Stop Time Testing
Stop time testing and safety validation
Ensure the positioning of your machinery safety devices is both legally compliant and operationally efficient, based on precise data-driven rather than guesswork.
What is stop time testing?
Stop time testing is a formal, quantitative process used to measure the precise time a machine takes to stop all hazardous motion. The test measures the total time, in milliseconds, from the moment a safety device (such as a light curtain, laser scanner, or two-handed control) is activated to the instant the hazardous motion completely ceases.
The entire purpose of this test is to gather the empirical data known as the ‘Overall System Stop Time’ (or T) to then allow us to use a prescribed formula to calculate and validate the minimum safe distance (S) required for machinery safeguards.
This formal measurement is the only reliable method to prove that a safeguard is positioned to ensure an operator cannot reach the hazard before the machine has completely stopped.
The importance of stop time testing
Stop time testing is a core process for managing the practical safety of your machinery. On one level it is simply about compliance and ensuring effective machinery safety.
However, it’s also far more than a compliance exercise. Stop time testing applications provide both practical and strategic benefits for safety, maintenance, and even operational efficiency:
- Validating operator safety: The test provides formal, documented verification that a safeguard is positioned at a sufficient distance from the hazard. It is the only way to prove that the time it takes an operator to reach the hazard is always greater than the time it takes the machine to stop.
- Providing predictive maintenance data: A machine’s stopping performance degrades over time due to wear on brakes, clutches, and valves. Periodic stop time testing establishes a performance baseline and tracks this degradation, allowing you to schedule maintenance before the stop time exceeds the safe limit and a failure occurs.
- Optimising production and ergonomics: To ensure compliance without precise data, companies often position safeguards (like light curtains) further away than is technically required, just to err on the side of caution. This inefficient design can lead to accidental “nuisance trips” (unnecessary stoppages caused by an operator inadvertently triggering a safety device during normal work) which in turn cause costly production delays. By measuring the true stop time, we can validate the correct minimum safe distance, improving workflow and ergonomics without compromising safety.
Stop time testing and your legal duties
The legal requirement for stop time testing is a direct consequence of an employer’s responsibilities under the Provision and Use of Work Equipment Regulations 1998 (PUWER).
A common and dangerous misconception is that a UKCA or CE mark is a guarantee of perpetual safety. A manufacturer’s mark simply attests that a machine was safe when supplied. The employer’s ongoing duty under PUWER is to protect their employees by ensuring the equipment remains safe for its entire lifecycle.
Over time, factors such as wear in mechanical, hydraulic, or pneumatic systems, or control system delays, can all cause a machine’s actual stop time to increase, invalidating the original manufacturer’s safety calculations.
PUWER Regulation 11 requires the employer to take ‘effective measures’ to prevent access to dangerous parts of machinery. For presence-sensing devices such as light curtains or laser scanners, known as electro-sensitive protective equipment (ESPE), the only way to empirically prove this measure remains ‘effective’ is to:
- Formally measure the machine’s current stopping time.
- Verify that the safeguard’s current position is compliant with this measured time.
Formal, periodic testing by a ‘competent person’ is the employer’s main proof of due diligence and ongoing PUWER compliance. This is an individual with the technical knowledge to follow a rigorous test methodology and correctly calculate the stopping time for different devices. This legal responsibility rests with the employer as the ‘duty holder’ under the regulations, even if they do not legally own the machinery (e.g., if it is leased).
Our stop time testing services
At Safety Systems Technology, we provide a full safety validation service. This is more than just a simple measurement. The physical stopping time alone is insufficient for compliance.
Our safety validation is a complete process. The true ‘Overall System Stop Time’ (T) is a sum of two parts: the mechanical stop time (which we physically measure and is affected by wear) and the fixed electronic response times of the safety components (Td, Ti). We add both together to get the compliant, accurate calculation.
Our machine safety specialists are competent persons who adapt their methodology to your specific assets, ensuring the data and conclusions are reliable, accurate, and compliant.
Our process comprises:
- Rigorous on-site testing: Our engineers use specialised, calibrated stop time analysers. We adhere to a strict methodology to test your machines under ‘worst-case’ conditions (e.g., at maximum operational speed, at the correct cycle position and at normal operating temperature). To ensure statistical validity and capture the true worst-case performance, tests are run a minimum of ten times. From this data, we use the longest measured stop time for our calculations, as this is the only value that ensures a safe and compliant validation.
- Bespoke validation protocols: The testing methodology is different for each type of safeguard. We apply distinct validation protocols as required by the standards for different devices.
- For light curtains and laser scanners: We measure the stop time (T). The protective measures must be positioned at the appropriate separation distance (S) from the hazard. This distance is calculated using the BS EN ISO 13855 (the standard which governs safeguard positioning) formula: S = (K x T) + C. Our calculation correctly incorporates the manufacturer’s device response times (Td, Ti) and the significant C factor (intrusion distance). This C value is determined by the safeguard’s resolution (e.g., ‘finger-safe’ vs. ‘hand-safe’), which dictates how far a hand or finger can penetrate before the safeguard is triggered.
- For safety mats: The test validates the depth of the mat. The C value in the formula is adapted to represent a standard human stride (e.g., 1200mm) to ensure an operator walking onto the mat cannot reach the hazard before it stops.
- For two-hand controls: These devices require two distinct tests. First, a positional stop time test (triggered on the release of the buttons) to validate the console’s location. Second, a functional test as per BS EN ISO 13851 (the standard covering design) to verify anti-defeat features and the mandatory 0.5-second (500ms) synchronicity between the buttons.
- For enabling devices: For these ‘hold-to-run’ controls, a positional stop time test is not applicable as the operator is already in the hazard zone. Our service involves a purely functional validation to ensure the 3-position logic (Off-On-Off) is working correctly, including the ‘panic’ stop function.
- Detailed compliance reporting: Following the on-site testing, we provide a formal, detailed validation report. This document is the employer’s legal record for PUWER compliance. It includes all raw data, machine identification, standards used and the final, clear calculations for S (Safety Distance) and T (Overall Stop Time), with a clear pass/fail verification for each safeguard.
Why choose Safety Systems?
Our expertise as machine safety specialists means we do more than just take a measurement:
- Competence and expertise: Our qualified engineers are legally ‘competent persons’ with thorough, practical and effective knowledge of the PUWER regulations and the full suite of EN/ISO standards, including the core standard BS EN ISO 13855 as well as BS EN ISO 13851.
- A complete safety perspective: We understand how stop time testing integrates with your wider safety systems, from machinery guarding and electrical safety to formal PUWER assessments and risk assessments. We can identify the root cause of a failing stop time, whether it’s a worn brake or a slow safety relay, and advise on the correct remedial action.
- Pragmatic and productive solutions: We provide a practical, cost-effective service that ensures your machinery is not only safe and legally compliant but also as efficient and productive as it can be.
- Testing services without disruption: Testing is planned to fit around customers’ production schedules. We carry out our work efficiently with minimum disruption.
Ensure your machinery is compliant and safe
Don’t risk operator safety on outdated data. As stop times change, a failure in compliance is a failure in safety. Contact our engineers to schedule your formal stop time testing.
Book your test
Why choose Safety Systems?
Our expertise as machine safety specialists means we do more than just take a measurement:
- Competence and expertise: Our qualified engineers are legally ‘competent persons’ with thorough, practical and effective knowledge of the PUWER regulations and the full suite of EN/ISO standards, including the core standard BS EN ISO 13855 as well as BS EN ISO 13851.
- A complete safety perspective: We understand how stop time testing integrates with your wider safety systems, from machinery guarding and electrical safety to formal PUWER assessments and risk assessments. We can identify the root cause of a failing stop time, whether it’s a worn brake or a slow safety relay, and advise on the correct remedial action.
- Pragmatic and productive solutions: We provide a practical, cost-effective service that ensures your machinery is not only safe and legally compliant but also as efficient and productive as it can be.
- Testing services without disruption: Testing is planned to fit around customers’ production schedules. We carry out our work efficiently with minimum disruption.
Ensure your machinery is compliant and safe
Don’t risk operator safety on outdated data. As stop times change, a failure in compliance is a failure in safety. Contact our engineers to schedule your formal stop time testing.