Performance Level (PL) and Required Performance Level (PLr)

Since 2006, the concept of machinery safety has required a two-tiered assessment that checks not only the structure of the safety control system but also the reliability of the safety control system.
As a result, the new criteria Performance Level (PL) and Required Performance Level (PLr) emerged. PL of a safety product must meet or exceed PLr of the equipment into which the safety product is to be integrated.

Evaluation of PL (Performance Level)

This is an index of ability of a safety control system which is determined for a machine designer. The index is called Performance Level (PL) and rated on a five-level scale from “a” to “e.”

Four parameters are used to evaluate the PL.

Category:
Structure of safety-related parts of control system

This classification is based on the architecture (structure) of the safety-related parts of a control system.
The requirements for the categories are basically the same as those in ISO 13849-1:1999, but the new elements I (Input device), L (Logic processing), and O (Output device) are used to more specifically indicate the basic architecture (structure) of each category.

Category B	Achievement of the functional objectives of safety-related parts of machine control systems Endurance of the anticipated stress (e.g. vibration, EMC)
Category 1	Category B + Use of well-tried and highly reliable components
Category 2	Category B + Safety functions are checked at appropriate intervals by the machine control systems
Category 3	Category B + A single fault does not impair the safety function, and single defects should be detected whenever possible
Category 4	Category B + A single fault is detected during or before the execution of the safety function, and when this is not possible, the accumulation of undetected defects must not impair the safety function

MTTFd (Mean time to dangerous failure)

This is the mean time until a system fails dangerously.
Each part has its own lifespan, but in reality it is calculated considering B10d (the number of operations required for 10% of a sample of the parts to fail dangerously in reliability engineering), which is determined by how frequently the part is used.

The calculation results can be classified into three types: Low, Medium, and High.

Mean time to dangerous failure (years)
Low: 3 ≦ MTTFd ＜ 10
Medium: 10 ≦ MTTFd ＜ 30
High: 30 ≦ MTTFd ≦ 100

DCavg (Average Diagnostic Coverage):
Reliability of a system including software

DCavg is an index that indicates the rate at which a safety-related system can detect dangerous failures using its self-diagnosis function.
It is calculated using the formula below:

(sum of detectable dangerous failure rates) ÷ (sum of all dangerous failure rate)

The calculation results can be classified into four types: None, Low, Medium, and High.

Average Diagnostic Coverage
None: DC ＜ 60%
Low: 60％≦ DC ＜ 90％
Medium: 90％≦ DC ＜ 99％
High: 99％≦ DC

CCF (Common cause failure):
A total score that quantifies design procedures and engineering methods that reduce common cause failures

CCF stands for common cause failure, which means multiple failures due to a single cause.
As a measure to prevent such systematic failures, a checklist is used to confirm the use of design procedures and engineering methods for reducing foreseeable common cause failures, and the level of CCF resistance is scored based on the total score of the check items.

Architectures of category 2 and above require a score of 65 or higher.

Performance Level of a safety product is determined with reference to category, DCavg, MTTFd, and CCF score.

PL of the safety products on this website have been determined based on the above.

Determination of PLr (Required Performance Level)

PLr (required performance level) is determined based on the following items, in the same way as risk assessment.

S: Severity of Injury
S1: Slight S2: Serious
F: Frequency and/or Exposure to Hazard
F1: Seldom-to-less-often and/or exposure time is short F2: Frequent-to-continuous and/or exposure time is long
P: Possibility of Avoiding Hazard or Limiting Harm
P1: Possible under specific conditions P2: Scarcely possible

PLr (required Performance Level) can be confirmed by using this chart. Please use the safety product with PL which is equal to or higher than PLr on the chart.

Performance Level (PL) and Required Performance Level (PLr)

Evaluation of PL (Performance Level)

Determination of PLr (Required Performance Level)

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