Last edit: 11/07/2023
The possibility of a Fault Masking reduces the values of the Diagnostic Coverage of the Safety Subsystem and therefore has an impact on the PL/PFHd of the safety function, according to EN ISO 13849-1.
ISO/TR24119 describes two possible methods to determine the DC value of the subsystem:
- 6.2 Simplified method for the determination of the maximum achievable DC
- 6.3 Regular method for the determination of the maximum achievable DC
With reference to the simplified method, the DC can be estimated directly form Table 1. Data to be considered are:
- N° of frequently (more then once per hour) used Movable Guards.
- N° of additional movable guards (used less then once per hour)
With there two information the table gives the DC of the safety subsystem.
Other info to be considered are:
- N° of operators
- Distances between the movable guards
- How easy is the access.
Hereafter is Table 1.
Example of calculation with the simplified method
The nearby figure shows a safeguarded working area. It is possible to enter the area by means of two interlocked guards. The frequency of opening has been reported for both of them.
Considering the frequency of access from Guard 1 (<1/h), the number of frequently used movable guards (thus, guards having frequency of access ≥ 1/h) is equal to zero. Since there is also Guard 2, there is an additional access to the previously considered one. Guard 2 is another "additional" access, because its frequency is lower than 1/h.
The data which have to be used in Table 1 are the following:
- number of frequently used movable guards (>1/h): 0
- number of additional movable guards: 2
Assuming that the number of operators and the distance or the recheability of the guards do not affect the result, the maximum DC level that can be obtained is "Medium"