How to Reduce Risks
The international standard ISO13849-1, which includes general principles for safety-related parts in control systems, was revised in 2006.
This was done because safety equipment parts and control methods have changed with the times, and conventional standards have not fully considered the safety of parts given their reliability and quality.
In the revised ISO 13849-1:2006, equipment is now classified by Performance Level (PL), which considers factors such as reliability, instead of classifying by category, which only determines the safety of machines via the original structure.
The parts of a machine that execute safety functions are known as the safety-related parts of the control system, and the levels that exist to assess the performance capability of the hardware and software related to these parts based on the size of the risk at hand, which is a necessary assessment, are called the Performance Levels (PL).
Required Performance Level (PLr)
To determine the performance level of safety-related control systems, one must do a risk assessment to determine the performance level required of the control system. This required performance level is called PLr and is classified and evaluated in 5 stages (from a to e).
Four Factors that Determine Performance Level
The PLr (a to e) determined by the previous evaluation is the minimum "performance level" required for the safety-related control system, and that performance level is comprised of the following (XNUMX) Category, (XNUMX) MTTFd, (XNUMX) DCavg, and (XNUMX) CCF. The total score of all of these must exceed the required performance level (PLr).
1. Category
The structure of the safety-related parts of the control system (the parts that perform the safety functions of a machine). The requirements for these are the same as in ISO13849-1: 1999, but the new elements I (input device), L (logical processor), and O (output device), allow the structure of the categories to be shown more concretely.
2.MTTFd (Mean Time To Dangerous Failure)
The average amount of time until a dangerous failure, classified into High, Medium, and Low.
3. Average of DC (Diagnostic Coverage)
The average diagnostic range, calculated by the total of the control system’s dangerous failure rate divided by the overall dangerous failure rate.
4.CCF (Common Cause Failure)
The total number of design procedures and engineering methods that are designed to reduce common cause failures.