ISO 13855

What is ISO 13855?

ISO 13855 is the international machinery safety standard that specifies how far safeguards such as light curtains, pressure-sensitive mats, two-hand controls and interlocking guards must be placed from a hazard zone, based on the speed at which a person or body part approaches that zone. At its core is a calculation rule: the distance must be large enough for the hazardous motion to have stopped before a hand, arm or whole body reaches the hazard. The current edition is ISO 13855:2024. CEN adopted it as EN ISO 13855:2024, drafted in support of the Machinery Directive.

A light curtain only protects if it is far enough from the hazard. ISO 13855 makes that distance calculable and verifiable.

🧠 Where does ISO 13855 fit in machinery safety?

European machinery standards are layered. ISO 12100 is the type A standard for risk assessment and risk reduction. Below it sit type B standards covering safety aspects common to many machines, and type C standards for a single machine type such as presses or robots. ISO 13855 is a type B1 standard: it governs one generic safety aspect, the safety distance.

The standard works together with others:

• ISO 13849-1 and IEC 62061 - determine how reliable the safety function must be (PL or SIL). ISO 13855 determines where the sensor of that function is placed.
• IEC 61496 - the product standard for electro-sensitive protective equipment (ESPE) such as light curtains and laser scanners. The response time from its datasheet feeds into the calculation.
• ISO 13857 - distances that stop fixed guards from being reached over, under or through an opening.
• ISO 14119 - interlocking devices for movable guards (interlocks). For doors without guard locking, ISO 13855 determines how far the door must be from the hazard.
• NEN-EN-ISO 13850 - the emergency stop. An emergency stop is a complementary measure and never replaces a correctly positioned safeguard.

The standard assumes persons aged 14 and older. Running, jumping and falling are outside the scope of the calculation.

📜 How has the standard evolved?

Within CEN, EN ISO 13855:2024 supersedes EN ISO 13855:2010. The presumption of conformity, however, depends on a reference in the Official Journal of the EU, and that listing was delayed for the 2024 edition, partly because of legal disputes over public access to harmonised standards. Check the current list to see which edition is cited. Many existing machines, manuals and vendor calculation tools still follow the 2010 formulas, so record in the technical file which edition you applied.

📐 How does the basic formula work?

The 2010 edition uses S = K × T + C:

Rule for K. For a perpendicular approach, you first calculate with K = 2000 mm/s. If S comes out above 500 mm, you may recalculate with K = 1600 mm/s, with a lower limit of 500 mm. S is never less than 100 mm.

T is the sum of all delays: the response time of the light curtain, the reaction time of the safety relay or safety PLC, any communication time over PROFIsafe or another safety protocol, and the machine’s overrun time to standstill. At K = 2000 mm/s, every 10 ms of extra latency costs 20 mm of extra distance. That is why deterministic behaviour of the control system can literally be expressed in millimetres here.

The 2024 edition rewrites the formula as S = (K × T) + DDS + Z. DDS is the reaching or intrusion distance that depends on the detection capability and the installation; it replaces the old supplements C and C_RO with separate values for reaching through, over and under. Z is a new supplement for effects such as measurement inaccuracy or brake wear. Also new: the calculation must account for a hazard point that keeps moving after the safeguard has been interrupted, new limits apply to vertical ESPE, and the calculation for two-hand controls has changed: without shrouding, the supplement rises from 250 mm (C) to 550 mm (DDS). For those values, use the standard text itself or a calculation tool that supports the 2024 edition.

🛠️ Which formulas apply to each type of safeguard?

For multi-beam access protection, the standard prescribes the beam heights: two beams at 400 and 900 mm (the low beam only if the risk assessment allows it), three beams at 300, 700 and 1100 mm, or four beams at 300, 600, 900 and 1200 mm. For a horizontal detection plane, the permitted resolution depends on height: d ≤ H/15 + 50 mm. An angled device is treated as a perpendicular approach above 30° and as a parallel approach below it.

If someone can reach over the top of a vertical light curtain, you also include the reach-over supplement from the table in the standard. Interpolation is not allowed: you take the higher table value, and then the larger of the conventional C and the reach-over value.

🏭 How do you calculate a safety distance in practice?

A worked example under the 2010 edition, for a vertical light curtain in front of a press brake:

1. Measure the stopping time. A stopping-time meter shows an overrun of 250 ms at maximum speed and maximum tool weight.
2. Add up all delays. Light curtain 15 ms + safety relay 20 ms + stopping time 250 ms gives T = 0.285 s.
3. Calculate with K = 2000. At 30 mm resolution: S = 2000 × 0.285 + 8 × (30 − 14) = 570 + 128 = 698 mm.
4. Recalculate with K = 1600. Because 698 > 500 mm: S = 1600 × 0.285 + 128 = 584 mm. That exceeds 500 mm, so the minimum distance is 584 mm.
5. Compare variants. With a 14 mm curtain the result is 2000 × 0.285 = 570 mm, recalculated 456 mm, so the lower limit of 500 mm applies. With two-beam access protection it becomes 1600 × 0.285 + 850 = 1306 mm.

The example shows why the choice of resolution and braking system determines floor space. A faster brake or control system directly yields a more compact machine.

Good practice in design and operation:

• Measure T under worst-case conditions - maximum speed, maximum load and a warm machine, and allow a margin for brake and clutch wear.
• Document the calculation in the technical file for CE marking, including the measurement report, the edition of the standard and the response times used.
• Repeat the measurement after brake maintenance, after software or parameter changes (include this in your MOC process), and periodically, for example during the annual inspection. A slower brake makes a previously correct distance unsafe.
• Prevent defeat - check that nobody can stand undetected between the safeguard and the hazard; add presence detection or a restart interlock where needed.
• Work safely when measuring - carry out stopping-time measurements and adjustments under Lock-out Tag-out where required.

🔄 What changes with the Machinery Regulation and mobile machines?

The Machinery Regulation (EU) 2023/1230 replaces the Machinery Directive from 20 January 2027. Harmonised type B standards such as EN ISO 13855 will remain the usual route to meeting the essential safety requirements. For employers in the Netherlands, the Arbowet also continues to apply, with an RI&E for the workplace.

The 2024 edition responds to robot cells, AGVs and AMRs. In mobile applications, the direction from which a person approaches is not always known. The standard therefore includes a calculation for dynamic separation distances, in which the protective field of a laser scanner scales with travel speed. A cobot with speed and separation monitoring follows the same principle: the robot must be able to stop before the person reaches it.

🧮 How do you calculate the safety distance yourself?

Enter the measured stopping time and the type of safeguard. The calculator applies the 2010 edition formulas, including the recalculation with K = 1600 mm/s and the minimum distances.

❓ Frequently asked questions

Which approach speed does ISO 13855 use?

ISO 13855 uses 2000 mm/s for hand and arm movement and 1600 mm/s for walking. For a perpendicular approach the calculation starts at 2000 mm/s; if the distance exceeds 500 mm, it may be recalculated at 1600 mm/s with a minimum of 500 mm.

What is the minimum safety distance under ISO 13855?

The safety distance under ISO 13855 is never less than 100 mm. When recalculating with K = 1600 mm/s, a lower limit of 500 mm also applies.

How does light curtain resolution affect the distance?

The finer the resolution, the smaller the supplement C. Under ISO 13855:2010, C is 0 for a 14 mm light curtain and 128 mm for 30 mm, and above 40 mm a fixed supplement of 850 mm applies because a whole arm can break the field.

Is ISO 13855 mandatory?

ISO 13855 is not legally mandatory, but EN ISO 13855 is a harmonised standard under the Machinery Directive. Applying an edition cited in the Official Journal of the EU gives a presumption of conformity with the related safety requirements; if you deviate, you must demonstrate yourself that your solution is equally safe.

What is the difference between ISO 13855 and ISO 13857?

ISO 13855 governs the distance of safeguards that respond to a person’s approach, such as light curtains and mats, and therefore takes time into account. ISO 13857 governs distances for fixed guards and openings so that a person cannot reach a hazard zone at all, regardless of time.

Do existing machines need recalculating under the 2024 edition?

A machine correctly built to ISO 13855:2010 does not automatically become unsafe because of the new edition. For a substantial modification, a new CE marking or a new machine, applying ISO 13855:2024 is the logical choice because it reflects the state of the art.

📌 In summary

ISO 13855 uses the formula S = K × T + C (2024 edition: S = K × T + DDS + Z) to determine how far a safeguard must be from the hazard, so that the machine stops before a person reaches it.

The standard links machine safety to control system performance: every millisecond of response time in sensor, safety PLC and brake counts. Together with ISO 13849 for the reliability of the safety function, IEC 61496 for the sensor and a documented stopping-time measurement, ISO 13855 forms the basis for a safe and demonstrably correct layout. See also functional safety and the broader topic of machine safety.