2
01
9-
11
10
Functional Safety NCN3-F31K2(M)-N*-B*3-S
Planning
3.2
Assumptions
The following assumptions have been made during the FMEDA:
• Failure rate based on the Siemens standard SN 29500.
• Failure rates are constant, wear is not considered.
• External power supply failure rates are not included.
• The safety-related device is considered to be of type
A
device with a hardware fault
tolerance of
0
.
• The device will be used under average industrial ambient conditions comparable
to the classification "stationary mounted" according to MIL-HDBK-217F.
Alternatively, operating stress conditions typical of an industrial field environment similar
to IEC/EN 60654-1 Class C with an average temperature over a long period of time
of 40
º
C may be assumed. For a higher average temperature of 60
º
C, the failure rates
must be multiplied by a factor of 2.5 based on experience. A similar factor must be used
if frequent temperature fluctuations are expected.
SIL 2 Application
Only use the device in SIL 2 applications in combination with a control unit according
to EN 60947-5-6 (NAMUR).
• The device claims less than 25 % of the total failure budget for a SIL 2 safety loop.
• For a SIL 2 application operating in low demand mode the total PFD
avg
value
of the SIF (
S
afety
I
nstrumented
F
unction) should be smaller than 1 x 10
-2
,
hence the maximum allowable PFD
avg
value would then be 2.5 x 10
-3
.
• For a SIL 2 application operating in high demand mode the total PFH value
of the SIF should be smaller than 1 x 10
-6
per hour, hence the maximum allowable
PFH value would then be 2.5 x 10
-7
per hour.
• Since the safety loop has a hardware fault tolerance of
0
and it is a type
A
device,
the SFF must be > 60 % according to table 2 of IEC/EN 61508-2 for a SIL 2 (sub) system.
