Vega VEGAPULS 61, Safety Manual

Page: 6 / 16

I f the internal diagnosis system detects a failure , the measuring
system goes into fault mode .
A dangerous undetected failure exists if the measuring system
switches neither to the de fi ned safe condition nor to the failure
mode when the process requires it .
I f the measuring system delivers output currents of " fail low " or
" fail high "
,
it can be assumed that there is a malfunction
.
T he processing unit must therefore interpret such currents as a
malfunction and output a suitable fault signal .
I f this is not the case , the corresponding portions of the failure
rates must be assigned to the dangerous failures . T he stated
values in chapter "
S
afety - relevant characteristics " can thus
worsen .
T he processing unit must correspond to the SIL level of the
measurement chain .
I
f the demand rate is only once a year
,
then the measuring
system can be used as safety - relevant subsystem in " low
demand mode
"
( IEC
61508
-
4
,
3
.
5
.
12
).
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 as if it is executing a safety
function in the mode with low demand rate
( IEC
61508
-
2
,
7
.
4
.
3
.
2
.
5
).
A n associated characteristic 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 umber values see chapter " S afety - related characteristics " .
I f the " low demand rate " does not apply , the measuring system
as safety
-
relevant subsystem in " high demand mode " should
be used ( 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
- related measurement
chain .
A n associated characteristic is the value PFH ( failure rate ).
N umber values see chapter " S afety - related characteristics " .
C on fi guration of the process -
ing unit
L ow demand mode
H igh demand mode
6
VEGAPULS series
60
-
4
…
20
m A / HART
F unctional safety
31338
-
EN
-
070510