Machine safety planning guide
Safety Light Curtain Installation Distance: What to Check Before Buying
The required separation distance is not printed on a light curtain label. It must be determined from the complete machine stopping time, human approach, protective-device geometry, reach possibilities, and application uncertainty before the mounting position and model are finalized.
- ISO 13855:2024 framework
- Stopping-time data chain
- Installation and purchasing checks
A shorter curtain response time can reduce distance, but it cannot compensate for an unmeasured machine stop or a layout that allows reaching around the field.
Include the sensor, safety logic, output devices, actuators, and mechanical run-down to the end of the hazardous state.
Perpendicular, parallel, angled, indirect, and moving-hazard cases do not share one universal layout calculation.
Calculate and validate reaching through, over, under, and around, plus any space where a person could remain behind the curtain.
Start with the right term
Do not confuse separation distance with sensing range
Safety light curtain installation distance normally means the separation distance between the sensing plane and the nearest hazardous point along the relevant approach path. Its purpose is to give the machine enough time to reach a safe state before a person can reach the danger.
This is different from the optical distance between the transmitter and receiver. A curtain may have enough operating range to span a wide opening and still be mounted too close to the hazard. Protective height is a third dimension: it determines the vertical area covered, not how far the field must sit from the danger point.
Current international framework
ISO 13855:2024 uses more than speed multiplied by time
For a static approach, the current high-level separation-distance structure is shown below. The actual supplements and geometry must be selected from the standard for the application, not guessed from a generic web calculator.
Dynamic cases can require an additional term for movement of the hazard relative to the person. A moving hazard cannot be treated as a fixed machine layout.
The final distance from safeguard to hazard along the assessed approach path.
Selected for the body part and approach case defined by the standard. It is not a measured conveyor or robot speed.
Time from protective-field interruption until the hazardous machine state has ended, including applicable tolerances.
The maximum applicable supplement for ways a person can reach the danger point through or around the protective arrangement.
Allowance for factors such as measurement uncertainty, reflection, or reduced braking performance where applicable.
Avoid an outdated shortcut
Why many older articles show a different formula
Manuals and articles written around ISO 13855:2010 often use S = K × T + C. That legacy method still explains older machine documentation, but it must not be presented as the complete 2024 calculation.
S = K × T + C
- C commonly represented an intrusion or reach allowance.
- Many device manuals still reproduce this method because they were issued before the 2024 revision.
- Useful for understanding legacy documentation, not for silently approving a new 2024 design.
S = (K × T) + DDS + Z
- Reach supplements are more structured and application dependent.
- Dynamic separation distance is addressed when the hazard position changes relative to the person.
- Vertical ESPE limits and other safeguard-placement rules were revised.
A corrected legacy example: why 480 mm is not the final answer
Assume a legacy 2010 finger/hand calculation with total time T = 0.22 s and detection capability d = 30 mm. This example is included only to correct a common arithmetic and rule-application error.
The value buyers often do not have
Total stopping time is a machine-system value, not a light curtain specification
A supplier can provide the curtain response time. Only the machine owner, integrator, or qualified safety specialist can establish the complete time to the end of the hazardous state under the assessed conditions.
Beam interruption is detected and the dual safety outputs change state.
Relay or safety PLC logic and downstream switching devices respond.
Actuators and mechanical motion reach the end of the hazardous state.
- Use measured worst-case performance. Brake wear, load, speed, pressure, temperature, and maintenance condition can change the stop.
- Document where the measurement starts and ends. A motor command going low is not necessarily the end of dangerous motion.
- Include tolerances required by the assessment. A nominal stop time without uncertainty can place the field too close.
- Revalidate after relevant changes. Control edits, drive replacement, brake work, speed changes, tooling, and added cascade devices can change T.
Geometry changes the calculation
Which direction can a person actually approach the hazard?
The distance must be assessed along the real route to danger. A vertical curtain approached at a right angle is only one case. Work platforms, stairs, feed openings, conveyors, and moving machine elements can create indirect or dynamic paths.
Perpendicular approach
The person moves toward a vertical sensing plane. Check reaching through the resolution, reaching over the top, crawling under, and access around both ends.
Parallel approach
The person moves over a horizontal protective field toward the hazard. Field height, stepping over, and entry from unprotected sides become central design questions.
Angled, indirect, or dynamic
Do not force an angled field or moving hazard into the perpendicular formula. Determine the actual approach and relative movement before selecting the calculation route.
Selection affects placement
Resolution, response time, and protective height must be checked together
Resolution describes detection capability. Response time describes how quickly the selected device changes its safety outputs. Protective height describes coverage. None of these can be inferred reliably from housing length alone.
| Selection factor | What it changes | What to verify before ordering | Common mistake |
|---|---|---|---|
| Detection capability | Which body part can be detected and which reach supplement may apply. | Smallest body part that can reach the hazard and any enabled blanking function. | Calling every 30 mm model a complete solution without checking reach-over and side access. |
| Device response time | Contributes to overall T and therefore to separation distance. | Exact model, protective height, operating mode, cascade arrangement, and data-sheet response time. | Assuming finer resolution is always faster. Beam count and model design can change response. |
| Protective height | Determines whether access over, under, or beside the field remains possible. | Hazard height, mounting reference, floor or platform level, and unprotected openings. | Ordering by total housing length instead of the effective protective field. |
| Operating range | Determines whether the emitter and receiver can maintain the optical link. | Actual span, contamination, vibration, mirrors, alignment margin, and the selected model manual. | Treating operating range as the distance from the curtain to the hazard. |
Typical 14 mm and 25–30 mm categories are often used for finger and hand detection respectively, but the final body-part classification, supplement, and mounting arrangement must follow the applicable standard and the exact device documentation.
Distance alone is not enough
Six installation details that can defeat an otherwise correct distance
A correct number does not create a safe installation if a person can bypass the sensing plane or if the optical field can be distorted.
A low upper edge or nearby fixture can let an arm reach the hazard without interrupting the protected field at the expected point.
If a person can enter and remain between the curtain and hazard, presence detection, guarded reset, or another protective measure may be required.
Machine tables, metal bins, glossy guards, and reflective material can redirect beams. Use the exact model manual and field width to determine clearance.
Flexible brackets and machine shock can reduce optical margin or cause nuisance trips. Confirm mounting stiffness and alignment access.
Blanking changes detection capability; muting temporarily suspends protection under controlled conditions. Neither is a shortcut for reducing separation distance.
The reset arrangement must not let a hidden person be trapped inside the safeguarded space or allow an unexpected automatic restart.
Interactive planning aid
Is your project ready for an installation-distance review?
Mark the information you already have. The tool identifies missing inputs so engineering and purchasing can request the right data before a model is released for order.
Data readiness
Complete inputs reduce late mounting changes and prevent the supplier response time from being mistaken for the full machine stop.
Confirm all eight input groups before finalizing the curtain model and mounting position.
Common project situations
What changes the installation plan in real machines?
Fast access, changing stop performance
A point-of-operation application usually demands careful stop-time measurement and strict control of reaching over, under, or around the field.
- Measure the hazardous stroke or run-down under worst-case conditions.
- Confirm the field covers every operator access path.
- Check whether the machine-specific standard adds requirements.
Limited mounting depth
When the calculated distance does not fit, the answer is not to move the curtain closer. Reduce the validated stopping time or redesign the guarding concept.
- Review drive safe-stop functions and mechanical braking.
- Compare exact model response times, not family headlines.
- Consider fixed guards or a different access arrangement.
Material must cross the field
Muting may support controlled material passage, but it introduces a separate safety-function design. It does not remove the need for distance and access validation.
- Define how material is distinguished from a person.
- Assess gaps beside and above the load.
- Validate timing, sequence, restart, and fault behavior.
Build a useful RFQ
Send engineering data, not only an opening width
A model recommendation is more reliable when the supplier can see the protection task, environment, integration, and space constraints. Final separation-distance approval remains with the responsible machine-safety team.
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Machine and hazardMachine type, dangerous motion, speed or load condition, and applicable machine standard.
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Measured stop dataMeasurement method, worst result, operating condition, date, and tolerance used.
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Dimensioned layoutHazard location, proposed field, opening size, floor or platform, fixtures, and side access.
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Detection requirementBody part to detect, protective height, operating span, and any blanking requirement.
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Electrical integration24 VDC supply, OSSD interface, safety relay or PLC, connector, cable length, and reset concept.
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Environment and orderDust, water, temperature, vibration, reflective surfaces, quantity, destination, and OEM needs.
From concept to acceptance
A practical installation and validation sequence
Define dangerous motion, exposed people, tasks, foreseeable misuse, and required risk reduction.
Establish T, select the correct approach case, and determine all applicable supplements.
Confirm detection capability, height, range, response, safety data, environment, and integration.
Mount at or beyond the required distance and close access over, under, around, or behind the field.
Test the complete safety function, record results, and repeat checks after relevant changes and at defined intervals.
Use each document for the right job
Which standards and documents should be checked?
A light curtain certificate does not determine the mounting position. Product design, machine control, safeguard placement, and local legal duties are connected but separate review layers.
Positions safeguards with respect to human approach. It is the central international reference for separation-distance planning discussed on this page.
View the ISO recordCovers electro-sensitive protective equipment requirements. IEC states that Part 1 does not define the detection-zone dimensions or its position relative to a specific hazard.
View IEC 61496-1:2020Use the selected model manual for response time, range, effective field, brackets, optical interference, reflective clearance, test procedure, and approved functions.
Explore XSZ safety light curtainsJurisdiction and machine type can add requirements. In the United States, OSHA 1910.212 is a general machine-guarding rule, not a universal light-curtain distance formula.
View OSHA 1910.212Prepare the right model data
Need a light curtain configuration for your machine layout?
Send the opening dimensions, proposed mounting drawing, required detection capability, measured stopping time, operating range, environment, voltage, safety-control interface, and quantity. XSZ can help compare suitable product configurations for your engineering review.
Frequently asked questions
Safety light curtain installation distance FAQ
What is safety light curtain installation distance?
It is the required separation between the protective field and the hazard along the assessed approach path. The distance must allow the complete machine safety system to detect entry and end the hazardous state before a person can reach the danger.
Can the light curtain supplier calculate the final distance?
The supplier can provide exact device data such as response time, detection capability, protective height, and operating range. The final calculation also needs measured machine stopping performance, approach geometry, reach possibilities, uncertainty, applicable standards, and a qualified machine risk assessment.
Is S = K × T + C still the current ISO formula?
It is widely found in documentation based on ISO 13855:2010. ISO 13855:2024 uses the current high-level static framework S = (K × T) + DDS + Z, with additional treatment for dynamic cases. Legacy manuals should be read in the context of their issue date and the applicable project requirements.
Does a faster light curtain always allow much closer mounting?
Not necessarily. Device response time is only one part of overall T. Mechanical run-down can dominate, and reach or application supplements can remain significant even when the sensor responds quickly.
How does 14 mm versus 30 mm resolution affect distance?
Detection capability influences the body part detected and the reach supplement used in the relevant calculation. It can also change the exact device response time. The final effect must be checked with the current standard and the selected model data rather than applying a fixed distance difference.
How far should reflective metal be from a safety light curtain?
There is no safe universal value for all models. Reflective clearance depends on the selected device, optical geometry, and emitter-to-receiver distance. Use the exact installation manual and check all reflective objects around the field, including machine tables, bins, glossy guards, and workpieces.
Can muting be used when there is not enough safety distance?
No. Muting temporarily suspends the protective function under a validated material-flow sequence; it is not a method for shortening the required separation distance. If space is insufficient, review stopping performance or redesign the guarding and access concept.
When should the installation distance be revalidated?
Revalidate after changes that can affect the safety function, stopping time, access path, or device configuration, and at the intervals defined by the risk assessment, machine documentation, validation plan, and applicable rules. Examples include control edits, brake work, speed changes, new tooling, and altered blanking or muting settings.
Technical references
Primary sources used for this guide
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ISO 13855:2024 official record Current edition status, publication date, scope, and withdrawal of ISO 13855:2010.
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Pilz summary of ISO 13855:2024 Public explanation of the revised formula, response-time components, reach supplements, application supplement, and dynamic approach.
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IEC 61496-1:2020 official record Scope of ESPE functional requirements and the explicit limitation that Part 1 does not determine detection-zone placement for a specific hazard.
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SICK senSe2 operating instructions Manufacturer example showing how stopping time, response time, resolution, reach-over, reflective surfaces, and field width enter a device-specific installation review.
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OSHA 29 CFR 1910.212 General United States machine-guarding requirement for protecting operators and other employees from machine hazards.