6 Safety Manual
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6.6.1 Safety integrity of the hardware
According to DIN EN 61508, a distinction must be made between systems of type A and systems of type B.
A subsystem can be considered to be type A if, for the components required to achieve the safety function,
- the failure behavior of all components used is sufficiently defined, and
- the behavior of the subsystem can be fully determined under error conditions; and
- reliable failure data from experience in the field exists for the subsystem to show that the assumed failure rates for detected
and undetected dangerous failures are achieved.
A subsystem can be considered to be type B if, for the components required to achieve the safety function,
- the failure behavior of at least one of the components used is not sufficiently defined, or
- the behavior of the subsystem cannot be fully determined under error conditions, or
- no sufficiently reliable failure data from experience in the field exists for the subsystem to support the utilized failure rates
for detected and undetected dangerous failures.
The type 707071 transmitter corresponds to a type B system.
The following table shows the achievable safety integrity level (SIL) depending on the safe failure fraction (SFF) and the hard-
ware fault tolerance (HFT) for safety-related type B subsystems.
For 707071 the following table applies:
Safe failure fraction (SFF) Hardware fault tolerance (HFT) for type B
012
<60 % Not allowed SIL 1 SIL 2
60to<90% SIL 1SIL 2SIL 3
90to<99% SIL 2SIL 3SIL 4
≥ 99% SIL 3SIL 4SIL 4
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