Safety mats – how to use them? These devices play a crucial role in machine safety systems. This article delves into what safety mats are, how they work, and the scenarios in which they should be implemented. We will also explore risk analysis in accordance with the ISO 12100 standard, emphasizing that safety mats should be a last resort when no other protective measures are feasible. Additionally, we will examine design requirements, including the machine’s stopping time as per ISO 13855 and the need to achieve appropriate safety levels PL (Performance Level, ISO 13849) or SIL (Safety Integrity Level, IEC 62061). Finally, we stress the importance of precise calculations and compliance with standards to avoid severe functional and legal consequences.
Table of Contents
What Are Safety Mats and How Do They Work?
Safety mats are pressure-sensitive devices used to detect the presence of individuals on a specific surface around machinery. They function like large floor-mounted emergency stop buttons. When a person (or object) steps on the mat, it changes the electrical signal, alerting the control system of someone’s presence. In essence, when an operator steps onto the mat, a safety signal is generated, triggering the immediate cessation of hazardous machine movement or activating an alarm.
Typically, safety mats consist of two conductive layers separated by an insulating spacer. This design ensures that when the mat is activated (stepped on), the layers come into contact, closing the electrical circuit. This signal is sent to a specialized safety relay module (safety controller), which promptly halts the machine’s movement. Thus, safety mats create a virtual detection zone—a flat surface whose breach (entry by a person) initiates a safety function that stops the machine. A complete system comprises the mat itself and its corresponding safety module; only this combination forms a functional operator presence detection system that responds to threats.
Where and When to Use Safety Mats – Risk Analysis According to ISO 12100
The decision to use safety mats should stem from a risk analysis conducted in accordance with the ISO 12100 standard (Safety of Machinery – General Principles for Design – Risk Assessment and Risk Reduction). This standard recommends that when designing a machine safety system, the first step should be to eliminate hazards at the source through appropriate design solutions. If eliminating a hazard is not possible, the next steps involve risk reduction. In the second stage, technical protective measures—such as fixed guards, interlocked movable guards, or other protective devices—are used to isolate or minimize access to hazardous areas.
Safety mats are classified as protective devices (other than guards) and can serve as one of these protective measures. Their specific application is particularly relevant when other protective methods prove unfeasible or insufficient. Safety mats can be used as the primary safeguard where the use of other protective devices is impossible or impractical. For example, if a fixed guard (e.g., fencing) or light curtain cannot be used because the operator needs free access to the work area, a safety mat may solve the problem. Mats are also used as supplementary protection—for instance, in conjunction with fencing (when there is a risk of bypassing the fence) or safety curtains to additionally monitor the floor area.
However, it is essential to remember that safety mats are a last-resort protective measure. Unlike physical guards that prevent access to the danger zone, mats allow entry into the hazardous area and then must quickly react to remove the threat (stop the movement). In other words, a guard prevents contact with a dangerous element, while a mat allows the operator to enter the zone and then must react very quickly to stop the machine before an accident occurs. Therefore, the use of mats must be justified—they should only be considered when no better protection method exists (e.g., when hazard elimination or effective guard installation is not possible). The risk analysis according to ISO 12100 should demonstrate that all preceding measures (design and organizational) have been exhausted, and the mat is necessary to further reduce the risk to an acceptable level.
Machine Stopping Time and Safety Distance – ISO 13855 Requirements
If the risk analysis indicates the necessity of using a safety mat, a critical design issue is the proper placement of the mat relative to the hazardous area. It is not enough to simply place the mat “where convenient”—it must be ensured that the machine can stop before the operator, who stepped on the mat, reaches the dangerous part of the machine. In other words, the safety distance between the mat and the hazard must account for the machine’s stopping time and the entire system’s reaction time. The standards provide clear guidelines on how to calculate this: ISO 13855 (Safety of Machinery – Positioning of Protective Equipment with Respect to Approach Speeds of Parts of the Human Body) contains formulas for determining the minimum distance of the mat from the hazard.
According to ISO 13855, the minimum distance S should be calculated based on the sum of the safety system’s reaction time (T1—including the time taken to detect pressure by the mat and the safety module’s response) and the machine’s stopping time (T2), multiplied by the appropriate constant for the human approach speed. For a person walking, a constant of K = 1600 mm/s is typically used, corresponding to an average walking speed. An additional constant distance (e.g., 1200 mm) depending on the detection height is also considered, although in the case of floor mats, this component is often constant (the mat is on the floor, so height H≈0). Consequently, a typical formula for the safety distance looks as follows:
S = 1600 [mm/s] × (T1 + T2) + 1200 [mm]The above equation (for example, derived from the standard) means that the longer the machine takes to stop or the longer the system’s reaction time, the further away the safety mat must be placed from it. Therefore, the system designer must know the reaction times—both of the mat itself (and the safety module) and the machine. The machine’s stopping time should be calculated and then measured for the worst-case scenario. Then, using the normative formula, the minimum distance S is calculated. Only then can we be confident that when the operator steps on the mat, they will not physically reach the hazardous area before the machine comes to a complete stop. It is worth emphasizing that failure to comply with these guidelines can have tragic consequences—if the mat is placed too close to the hazard (without proper consideration of stopping time), the machine may not stop in time.
Safety Levels PL and SIL – ISO 13849-1 and IEC 62061 Requirements
The mere presence of a safety mat does not guarantee that the protective system will be sufficiently reliable. Mats are part of a broader safety-related control system—including the sensor (mat), logic module (safety relay or controller), and actuators stopping the machine. The entire chain must meet a specified safety assurance level appropriate for the required risk reduction. The standards ISO 13849-1 (Safety of Machinery – Safety-Related Parts of Control Systems – Part 1: General Principles for Design) and IEC 62061 (Functional Safety of Machinery – Electrical, Electronic and Programmable Electronic Control Systems) define how to classify and verify such systems for reliability. In practice, parameters PL (Performance Level) according to ISO 13849-1 or SIL (Safety Integrity Level) according to IEC 62061 are used for this purpose.
The PL level is denoted alphabetically from a (lowest) to e (highest), while SIL is numerically from 1 to 3 (or 4, depending on the standard—in machinery, up to SIL 3). The required level depends on the risk assessment—the higher the risk (frequency of exposure, possibility of bypassing protection, severity of potential injuries), the higher the required PL or SIL for the safety function. For a safety mat, this means that, for instance, with a very dangerous machine, it may be necessary to design the system to achieve PL d or PL e (equivalent to SIL 2/SIL 3). Achieving such a high level requires meeting several requirements: the mat must have an appropriate internal structure (e.g., two redundant signal paths), the safety module should monitor its circuits (detect faults, such as wire breaks or contact shorts), and the entire system must be resistant to single failures (category 3 or 4 according to ISO 13849-1). The designer is responsible for selecting components and the control system architecture to achieve the required level of reliability.
It is important to note that the existence of standards does not impose a specific PL or SIL for the mat—the machine designer determines the required level of protection through risk analysis. They must then design and evaluate the entire safety system according to the relevant standards to ensure that this level is achieved. In other words, if the analysis indicates a need for PL d, the mat and control system must be configured and connected to meet the PL d criteria (e.g., have two channels, diagnostics, appropriate component reliability, required MTTFd, DC, etc.). Similarly, for SIL—the system must undergo an evaluation process (e.g., using IEC 62061 guidelines), considering the probability of dangerous failures per hour and other factors affecting the SIL level.
Do Not Install Safety Mats “By Eye” – Consequences of Design Errors
When designing systems with safety mats (similar to safety sensors), one must not rely on “by eye” methods—every aspect must be supported by calculations and reference to standards. Intuitive or random placement of mats, without checking stopping times and minimum distances, risks leaving unprotected gaps in the safety system. Similarly, ignoring PL/SIL requirements (e.g., connecting the mat haphazardly, without the appropriate safety relay or without reliability analysis) can result in the system failing when truly needed. The consequences of such negligence are twofold:
- Functional (safety) consequences – poorly designed protection may not act in time or fail in an emergency. In the worst case, this means endangering the lives or health of operators, despite the apparent presence of mats. For example, if the mat is placed too close to a moving mechanism, the operator may be struck before the machine stops. If the control system does not achieve the required reliability level, there is a risk that a single failure (e.g., contactor sticking, broken wire) will prevent the mat from shutting down the machine the next time the operator enters the zone—potentially leading to an accident.
- Normative and legal consequences – failing to comply with standard requirements (ISO 13855, ISO 13849, IEC 62061, etc.) means that the machine does not meet the essential safety requirements prescribed by law. In the European Union, such a machine should not receive CE marking or be placed on the market, and if it is already in operation, its use is illegal. In the event of an accident, an inspection or prosecutor may find negligence in the form of deviations from normative guidelines—resulting in legal liability for the designer, supplier, or employer. In other words, using safety mats without strict adherence to standards can result in not only safety threats but also legal consequences (e.g., sanctions, compensation claims).
In summary, safety mats are a valuable component of a machine’s safety system, but only when their application is thoroughly considered and supported by analysis. They should be treated as the last line of defense—used consciously, correctly calculated for placement, and ensuring the appropriate level of functional safety. The following table summarizes the key standards discussed in the article and the scope of their requirements: