This calculator determines the required safety distances EN ISO 13855 (approach speed and detection capability of protective equipment) sets for the design of safeguarding.
Calculate Safety Distance under EN ISO 13855
A light curtain only protects if it is positioned far enough from the hazard point. The standard EN ISO 13855 specifies how to determine this distance: from the speed at which a person approaches the hazard point, from the time the safeguard and machine need to come to a stop, and from the distance a hand can penetrate into the protective field before it is detected.
Using the calculator below, you can determine the required safety distance for vertical and horizontal protective fields, for multi-beam photoelectric safeguards, and for two-hand control devices. You receive the full calculation, automatic notices on reaching over and reaching under, and a print-ready calculation sheet for your technical documentation.
What the safety distance is for
Non-contact safeguards do not physically stop anyone. They trigger a stop signal – and time passes before the hazardous movement actually comes to a stop. During that time, the hand keeps moving. The safety distance is precisely the distance that prevents the hand from reaching the hazard point before the movement has come to a standstill. It is therefore neither a rule of thumb nor a question of available space, but the calculated result of speed and time.
The basic formula: S = K × T + C
All application cases of EN ISO 13855 follow the same structure:
S = K × T + C
K – Approach speed
The standard uses two values. 2000 mm/s applies to hand and arm movement over short distances, 1600 mm/s to the approach of the whole body. For vertical protective fields with a detection capability of up to 40 mm, the calculation is first made with 2000 mm/s. If this produces a distance above 500 mm, it may be recalculated with 1600 mm/s – but the result may not fall below 500 mm. This switch is regularly overlooked in practice and leads to distances that are either unnecessarily large or impermissibly small.
T – Total time to standstill
T consists of the response time of the safeguard, including the evaluation unit, safety relay and control system, plus the machine’s stopping (overrun) time. The response time is given in the datasheet; the stopping time is not: it must be measured – at the most unfavourable operating point, with the largest tool mass and at the highest speed.
C – Penetration depth
The penetration depth describes how far a body part can penetrate the protective field without being detected. It depends on the detection capability: for a resolution of up to 40 mm, C = 8 × (d − 14 mm) applies; for coarser resolutions, detection of the whole body is assumed and C = 850 mm is used. A light curtain with a resolution of 30 mm therefore already has a penetration depth of 128 mm before time even comes into play.
The four application cases at a glance
- Vertical protective field, perpendicular approach – the classic light curtain on presses, punching machines, loading stations and robot cells.
- Horizontal protective field, parallel approach – pressure-sensitive mats, low-level protective fields and safety laser scanners. Here, S = 1600 × T + (1200 − 0.4 × H) applies; the protective field height H must not exceed 1000 mm.
- Multi-beam and single-beam safeguards – access guards with beam heights fixed by the standard: 750 mm for a single beam, 400/900 mm for two, 300/700/1100 mm for three, and 300/600/900/1200 mm for four beams.
- Two-hand control device – S = 1600 × T + 250 mm, but at least 100 mm.
Five mistakes that most commonly occur in practice
- The stopping time is estimated instead of measured. A value from the machine’s manual applies to the as-delivered condition – not to a brake after five years of operation.
- No allowance for wear. Brakes get slower over time. An allowance added to the measured total time is not a normative value, but in practice it is the difference between a distance that remains correct for years and one that fails the next inspection.
- Reaching over is not assessed. A protective field whose upper edge is below 900 mm can be reached over. EN ISO 13857 must additionally be applied for this.
- Reaching under is not assessed. If the lower edge is above 300 mm, there is a path underneath the protective field.
- Presence within the hazard zone is overlooked. Multi-beam safeguards secure access, not presence. Without a start-up and restart interlock that must be acknowledged from outside the hazard zone, the result is a trap rather than a safeguard.
Measuring the stopping time correctly
The stopping time is the only variable in the formula that changes over the machine’s service life. It must therefore not only be determined once at commissioning, but monitored during recurring inspections. It is measured with a stopping-time measuring device under the most unfavourable operating condition, several times, and the largest measured value is used. If the machine changes – a new tool, higher speed, a different brake – the measurement must be repeated and the safety distance reassessed.
Frequently asked questions
Which version of EN ISO 13855 applies?
Both: the calculator can be switched between EN ISO 13855:2010 and ISO 13855:2024. For the presumption of conformity with the Machinery Directive, the 2010 version listed in the EU Official Journal is authoritative; at ISO level, however, it has been withdrawn since the 2024 edition was published.
The 2024 edition calculates according to S = (K × T) + D_DS + Z and considers reaching over, reaching through and the newly introduced reaching under separately; the largest of the three values is authoritative. Three changes have the strongest practical impact: for two-hand control devices, the reach distance increases from 250 to 550 mm. For the horizontal protective field, the height-dependent deduction is dropped, and the value is now uniformly 1,200 mm. And reaching under is practically never included in existing calculations. Anyone planning today should cross-check both versions – that is exactly what the switch is for.
Does the calculated distance still apply after a modification?
No. Any intervention in the drive, brake, control system or tool changes the stopping time and therefore the required distance. In the case of a substantial modification, it must additionally be checked whether new CE marking is required.
What to do if the required distance cannot be implemented?
There are usually three options: shorten the stopping time (faster brake, direct shutdown), improve the resolution of the safeguard, or change the protection concept – for example with an interlocked guard or a combination of a protective field and an access guard. Which approach makes economic sense is usually only revealed by the risk assessment.
Is the calculation sufficient as evidence?
It is one building block. Also to be demonstrated are the required Performance Level under EN ISO 13849-1, validation of the measures implemented, and inclusion in the technical documentation. The calculator’s calculation sheet provides a traceable record of the calculation for this purpose.
Support with implementation
If the calculated distance cannot be achieved by design, if the stopping time needs to be measured, or if it is unclear after a modification whether a new conformity assessment is required, 4S – Safety Services can support you. We prepare risk assessments, support CE conformity, carry out validation after measures have been implemented, and prepare the technical documentation.
Have your situation reviewed – get in touch
4S – Sascha Steinkrauß Safety Services e.U., 1230 Vienna · 4S@4S.at · +43 664 9123780
Note: The calculator supports the application of EN ISO 13855 and replaces neither the risk assessment nor case-by-case verification. Only the wording of the standard is authoritative.
Are you delivering machinery into German-speaking markets (Germany, Austria, Switzerland)? 4S can also act as your German-speaking point of contact on site and support the entire project from a safety-engineering perspective. Technical documents are prepared directly in German or English; a Spanish or Swedish version is translated on request.