I
f the ratio of the internal diagnostics test rate of the measuring
system to the demand rate exceeds the value
100
,
the
measuring system can be treated in the way it is executing a
safety function in the mode with low demand rate
(
IEC
61508
-
2
,
7
.
4
.
3
.
2
.
5
).
C
orresponding characteristics is the value
PFD
avg
(
average
P
robability of dangerous
F
ailure on
D
emand
).
I
t is dependent
on the test interval
T
P
roof
between the function tests of the
protective function
.
N
umbers see paragraph
"
S
afety
-
technical characteristics
"
.
I
f the
"
low demand mode
"
does not apply
,
the measuring
system must be used as safety
-
relevant subsystem in
"
high
demand
"
(
IEC
61508
-
4
,
3
.
5
.
12
).
T
he fault tolerance time of the complete system must be higher
than the sum of the reaction times or the diagnostics test
periods of all components in the safety chain
.
C
orresponding characteristics is the value
PFH
(
failure rate
).
N
umbers see paragraph
"
S
afety
-
technical characteristics
"
.
T
he safe condition of the measuring system is the switched o
ff
status
(
quiescent current principle
):
l
R
elay output
–
relay deenergised
l
T
ransistor output
–
transistor blocks
A
fail
-
safe failure
(
safe failure
)
exists if the measuring system
changes to the de
fi
ned safe condition without demand of the
process
.
I
f the signal conditioning instrument detects currents
<
3
.
6
m
A
or
>
21
.
6
m
A
,
the signal conditioning instrument takes on the
safe condition
.
A
dangerous undetected failure exists if the measuring system
does not go to the de
fi
ned safe condition when required by the
process
.
T
he processing unit must evaluate the output circuit of the
measuring system by taking the quiescent current principle
into account
.
T
he processing unit must correspond to the
SIL
level of the
measuring chain
.
H
igh demand mode
S
afe condition and fault
description
C
on
fi
guration of the proces
-
sing unit
SU
501
E
x
-
S
ignal conditioning instrument
27
F
unctional safety
27953
-
EN
-
050616
