8.2
Probability of failure of the safety functions (PFH value)
The specified probability of failure values apply under the following preconditions:
● The mission time of the inverter has not expired.
● The test interval has not expired.
Mission time
You may not operate inverters with integrated safety functions for longer than 20 years. The
20 years starts when the device is delivered. The mission time cannot be extended. This is
the case even if a service department checks the inverter – or in the meantime, the inverter
was decommissioned.
Test interval
You must test the safety-related circuits of the inverter at least once every year. This process
is called "forced checking procedure" or "test stop".
Regularly testing the safety functions (Page 214).
Probability of failure
Table 8-1
Probability of failure of the safety functions
Safety function
PFH, PFH
D
1)
PFD
2)
STO, SBC
3)
, SS1, SLS, SDI, SSM
5×10
-8
1/h
5×10
-3
1)
PFH, PFH
D
: Probability of failures per hour.
PFH according to IEC 61800‑5‑2, PFH
D
according to IEC 62061.
2)
PFD: For a low demand rate of the safety function: Mean probability of dangerous failure when the
safety function is demanded according to IEC 61508.
3)
Including Safe Brake Relay
The machine manufacturer must specify a probability of failure in the form of a PFH value for
every safety function of a machine.
The integrated inverter safety functions are only ever part of a complete machine safety
function. A complete safety function comprises the following components, for example:
● A dual-channel sensor for detecting an open protective door.
● A central fail-safe controller for processing the sensor signal further.
● An inverter for safely stopping (SS1) a motor if a protective door is opened.
IEC 62061 explains how to calculate the PFH
D
value for the complete safety function from the
PFH
D
values of the components used for the safety function.
You can find a free-of-charge tool in the Internet to calculate the PFH
D
value of a complete
machine:
Safety Evaluation Tool (
www.siemens.com/safety-evaluation-tool
).
Application examples for the probability of failure PFH and PFD are available in the Internet:
System properties
8.2 Probability of failure of the safety functions (PFH value)
Safety Integrated - SINAMICS G110M, G120, G120C, G120D and SIMATIC ET 200pro FC-2
368
Function Manual, 01/2017, FW V4.7 SP6, A5E34261271B AD