JUMO 14597 safetyM STB/STW Ex - Safety Temperature Limiter and Safety Temperature Monitor Operating Manual - Manual (Page 50)

6 Safety Manual
2013-04-01 44
6.9.1 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 failure 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 failure 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 JUMO safetyM STB/STW Ex corresponds to a type B system.
The following table shows the achievable Safety Integrity Level (SIL) in dependency on the safe failure fraction (SFF) and the
hardware fault tolerance (HFT) for safety-related type B subsystems.
Table 8: For JUMO safetyM STB/STW Ex
Safe failure fraction (SFF) Hardware fault tolerance (HFT) for type B
0 1 2
< 60 % Not allowed SIL1 SIL2
60 to < 90 % SIL1 SIL2 SIL3
90 to < 99 % SIL2 SIL3 SIL4
≥ 99 % SIL3 SIL4 SIL4