Application manual for PSRclassic and PSRmultifunction, and the modular safety relay system
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102597_en_03
Figure 2-3
Safety-related block diagram (in accordance with EN 13849-1)
2.2.4
Determination of the achieved PL for each subsystem
A characteristic value when determining the performance level is the PFH
D
value, the sta-
tistical “average frequency of a dangerous failure per hour”. The safety characteristics can
be found in the DB EN SAFETY CHARACTERISTICS data sheet or the SISTEMA library.
The diagram below shows the basic relationship between PL and the safety characteristics
category, DC, and MTTF
D
.
Figure 2-4
Relationship between PL, category, DC, and MTTF
D
(in accordance with
EN 13849-1)
The category is an important parameter when determining the PL. The category term has
been taken from the previous standard EN 954-1. The requirements for the categories are
listed below.
L
I
O
I
m
I
m
a
b
c
d
e
Kat. B
Kat. 1
Kat. 2
Kat. 2
Kat. 3
Kat. 3
Kat. 4
MTTF
d
MTTF
d
MTTF
d
10
-5
10
-6
10
-7
10
-8
PFH
D
Dc
avg
Dc
avg
Dc
avg
Dc
avg
Dc
avg
Dc
avg
Dc
avg
None
None
Low
Medium
Low
Medium
High
Low
Medium
High
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