Sample application
ID 442181.07
20
Operating manual ASP 5001
6
WE KEEP THINGS MOVING
6.2
Calculations of the failure probability
The calculation for the STO and SS1 safety functions were carried out on the basis of the safety-related block
diagrams represented in Fig. 6-2 Safety-related block diagram that were derived from the principal circuit
diagram (see section 5.1).
Version 1.1.2 of the SISTEMA software assistant was used for a part of the calculations.
Fig. 6-2Safety-related block diagram
Switching can be divided into three subsystems. Here, the safety device with the position switches (B1/B2)
forms the SB 1 subsystem, the safety switching device (K1) forms the SB 2 subsystem and switchoff paths in
xDS 5000 (T1a/T1b) are combined in the SB 3 subsystem.
The K1 safety switching device is a finished product that meets the requirements of category 4 and PL e. The
failure probability is specified by the manufacturer as PFH = 2.31
10
-9
1/h.
For both other SB 1 and SB 3 subsystems, the failure probability (PFH) is calculated as follows.
In order to determine the failure probability for the overall safety function of the sensor to the actuator, the PFH
values of the individual subsystems are added at the end.
MTTF
d
•
SB 1: An exclusion of faults for the electrical contact is possible for the B1 position switch with positive
opening operation. For the electrical NO contact of B2, a B
10d
value of 2 000 000 cycles is assumed. This
also applies for the mechanical part of B1 and B2. For 365 workdays, 16 work hours and a cycle time of
5 minutes (300 s), n
op
= 70 080 cycles/year for these components.
MTTF
d
for the B1 position switch is 285 years and 143 years for B2.
Both values are reduced to 100 years in SISTEMA (high).
•
SB 3:
A MTTF
d
value of 100 years can be assumed for the subsystem using the table in section 3.3. for a cycle
time of 5 minutes.
B1
B2
K1
T1a
T1b
sub system
input
SB1
parts of
sub systems
parts of
sub systems
sub system
logic
SB2
sub system
output
SB3
