NGC-20 INSTALLATION MANUAL
19
4.6.1 safety integrity of the DigiTrace nGC-20 hardware
According to IEC 61508-2:2000, a distinction must be made between Type A systems and
type B systems. A sub-system can be viewed as a Type A system if, for the components
that are necessary in order to achieve the safety function:
1. The failure mode of all components that are used is adequately defined, and
2. The response of the sub-system in fault conditions can be completely determined,
3. Reliable failure data based on field experience are available for the sub-system,
to demonstrate that the assumed failure rates for recognized and unrecognized
hazardous failures can be achieved.
A sub-system can be viewed as a Type B system if, for the components that are necessary
in order to achieve the safety function:
1. The failure mode of at least one component that is used is not adequately defined, or
2. The response of the sub-system in fault conditions cannot be completely determined, or
3. No adequately reliable failure data based on field experience are available for the
sub-system, to support the assumed failure rates for recognized and unrecognized
hazardous failures.
The DigiTrace NGC-20-CL-E temperature control system with limiter corresponds to a
Type A system.
4.6.2 PFDavg safety function
The limiter sensor, limiter electronics and the limiter relay together form the safety
related system that performs a safety function. The “average probability of the hazardous
failure of a safety function for the entire safety-related system” (PFDavg) is usually
divided among the subsystems. An external device e.g. an external power contactor
installed in a panel, is specific to the installation, and shall, in accordance with the
standards for the safety loop, be considered separately.
Table 4: Safety Integrity level
MTTR = Mean time to repair
4.6.3 sIl related to sFF and hFT
The following table presents the achievable safety integrity level (SIL), depending on the
safe failure fraction (SFF) and the hardware failure tolerance (HFT) for Type A safety-
related sub-systems.
Table 5 is valid for the DigiTrace NGC-20-CL-E:
Table 5: Relation SFF to HFT
Safe failure fraction (SFF)
Hardware fault tolerance (HFT) for Type A
0
1
2
SFF < 60%
SIL 1
SIL 2
SIL 3
60 < SFF < 90 %
SIL 2
SIL 3
SIL 4
90% < SFF < 99%
SIL 3
SIL 4
SIL 4
99% < SFF
SIL 3
SIL 4
SIL 4
Type
SIL level
Architecture
Proof check
interval
MTTR (hrs)
PFD avg.
HTF
SFF
DigiTrace NGC-20-CL-E
SIL 2
1oo1D
1 year
24
3.017E-3
0 (1oo1)
95.03%
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