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VACON® NXP Advanced Safety Options
Operating Guide
Emergency
stop switch
(SIL3,
PLe)
Channel 1
Channel 2
Illustration 5: A Logical Presentation of the STO Safety Function
In this example case, the STO function has one activation per day, and a lifetime of 20 years. For the emergency stop switch, β = 10%
is used as the susceptibility to common cause failure between the channels. No proof test is executed during the lifetime. The exam-
ple system is limited to Category 3 because the Category 3 element OPTAF option board is used as a single final element.
Table 3: An Example of System Level Calculations for the STO Safety Function
Subsystem
Emergency stop switch
OPTBL
OPTAF
Overall safety system (for STO)
This value is calculated directly from the values provided by the manufacturer. The diagnostic capabilities of OPTBL have not been taken into ac-
1
count. The calculation formula: PFH
The calculation formula: PFD
2
avg
The OPTBL executes "Cross monitoring of inputs without dynamic test", DC: 0%...99%, depending on how often a signal change is done by the
3
application. A DC of 90% is assumed with the once a day activation.
The calculation formula: MTTF
4
d
OPTAF manual: DC
= low, using the lower end of the possible range (60%...90%)
5
avg
Sum of the individual PFH
values.
6
d
Sum of the individual PFH
values.
7
avg
The calculation formula:
8
DC Switch
+
MTTF d Switch
MTTF d OPTBL
DC avg STO =
1
+
MTTF d Switch
MTTF d OPTBL
According to EN ISO 13849-1, the MTTF
9
MTTF d STO =
1
+
MTTF d Switch
MTTF d OPTBL
When designing systems according to IEC-61508, the requirement for the value of the Safe Failure Fraction (SFF) is considered on
subsystem level, not on system level.
26 | Danfoss A/S © 2021.06
Stop (STO) request
Stop (STO) request
SIL, PL
SIL3, PLe
SIL3, PLe
SIL3, PLe
SIL3, PLe
2
= (1- β)
x λ
x λ
x T
+ β x (λ
d
ch1
ch2
1
= (PFH
x T
)/2.
d
M
= B10
/ (0.1 x cycles per year).
d
DC OPTBL
DC OPTAF
+
MTTF d OPTAF
1
1
+
MTTF d OPTAF
must be limited to a maximum limit of 100 years per channel. The calculation formula:
d
1
1
1
+
MTTF d OPTAF
N O T I C E
OPTBL
(SIL3, PLe,
Cat 4)
STO
STO
PFH
PFD
d
avg
-9
-4
4.2 x 10
3.7 x 10
(1)
-11
-6
6.45 x 10
5.61 x 10
-9
-4
2.7 x 10
1.3 x 10
-9
-4
6.94 x 10
5.06 x 10
(6)
+ λ
)/2, where λ
= (0.1 x cycles per hour) / B10
ch1
ch2
ch
Overview of the System
OPTAF &
AC drive
(SIL3, PLe,
Cat 3)
DC
[%]
MTTF
avg
90
2849.3
(2)
(3)
99
373
60
1918
(5)
74
281
(7)
(8)
).
d
AQ319736045637en-000101 / DPD01798
[a]
d
(4)
(4)
(9)
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