SAFETY MANUAL SIL KFD0-RSH-1.4S.PS2
Planning
22
5538
201
1-
04
7
2.2
Assumptions
The following assumptions have been made during the FMEDA analysis:
■
Failure rates are constant, wear out mechanisms are not included.
■
The stress levels are average for an industrial environment and can be
compared to the Ground Fixed Classification of MIL-HNBK-217F.
Alternatively, the assumed environment is similar to:
• IEC 60654-1 Class C (sheltered location) with temperature limits within
the manufacturer's rating and an average temperature over a long period
of time of 40
º
C. Humidity levels are assumed within manufacturer's
rating. For a higher average temperature of 60
º
C, the failure rates should
be multiplied with an experience based factor of 2.5. A similar multiplier
should be used if frequent temperature fluctuation must be assumed.
■
Failure rate based on the Siemens SN29500 data base.
■
It was assumed that the appearance of a safe error (e. g. output in safe state)
would be repaired within 8 hours.
■
During the absence of the device for repairing, measures have to be taken to
ensure the safety function (for example: substitution by an equivalent device).
■
For high currents and high ambient temperature the de-rating given in the data
sheet needs to be considered.
■
The input of the device must be connected to a safety PLC which has
minimum the SIL needed in the loop.
■
The device shall claim less than 10 % of the total failure budget for a
SIL3 safety loop.
■
For a SIL3 application operating in Low Demand Mode the total PFD
avg
value
of the SIF (
S
afety
I
nstrumented
F
unction) should be smaller than 10
-3
, hence
the maximum allowable PFD
avg
value would then be 10
-4
.
■
For a SIL3 application operating in High Demand Mode of operation the total
PFH value of the SIF should be smaller than 10
-7
per hour, hence the
maximum allowable PFH value would then be 10
-8
per hour.
■
Since the circuit has a Hardware Fault Tolerance of
0
and it is a type
A
component, the SFF must be > 90 % according to table 2 of IEC 61508-2 for
SIL3 (sub)system.
