With the increasing complexity and automation of machinery and production equipment, safety has become increasingly important. Effective and reliable safety technologies are the foundation for ensuring the safety of equipment and personnel. When discussing machinery safety, safety levels are an unavoidable topic. How can we quickly calculate the corresponding safety level of a safety circuit?
When discussing safety levels, standards and directives are naturally involved. In Europe, safety-related issues are primarily emphasized and articulated through regulations such as the Machinery Directive. The Machinery Directive contains explicit requirements for the design and manufacture of machinery and control systems, and harmonized standards are used to more precisely and effectively implement these requirements. For example, EN ISO 13849-1 can be used to verify whether safety-related components of a control system comply with the general health and safety requirements stipulated in the Machinery Directive.
Reduce expected risks
EN ISO 13849-1 applies to applications in electrical, electronic, programmable, mechanical, and hydraulic control systems, with the goal of reducing risk to an acceptable level. It is mainly divided into five levels:
Performance level PL
Performance level (PL) describes the ability of a control system's relevant safety components (SRP/CS) to perform safety functions and achieve the expected risk reduction under foreseeable conditions. A required performance level (PLr) must be determined for each safety function of the machine. For example, preventing a collision hazard in a moving mechanism requires ensuring its safe stop when an emergency stop button is pressed or a safety door is opened. Therefore, stopping the motor by controlling the relevant contactor via the emergency stop button is a safety function, and stopping the motor by controlling the same relevant contactor via the safety door is also a safety function.
SRP/CS 1: Emergency Stop – Safety Controller – Contactor K1/K2
SRP/CS 2: Safety door – Safety controller – Contactor K1/K2
Both of these safety functions require a defined performance level (PLr). For specific machinery, the Class C harmonized standard can be referenced.
S/F/P "Three Musketeers"
Assuming safety functions are unavailable, and based on a thorough understanding of mechanical limitations and hazard identification, a risk assessment is conducted to determine whether the risk reduction of the equipment is sufficient. If risk reduction still needs to be achieved through safety functions, then the risk diagram in Annex A of EN ISO 13849-1 (as shown below) is used to determine the appropriate risk reduction PL for each safety function. PLr can be understood as the "rated value" for achieving the safety function, derived from a combination of the three parameters S/F/P.
After determining the PL value of a certain safety function, the electrical circuit design and component selection can proceed. At this point, the factors to be considered regarding safety components include:
Structure Category
Mean Time to Failure (MTTFd)
Diagnostic coverage DCavg
Common Cause Failure CCF
The above factors can also be used to verify whether existing equipment meets certain safety requirements.
More powerful software tools
Using PAScal, a software tool provided by Pilz, to calculate the loop safety level of the control system for safety-related components is not only easy to operate but also has an intuitive interface, greatly reducing the time cost of calculation.
The following diagram illustrates an example of a PSENcode safety coded switch connected to a PNOZsigma safety relay to control a dual contactor to cut off a motor:
1. A graphical editor simulates security features and defines their structure, simplifying the usage of components.
2. The calculation panel clearly displays the main parameter information for each component.
3. Users can import standard database formats from various original documents, and can also create and manage their own databases, ultimately generating multilingual reports for the project without incurring any additional costs.
In addition, Pilz also offers the PASmsi mobile app, which allows users to verify secure computing anytime, anywhere.
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