Stopping time changes everything
What EN ISO 13855:2024 introduces into the calculation of Safety Distances
The Safety Distance depends on what actually happens
The Safety Distance is calculated, verified, documented. Then the machine undergoes a maintenance operation. Or a component wears out. Or the control software is updated. The stopping time changes. The distance calculated at commissioning may no longer be valid.
EN ISO 13855:2024, which defines the criteria for positioning safeguards with respect to the approach of the human body to hazard zones, does not only change the calculation formula. It changes the way stopping time is conceived and managed, requiring its assessment and control across the entire machine lifecycle.
The standard applies to safety light curtains, laser scanners, pressure-sensitive mats, two-hand control devices and interlocked guards: all safeguards that act before the operator can reach the hazard zone.
The formula and the parameter that governs everything
The separation distance is defined by the formula:
S = (K × T) + DDS + Z
S is the Safety Distance. K is the approach speed. T is the overall response time of the system. DDS is the reaching distance associated with the protective device. Z gathers the correction factors specific to the scenario.
The significant change is not in the structure of the formula. It is in the parameter T: EN ISO 13855:2024 requires a systemic reading of the entire functional safety chain. T is not the response time of the individual sensor. It is the overall time that elapses between the detection of the hazardous condition and the machine reaching the safe state.
Every link in the chain, detection, signal processing, mechanical action, contributes to the value of T. If one of these links changes, T changes. And with T, S changes.
Contextual distances: the direction of approach changes everything
The standard abandons the idea that the Safety Distance is a single, invariable value.
EN ISO 13855:2024 distinguishes scenarios in which the direction of approach to the hazard zone is known from those in which it is not predictable. The standard recognises that approach is not a uniform event: it varies with the geometry of the installation and the operating modes.
In open areas, robot cells or installations with multiple access points, the direction of approach is not controlled. The Safety Distance must be dimensioned for the most critical case.
The Safety Distance is no longer a geometric parameter derived from an ideal situation: it must be consistent with the real configuration of the machine-operator system.
From point-based systems to protection volumes
Modern electro-sensitive protective equipment (ESPE) no longer detects a point or a line: it detects a volume.
3D laser scanners, integrated vision systems and advanced area sensors work on three-dimensional protective fields with dynamic triggering logic.
EN ISO 13855:2024 incorporates this conceptual evolution: the shift from logic based on a detection point or line to logic based on a dynamic safety volume.
In this context, distance is no longer just a geometric parameter. It must be assessed with respect to the real shape of the protective field and its triggering logic. A 3D field that adapts to the operator’s posture requires a different calculation approach from a single-beam light curtain.
The response time also includes the operator
The standard updates the criteria for two-hand control devices and single control devices, and for the position of interlocked guards. It also specifies the requirements for the positioning of safety-related manual control devices (SRMCD).
The central element is not only the revision of the calculation criteria. It is the recognition of the variability of human behaviour.
In real conditions, the operator’s intervention time is not constant: workstation ergonomics, operating frequency, fatigue, machine layout and production context directly affect the response of the human-machine system.
This leads to a more realistic modelling of the overall response time, which must include not only the technical system but also the human component.
The protected space as a dynamic relationship
EN ISO 13855:2024 introduces a more rigorous definition of the concepts of protected space and access to hazard zones.
The focus shifts from physical distance alone to the relationship between available space, possibility of access and the state of the safety function. Concepts such as whole body access, reachability from different positions and the relationship between the hazard zone and the dynamic state of the system are clarified.
This approach reduces the interpretative ambiguities present in previous versions and reinforces a systemic view of machine safety: safety is not a property of the individual device, but a relationship between the installation, the operator and the operating conditions.
Stopping time is not a fixed value
This is the point that EN ISO 13855:2024 places at the centre.
The parameter T is the central element of the model.
The Safety Distance depends directly on the time needed for:
- the system to detect the hazardous condition
- the signal to be processed
- the machine to reach a safe state
Even small variations in this time can generate significant differences in the required Safety Distance.
The stopping time of a machine varies over time. Brake wear, changes in the masses involved, software updates, component replacement: each of these events can significantly alter T.
The distance remains unchanged from the one calculated at commissioning.
What happens in industrial practice
In practice, the stopping time is often:
- derived from the manufacturer's data
- assumed as a fixed value in the Risk Assessment
- verified only at initial commissioning
- rarely re-checked after modifications or maintenance operations
The behaviour of the machine does not remain constant over time.
Brake wear, variations in masses, software updates or component replacement can significantly affect the real stopping time.
It follows that a Safety Distance that is correct at commissioning may no longer be so during the useful life of the machine. With EN ISO 13855:2024, this approach reveals all its structural limits.
C-MON-SAFE: from periodic checks to continuous monitoring
To meet this need, Accessafe has developed C-MON-SAFE, a platform dedicated to the continuous monitoring of the safety functions of machines and installations.
The system was not designed solely as a tool for measuring stopping time. It is a solution for the structured supervision of safety performance across the entire machine lifecycle.
Within this architecture, stopping-time monitoring is one of the main applications, particularly relevant in relation to the criteria introduced by EN ISO 13855:2024.
The measurements taken at commissioning or validation are integrated into the system, creating a technical baseline for comparison over time. Through instrumental measurements and historical data analysis, C-MON-SAFE makes it possible to highlight progressive variations in the performance of the safety functions with respect to the reference values used at the design stage.
This makes it possible to move from logic based on periodic checks to a model of continuous observation of safety performance.
Accessafe support: from instrumental measurement to continuous monitoring
Accessafe carries out instrumental measurements of stopping times at commissioning, revamping and periodic checks.
The measured value is objective, repeatable and directly usable as:
- Instrumental measurement of stopping time / Objective data on the real behaviour of the machine, the basis for the correct application of EN ISO 13855:2024.
- Validation of Safety Distances / Verification that the installed distances are consistent with the machine's actual T, at commissioning or revamping.
- Technical baseline for C-MON-SAFE / Integration of the measured value into the platform as a reference for continuous comparison over time.
- Continuous monitoring of safety functions / Structured supervision of safety performance across the entire machine lifecycle.
- Periodic checks / Re-checking of the stopping time after modifications, extraordinary maintenance or software updates.
Frequently asked questions
What changes compared with EN ISO 13855:2010?
The 2024 version introduces a systemic reading of the parameter T (overall response time), which now represents the entire functional safety chain and not just the time of the individual device.
It adds the factor Z for contextual correction factors, distinguishes access scenarios with known and unknown direction, and incorporates the evolution of ESPE towards volumetric protection systems.
It also updates the criteria for the variability of human behaviour in the use of control devices.
What is the overall response time T?
T is the total time that elapses between the moment the system detects the hazardous condition and the moment the machine reaches a safe state.
It includes the response time of the protective device, the processing time of the safety system and the stopping time of the machine.
Any change that alters one of these components alters T, and with T, the required Safety Distance.
How often should the stopping time be re-checked?
EN ISO 13855:2024 does not set a frequency, but indicates that safety depends on reliable knowledge of the machine’ sactual T over time.
Correct practice provides for verification after every substantial change (mechanical, software,components), after extraordinary maintenance operations and as part of periodic safety checks.
Continuous monitoring with C-MON-SAFE makes it possible to detect progressive variations without waiting for the periodic check.
Does C-MON-SAFE replace the instrumental measurement of stopping time?
No. They are two complementary levels.
The instrumental measurement provides the objective data on the real behaviour of the machine: it is the starting point, indispensable for the correct application of EN ISO 13855:2024. C-MON-SAFE integrates that data as a baseline and compares it over time, detecting variations before they become a safety problem.
The instrumental measurement defines the reference; C-MON-SAFE keeps it up to date.
Does the standard also apply to machines already installed?
Yes. EN ISO 13855:2024 is relevant not only for new installations, but for any machine in which the protective devices, the components of the safety system or the operating conditions are modified. In these cases, verifying the Safety Distance against the actual T is necessary to maintain conformity.
