Performance Level
It is the measure that expresses the risk reduction capability of a safety function in five steps from a to e; a indicates the lowest and e the highest capability. The required level is determined by risk assessment, while the achieved level is calculated from the circuit architecture and component data.
The scale is based on the probability of dangerous failure per hour; each step corresponds to a probability range. In practice two separate quantities are compared: the level required by the function and the level provided by the design. The second must be equal to or higher than the first.
- The three factors that determine the required level: the severity of injury, the frequency of exposure to the hazard, and the possibility of avoiding the hazard.
- The factors that determine the achieved level: the category, the channel lifetime, the diagnostic coverage, and the measures against common cause failure.
- The level is determined not by a single component but by the whole chain from sensor to logic to output.
For the emergency stop function the general minimum expectation is referred to as level c; on a radio remote controlled crane the common target for machine stopping is level d. Since the weakest link in the chain determines the result, you cannot reach the expected level by driving a high-level safety relay with an ordinary contactor.
The context in which this term is used: 433 MHz or 2.4 GHz?