Understanding CAT, SIL and PL Safety Ratings

In the world of industrial machine building, safety is one of the most important things to consider. Although many factors come into play when determining the best type of safety component to use, three of the most common rating terms you’ll hear are CAT, SIL, and PL. The following is brief summary of these ratings.

SIL (Safety Integrity Level)) is a ranking method used by the International Electrotechnical Commission (IEC) under standards 61508 and 62061 to assign a risk level to possibly dangerous situations or machinery and the extent that a safety system mitigates that risk. It is based on the probability of a dangerous event occurring, and the factor by which a safety system reduces that probability. It is important to note that the SIL rating is applied only to the system as a whole, and not to individual components, although safety components can be rated as to what SIL level they are suitable to be used with.

A system with a higher SIL rating is more likely to prevent a dangerous event and is typically used with machinery and processes that pose a significant risk of severe or fatal injuries to workers. SIL ratings are assigned through analysis of the risks of individual components of the machinery to be protected, and the layers of protection that a safety system uses to prevent one component’s failure causing a dangerous event to occur. SIL ratings can be assigned by a third party, such as CSA, or can be self-assigned if it can be documented that a rigorous design and development procedure was enacted, and if the company meets IEC 61508 or 62061 functional safety capability and management standards.

PL (Performance Level) Rating provides a classification system for industrial risk, ranking from ‘a’ through ‘e’, with ‘Ple’ as the highest risk possible. PL ratings are certified by the International Organization for Standardization (ISO) under standard 13849-1 and 13849-2, with 13849-1 establishing general principles for machine safety system design and 13849-2 validating the implementation of those principles in the system in question. As with SIL rankings above, the PL is primarily concerned with how often a dangerous situation will occur, and how likely the safety system is to prevent harm caused by the dangerous situation. One aspect affecting PL and SIL ratings is the reliability of the safety system in preventing a dangerous situation each time it is called upon to do so. After the risks of a particular machine have been assessed and a safety system has been designed and tested, a PL can be assigned.

This is based on three criteria as outlined in the above flow chart:

• Severity of Injury (S1 or S2- S2 being serious injury)
• Frequency and/or exposure time to the hazard (F1 or F2- F2 being frequent or continuous exposure)
• Possibility of avoiding the hazard or limiting the harm (P1 or P2- P2 being ‘barely possible’ to avoid the hazard or limit the harm)

CAT (Safety Category) Ratings are designed to help protect workers from injury and death. These categories are directly related to SIL and PL ratings of a machine. SIL and PL ratings are determined by a series of risk assessments and calculations.

Most machinery falls into one of four Categories:

• Machine Safety Category 1- Minimal Risk requiring basic safety measures. An example of items in this category includes hand tools, which rely on operator awareness and vigilance.
• Machine Safety Category 2- Moderate Risk requiring additional safety features. An example of items in this category includes household appliances, which may rely on an emergency stop button.
• Machine Safety Category 3- High Risk requiring redundant safety features to prevent dangerous failures. This category includes basic industrial machinery.
Machine Safety Category 4- Serious Risk with the highest level of safety required. This category includes machines that have extensive safety measures such as redundancy, self-checking, and diagnostic capabilities. An example of an item at this level would be a complex automated system.

The REER Safety Guide lays out the ground rules for CAT, SIL and PL ratings, how to determine each and how they relate to one another. Find the guide in the Document section below. Flotronics Automation also has knowledgeable experts who can help you select safety components for your industrial equipment. Feel free to contact us to review your application.

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