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Functional Safety HiC283*
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 ºC may be assumed. For a higher average temperature of 60 ºC, 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.
•
Do not use more than one input channel of a multi-channel device for a safety loop,
as the channels of the device use common components. Observe that a multi-channel
device contains components that affect the behavior of all channels.
SIL 2 Application
•
To build a SIL safety loop for the defined SIL, it is assumed as an example that this device
uses 10 % of the available budget for PFD
•
For a SIL 2 application operating in low demand mode the total PFD
of the SIF (Safety Instrumented Function) should be smaller than 10
hence the maximum allowable PFD
•
For a SIL 2 application operating in high demand mode the total PFH value
of the SIF should be smaller than 10
would then be 10
•
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.
avg
value would then be 10
avg
-6
per hour, hence the maximum allowable PFH value
-7
per hour.
/PFH.
avg
-2
,
-3
.
value
11
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