In machine safety, it is common to come across a statement that, in principle, seems unquestionable: the safety distance has already been calculated.

In many cases, this is true. The calculation has been performed correctly, the formula applied is appropriate and the solution seems consistent with the regulatory criteria. However, there is one aspect that is not always reviewed in the same depth: the data used for that calculation.

The downtime measurement according to EN ISO 13855 appears precisely at this point. Not because the calculation is wrong, but because the starting value may not reflect the actual behavior of the machine.

In well-structured projects, this verification is performed during the validation phase, when the machine is already built and there is still room for adjustment. However, in many installations, measurement is not considered until a doubt, a modification or an external requirement arises.

And this is where the approach changes. Because the difference between an estimated data and a measured data can completely condition the validity of the adopted solution.

EN ISO 13855: a simple formula with critical dependency

EN ISO 13855 establishes the criteria for positioning protective devices according to the time required to stop a hazard. Its purpose is to ensure that, once an intrusion is detected, the security system acts before contact is made with the hazardous area.

The expression used is well known:

Basis of calculation

S = (K × T) + C

Where:

• S is the minimum safety distance
• K is the approach velocity of the human body.
• T is the total system downtime
• C is the additional distance associated with the type of detector.

From a mathematical point of view, the formula is simple. However, its result depends directly on the quality of the data entered as downtime.

Calculation is not usually the problem

In practice, the calculation is rarely incorrect. The normative criteria are clear and, when applied correctly, the result is consistent.

The question is different.

The value of T is not always obtained from an actual measurement. In many cases, it is based on:

• Theoretical values
• Manufacturer’s data
• Conservative estimates
• Initial conditions that no longer hold

This implies that the calculation may be formally correct, but not necessarily representative of the actual behavior of the machine.

Difference between theoretical data and actual plant performance

One of the main difficulties in machine safety is that the actual behavior evolves over time, while the calculation tends to remain unchanged.

In the plant, conditions change naturally. And those changes directly affect downtime.

Factors that modify downtime

Without the need for major interventions, it is common to find variations derived from:

• Adjustments in drives or working speeds
• Modifications in braking systems
• Changes in valves or pneumatic circuits
• Integration of new control systems
• Progressive wear of components
• Differences between apparently equivalent machines

These factors are not always evident from the documentary point of view, but they do affect the dynamic behavior of the system.

Direct impact on safety distance

When the stopping time increases, the minimum safety distance also increases.

This can lead to situations such as:

• Barriers too close to the risk zone
• Control devices, such as double pushbuttons, incorrectly positioned
• Solutions that are no longer justifiable from a regulatory point of view

In these cases, the installation is not necessarily insecure in appearance, but it may no longer be properly validated.

When is downtime actually measured

It is important to clarify that measurement is not part of the design phase as such, since it requires the physical existence of the machine.

What is part of the design is the decision as to when and how this data will be validated.

Measurement in validation phase

In well-planned projects, measurement is performed during:

• FAT (Factory Acceptance Test)
• SAT (Site Acceptance Test)
• Start-up

At this point, the system is already built and can be evaluated under real conditions. In addition, there is still room for adjustments if the behavior does not match the predicted performance.

This is, from a technical point of view, the right time.

Measurement in operation: the most common scenario

However, in many installations the measurement is not carried out at this stage.

It appears later, usually when one of these situations occurs:

• An incident or near miss
• An investigation or legal requirement
• A modification to the machine
• An operational constraint affecting production
• An audit that requires objective evidence

In these cases, measurement ceases to be a preventive validation and becomes a reactive necessity.

The role of measurement in retrofit projects

This point is especially relevant in existing machinery, where physical and operational constraints condition the implementation of safety solutions.

The usual problem

It is common to find situations in which, when applying the normative calculation with conservative data, the resulting distance does not fit the available space.

This can lead to complex solutions or to considering the suitability as unfeasible without major modifications.

What actual measurement provides

When the downtime is measured under actual conditions, the calculation is adjusted to the effective response of the system.

This allows:

• Redefining distances on a technical basis
• Adapt the solution to the existing environment
• Avoid unnecessary interventions
• Maintaining the balance between safety and operations

It is not a matter of reducing requirements, but of basing them correctly.

Downtime as a result of the complete system

Another relevant aspect is that the downtime does not depend solely on the mechanical element.

It is the result of the overall behavior of the security system.

Elements involved in total time

In practice, downtime includes the contribution of:

• Detection devices (barriers, sensors)
• Relays or safety systems
• Control logic
• Power cut-off elements
• Braking systems
• Mechanical dynamics and inertia

Therefore, any modification in these elements may alter the final result.

Conditions for reliable measurement

For the measurement to be useful from a technical point of view, it must be carried out with appropriate criteria.

Basic requirements

A valid measurement must guarantee:

• Use of certified and calibrated equipment
• Specific instrumentation for downtime
• Sufficient repetition of tests
• Traceability of results
• Consistency with regulatory implementation

This point is especially relevant when the result is used to justify design or suitability decisions.

What does measurement actually provide

Beyond compliance, measurement provides objective data on which to make decisions.

Technical validation

It allows to verify that the implemented solution responds to the real behavior of the system.

Feasibility of solutions

It makes it easier to adapt safety measures to actual machine conditions, especially in existing environments.

Optimization with criteria

Avoid unnecessary oversizing, always within a technically justified framework.

Conclusion

The calculation may remain unchanged for years. However, the behavior of the machine does not.

When the stopping time is not validated, the safety distance becomes dependent on data that may no longer be representative.

Therefore, the measurement of downtime according to EN ISO 13855 should not be understood as a spot check, but as a key element to ensure that the calculation remains valid under real conditions.

Here you can learn the details of a real case project for downtime measurement in industrial presses.