting frequent manual part insertion without a mechanical gate, with muting for pallets
- Conveyor entry/exit apertures — detecting unauthorised access through conveyor openings in automated warehouses
- Collaborative robot (cobot) applications — providing a supplemental safety stop zone around the cobot work area
- Laser and plasma cutting enclosures — front-panel beam curtain preventing accidental reach-in during the cutting cycle
Key facts
- Safety light curtains are classified by IEC 61496-1 as Type 2 (lower diagnostic coverage) or Type 4 (self-monitoring, highest diagnostic coverage, suitable up to SIL 3 / PLe per ISO 13849-1).
- The OSSD (Output Signal Switching Device) outputs are dual-channel PNP transistors that both output +24 V when the field is clear and simultaneously switch to 0 V when a beam is broken, allowing safety controllers to detect single-channel failures.
- Resolution (minimum detectable object diameter) determines the body part that can be reliably detected: 14 mm for finger protection, 30 mm for hand detection, 90 mm for body/presence detection.
- Safety distance (minimum distance from the curtain to the hazard) is calculated using ISO 13855: S = K × (Ts + Tc + Tr + Tbm), where K is hand approach speed (typically 2000 mm/s), and T values are system response times.
- Muting is a deliberate, temporary suspension of the safety function to allow material (e.g., a pallet) to pass through the light curtain without stopping the machine; muting requires additional sensors and logic to prevent misuse.
- Fixed blanking allows a portion of the protective field to be permanently suppressed to accommodate a fixture or workpiece, reducing the effective protection field height.
- The reference designator for safety light curtains in safety circuit documentation is often LC (light curtain) or ESPE; no single IEC 60617 designator is defined.
Frequently asked questions
What does the safety light curtain symbol mean in a circuit diagram?
The safety light curtain symbol represents an infrared beam array that detects intrusion into a machine hazard zone and switches its OSSD outputs from +24 V to 0 V when any beam is broken. The symbol shows where the dual-channel safety signal enters the safety relay or safety PLC in the circuit.
What does a safety light curtain symbol look like?
The safety light curtain symbol is a narrow vertical rectangle labelled 'Safety Light Curtain' or 'Light Guard' with a +24V supply pin at the top and an OSSD pin (or OSSD1 and OSSD2 for dual-channel) at the bottom. In physical layout diagrams, the transmitter and receiver may be shown as two facing vertical bars with dashed lines representing the beam field.
What is OSSD on a safety light curtain?
OSSD stands for Output Signal Switching Device — the dual-channel PNP transistor outputs that switch between +24 V (field clear) and 0 V (field interrupted). Two independent OSSD outputs (OSSD1 and OSSD2) are provided so a safety relay or safety PLC can perform cross-channel monitoring and detect a failed output transistor.
What standard governs safety light curtains?
IEC 61496-1:2020 defines general requirements for electro-sensitive protective equipment. IEC 61496-2 specifically covers active opto-electronic protective devices (AOPDs) including light curtains. Type 4 devices per IEC 61496-2 achieve the highest diagnostic coverage and are suitable for SIL 3 / PLe safety functions.
What resolution safety light curtain do I need for finger protection?
A minimum resolution of 14 mm (minimum detectable object diameter) is required for finger protection per IEC 61496-2. For hand detection, 30 mm resolution is acceptable. For body presence detection, 90 mm resolution is sufficient. The required resolution is determined by the body part risk analysis under ISO 13857.
What is the difference between Type 2 and Type 4 safety light curtains?
Type 2 light curtains (IEC 61496-2) have lower diagnostic coverage (DC ≥ 60%) and are suitable for SIL 1 / PLc applications. Type 4 devices have the highest diagnostic coverage (DC ≥ 99%) through continuous self-monitoring of the beam array and OSSD circuits, achieving SIL 3 / PLe — required for high-risk machinery such as power presses.
What is safety distance for a light curtain?
Safety distance is the minimum distance between the light curtain detection plane and the nearest hazard point, calculated per ISO 13855: S = K × (Ts + Tc + Tr), where K = 2000 mm/s (hand approach speed), Ts = machine stopping time, Tc = control system response time, and Tr = light curtain response time. The calculated distance ensures the machine stops before a hand can reach the hazard.
Related symbols
- CO Detector symbol
- Door Interlock Switch symbol
- Emergency Stop symbol
- RCD / RCCB symbol
- Safety Interlock Switch symbol
- Smoke Detector symbol
Physical breadboard, PCB, and panel mapping
No universal physical profile is published for this symbol yet. Create a versioned custom part from the exact manufacturer dimensions and terminal map before fabrication.
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