# Safety light curtain guide for robot cells in India > Specify a light curtain properly: device types, standards, the ISO 13855 distance formula with a worked example, muting rules, wiring and validation. Canonical: https://roboticsengineering.in/robot-safety-light-curtain-guide-india/ | Published: 2026-10-28 | Updated: 2026-10-28 | Author: Jignesh Panchal **A light curtain is not a sensor you add at the end of a project.** It is a safety function with a required performance level, a calculated mounting distance and a validation record. This guide explains the device types, the standards that apply, the separation distance formula, how to choose resolution and height, and the installation mistakes that leave a cell unsafe while looking protected. ## Which device fits the opening | Device | Detects | Best for | | --- | --- | --- | | Safety light curtain | Body or hand crossing a line | Regular access openings | | Safety laser scanner | Presence in an area, configurable zones | Irregular areas, AGV paths, muting | | Safety interlock switch | Door or guard position | Physical guards and gates | | Safety mat | Person standing in a zone | Floor areas with no barrier | | Safety edge | Contact on a moving edge | Closing doors and gates | Light curtains are the common choice for a robot cell opening because they allow frequent access without a physical door. They are also the device most often installed at the wrong distance. ## The standards that apply - **ISO 10218** — safety requirements for industrial robots and robot systems. - **ISO 13849** — safety-related parts of control systems, performance levels PL a to PL e. - **IEC 62061** — functional safety of machinery, SIL 1 to SIL 3. - **ISO 13855** — positioning of safeguards based on approach speed. - **ISO 13857** — safety distances to prevent reaching hazardous zones. - **ISO/TS 15066** — collaborative robot force and speed limits. The risk assessment decides the required performance level. A type 4 curtain with PL e rated output is common for robot cells, but the device rating alone does not make the function PL e. The relay, wiring, monitoring and reset behaviour all count. ## The separation distance formula ISO 13855 gives the minimum distance between the detection plane and the hazard. The basic form is S = K × T + C, where S is the distance in millimetres, K is the approach speed, T is the total stopping time and C is an additional distance that depends on the detection capability. ``` S = K * T + C K = 2000 mm/s for a walk-in approach T = sensor response + machine stopping time (seconds) C = 8 * (d - 14) for detection capability d up to 40 mm S minimum is 100 mm ``` Example: a curtain with 14 mm resolution (so C = 0), total response and stopping time of 0.25 s gives S = 2000 × 0.25 = 500 mm. Mount the curtain at least 500 mm from the hazard. If the calculated distance exceeds 500 mm, the standard allows a lower K of 1600 mm/s, but the exact rules depend on the configuration. Do not compute this once and reuse it on another cell. Stopping time changes with payload, speed, brake wear and the robot’s own stopping category. Measure the actual stopping time, or get it from the robot manufacturer for your payload and speed. ## Resolution and protective height Resolution is the smallest object the curtain detects reliably. A 14 mm resolution is finger detection, 30 mm is hand detection and 70 mm is body detection. Smaller resolution is safer but requires a larger separation distance and costs more. - 14 mm for openings where fingers could reach the hazard. - 30 mm for hand access and general cell openings. - 70 mm for body detection where the opening is large. - Protective height must cover the full opening, including above and below. - Watch for reach-over and reach-around with nearby structures. ## Muting, blanking and cascading Muting temporarily ignores the curtain so a pallet can pass. It must be designed so that a person cannot enter while muting is active. Fixed blanking ignores a permanent obstruction; floating blanking tolerates a moving one. Both reduce protection and need a documented risk assessment. Cascading joins several curtain segments into one safety output. It simplifies wiring but means a break in any segment stops the cell, which may be exactly what you want. ## Installation mistakes | Mistake | Consequence | | --- | --- | | Mounted too close to the hazard | A person is still moving when the robot reaches them | | Resolution quoted but alignment poor | Beams miss small hands or mis-trigger | | Reset button inside the guarded area | Someone can restart the cell from inside | | Curtain used as the only guard | Reach-over or reach-around remains possible | | Muting without entry detection | Person enters while hazard is live | | Dirty lenses | Nuissance stops, then someone bypasses the device | | No validation record | No evidence the function works as designed | ## Wiring and reset behaviour The curtain’s safety output should feed a safety relay or safety input, not a standard PLC input. The reset must be manual, outside the guarded area, with a clear view of the hazard. Automatic reset is only acceptable where the risk assessment permits it and access cannot be gained without tripping the device. - Use the manufacturer’s safety relay or a rated safety controller. - Keep safety wiring separate from standard control wiring. - Use dual-channel wiring with cross-monitoring where required. - Prevent reset until the hazard area is clear. - Label the reset and record the function test. ## Validation and periodic checks ISO 13849 and the machinery directive expect proof that the safety function works. Validation means testing the whole chain, not just the curtain: break a beam with a test rod at the top, middle and bottom of the protected height and confirm the robot stops in the measured time. - Record the test rod diameter and the positions tested. - Measure stopping time with a stop-time analyser. - Confirm the reset only works from outside the guarded area. - Test muting with a person-sized obstruction, not only with a pallet. - Repeat the check after any change to payload, speed or tooling. - Retest at a planned interval and after brake or drive service. Keep the risk assessment, the wiring drawing, the device certificate and the validation record together. An auditor will ask for all four, and a missing document is treated as a missing safety function. ## Choosing a device in India Buy from an authorised distributor with the declaration of conformity, the type approval and the manual in English. Grey-market devices with no traceable certificate are a liability in an audit and a risk to people. - Confirm the safety category, PL or SIL for the function, not just the device. - Check spare parts and replacement curtain availability. - Ask for the response time, which goes into the distance calculation. - Confirm IP rating for dust, coolant or washdown. - Plan the validation test and record the result. Budget for the safety relay, wiring, mounting and validation, not just the curtain. The device is often the smaller part of the installed cost, and the validation is what makes it a safety function rather than a decoration. Related: [robot safety standards in India](https://roboticsengineering.in/robot-safety-standards-india/), [integration checklist](https://roboticsengineering.in/robot-integration-checklist/), [cobot vs industrial robot](https://roboticsengineering.in/cobot-vs-industrial-robot/), [industrial robot cost](https://roboticsengineering.in/industrial-robot-cost-india/), [maintenance checklist](https://roboticsengineering.in/robot-maintenance-checklist/). **What is a safety light curtain?** An optoelectronic guard that detects a person or body part crossing a protected plane. It sends a stop signal to the machine's safety circuit when a beam is broken. **How far should a light curtain be from the hazard?** The distance comes from ISO 13855: S = K × T + C, where T is the total response and stopping time and C depends on the detection capability. Use the measured stopping time for your payload and speed. **What resolution do I need?** 14 mm for finger detection, 30 mm for hand detection and 70 mm for body detection. Smaller resolution needs a larger separation distance. **Can a light curtain connect to a normal PLC?** No. Safety outputs must go to a safety relay or safety-rated input. A standard PLC input is not a safety function. **What is muting on a light curtain?** A temporary, controlled suspension of the guard so material can pass. It must be designed so a person cannot enter the hazard zone while muting is active. Where the protected shape is irregular, the [safety laser scanner guide](https://roboticsengineering.in/safety-laser-scanner-guide-india/) covers the alternative. The light curtain is only half the circuit, so wire it through the [dual-channel safety relay](https://roboticsengineering.in/robot-safety-relay-wiring-guide/) as described here. A light curtain can replace a gate on a frequent-access opening, and the [fencing cost guide](https://roboticsengineering.in/robot-safety-fencing-cost-india/) compares the trade-offs. A light curtain is only as good as the logic behind it, so match the curtain with the [safety controller](https://roboticsengineering.in/safety-plc-vs-safety-relay-guide/) you actually install. If the risk assessment rules out power and force limiting, a light curtain may be the fallback; see [cobot safety methods compared](https://roboticsengineering.in/cobot-safety-iso-ts-15066/). ## Sources and further reading - [ISO 10218](https://www.iso.org/standard/76922.html) — robot safety requirements. - [ISO 13849](https://www.iso.org/standard/69883.html) — safety-related control system performance levels. - [IEC](https://www.iec.ch/) — IEC 62061 functional safety standard. - [SICK](https://www.sick.com/) — safety light curtain and scanner documentation.