Pilz PSS SB HYD, Basic license - Manual (Page 29)

3-5 Modular Block System: Hydraulic Presses
Fault detection techniques
Various options are available to the user to help detect and rectify errors in
the process periphery units:
• Errors can be detected via the feasibility checks in the application pro-
gram, e.g. by using redundant input devices.
• Errors can be detected by pulsing the signal lines using the test pulses
from the PSS DI2O T module.
• Errors can be detected by using the automatic tests in the output
modules.
• Avoid potential errors by using appropriate wiring.
Additional information on how to exclude potential errors can be found in
the directive VDI 2854 (safety requirements on automated manufacturing
systems).
Feasibility test
Redundant input devices for safety functions must undergo a feasibility
test within the application program.
Example:
A two-hand button has a N/C and a N/O contact. Once the application
program has detected the open N/C contact, it must check that the N/O
contact closes within an appropriate period of time. The function test
must be carried out for all possible combinations of the two contacts.
The plant must be stopped immediately if a feasibility error occurs. Similar
tests will also be required for other input devices.
If safety valves have feedback contacts, these must be evaluated
accordingly.