What do PL d and Category 3 mean in ISO 13849?
EN ISO 13849-1 classifies the safety-related parts of control systems with Performance Levels (PL) from a to e and Categories B, 1, 2, 3, 4. Category 3 requires that a single fault does not cause the loss of the safety function; PL d is a high level of risk reduction. For the radio-controlled stop function of a crane, the target commonly used in the sector is Category 3 / PL d.
The standard thinks along two axes. The Category describes the architecture, while PL describes the reliability that architecture achieves.
The categories:
- B and 1: single channel; safety rests on the correct selection and robustness of the components.
- 2: single channel but the function is tested periodically.
- 3: a single fault must not lead to the loss of the safety function; most faults are detected.
- 4: a single fault is detected and faults do not accumulate.
The PL scale runs from a to e and corresponds to the probability of dangerous failure per hour. The required PL for a safety function is found on the risk graph by assessing the severity of injury, the frequency of exposure to the hazard and the possibility of avoidance.
The practical implication for hoisting applications: for the emergency stop function the general minimum expectation is PL c, though the risk assessment may require higher; for the radio-controlled machine stop function Category 3 / PL d is frequently targeted in the sector. This means that a single-channel relay output will not suffice on its own and a second independent breaking path will be required.
This is a general explanation of the standard; a product's PL level is valid only to the extent it is declared in the manufacturer's validation documentation.
Full article: Wireless Crane Remote Control Selection Guide