General description of functional safety
2.2 Safety integrity level
Safety Manual
Operating Instructions, 06/2012, A5E03433511-05
11
The following table shows the achievable Safety Integrity Level (SIL) for the entire safety-
instrumented system for type B subsystems depending on the proportion of safe failures
(SFF) and the hardware fault tolerance (HFT). Type B subsystems include sensors with
positioners, actuators with complex components, e.g. microprocessors (see also IEC 61508,
Section 2).
Table 2- 2 Achievable Safety Integrity Level (type B subsystems)
HFT
SFF
0 1 (0) * 2 (1) *
< 60 % Not allowed SIL 1 SIL 2
60 ... 90 % SIL 1 SIL 2 SIL 3
90 ... 99 % SIL 2 SIL 3 SIL 4
> 99 % SIL 3 SIL 4 SIL 4
* As per IEC 61511-1, Section 11.4.4
Operational reliability
According to IEC 61511-1, Section 11.4.4, the hardware fault tolerance (HFT) can be
reduced by one (values in brackets) for transmitters and final controlling elements with
complex components if the following conditions are applicable to the device:
● The device is proven in operation.
● The user can configure only the process-related parameters, e.g. control range, signal
direction in case of a fault, limiting values, etc.
● The configuration level of the firmware is blocked against unauthorized operation.
● The function requires SIL of less than 4.
The device fullfils these conditions.
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