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Conditions for UL 508C
IMS-MN-LMDA42C_A.pdf
LMD CANopen
Conditions for UL 508C
The following conditions must be met if the product is used to comply with UL 508C.
Ambient temperature during operation:
• Surrounding air temperature [°C] 0 ... +50.
Pollution degree:
• Use in an environment with pollution degree 2.
Power supply:
• Use only power supply units that are approved for over-voltage category III.
• Wiring - Use only 60/75°C copper conductors.
Safety Integrity Level (SIL)
Probability of Failure/Hour (PFH)
at high or continuous demand
4
≥
10
-9
... <10
-8
3
≥
10
-8
... <10
-7
2
≥
10
-7
... <10
-6
1
≥
10
-6
... <10
-5
Hardware Fault Tolerance (HFT) and Safe Fail Fraction (SFF):
Depending on the SIL for the safety system, the IEC 61508 standard requires a specific
hardware fault tolerance HFT in connection with a specific proportion of safe failures SFF (safe
failure fraction). The hardware fault tolerance is the ability of a system to execute the required
safety function in spite of the presence of one or more hardware faults. The SFF of a system is
defined as the ratio of the rate of safe failures to the total failure rate of the system. According
to IEC 61508, the maximum achievable SIL of a system is partly determined by the hardware
fault tolerance HFT and the safe failure fraction SFF of the system.
SFF
HFT Type A Subsystem
HFT Type B Subsystem
0
1
2
0
1
2
< 60%
SIL1
SIL2
SIL3
—
SIL1
SIL2
60% ...<90%
SIL2
SIL3
SIL4
SIL1
SIL2
SIL3
90% ...<99%
SIL3
SIL4
SIL4
SIL2
SIL3
SIL4
≥
99%
SIL3
SIL4
SIL4
SIL3
SIL4
SIL4
Fault Avoidance Measures
Systematic errors in the specifications, in the hardware and the software, usage faults and
maintenance faults of the safety system must be avoided to the maximum degree possible.
To meet these requirements, IEC 61508 specifies a number of measures for fault avoidance
that must be implemented depending on the required SIL. These measures for fault avoidance
must cover the entire life cycle of the safety system, i.e. from design to decommissioning of the
system.
