User Documentation

Notes on configuring
1373930000/02/06-2017 9
3. Notes on configuring
3.1 Low demand mode of
operation
The SIL3 relays from the SAFESERIES are used in
low demand mode, when their demand frequency is
no more than five times per year and no more than
double the repeated testing frequency (refer to
DIN EN 61508-4, 3.5.12).
The corresponding parameter is the value
PFD
avg
= 3.07 ⋅ 10
-6
, which is valid for a testing
interval T
proof
of 12 years.
3.2 High demand mode of
operation
If the “low demand mode of operation” usage cannot
be applied, then the SIL3 relay should be used as a
safety-critical sub-system operating at high demand
mode or continuous mode (DIN EN 61508-4,
3.5.12). The following values are valid for the
frequency of demand and the corresponding PFH
value:
Once per month PFH = 6,83 ⋅ 10
-11
h
-1
Once per week PFH = 1,83 ⋅ 10
-10
h
-1
Once per day PFH = 1,08 ⋅ 10
-9
h
-1
3.3 Types of malfunctions
A safe failure is not able to render a technical safety
system dangerous or non-functional. The SIL3 relay
passes to a predefined safe state.
A dangerous, undetected failure has the potential to
render a technical safety system dangerous or non-
functional. The SIL3 relay does not pass to a
predefined safe state.
3.4 Test intervals
The test interval is the time between complete
repeated tests.
Incidental hardware errors can be detected within
this time period (T
proof
= 12 years).