• The STO is a type A safety component as defined in IEC 61508-2.
• Relevant failure modes:
• The STO trips spuriously (safe failure)
• The STO does not activate when requested
• A fault exclusion on the failure mode “short circuit on printed circuit board”
has been made (EN 13849-2, table D.5). The analysis is based on an
assumption that one failure occurs at one time. No accumulated failures
have been analyzed.
• STO response times:
• STO reaction time (shortest detectable break): 1 ms
• STO response time: 5 ms (typical), 10 ms (maximum)
• Fault detection time: Channels in different states for longer than 200 ms
• Fault reaction time: Fault detection time + 10 ms.
• Indication delays:
• STO fault indication (parameter 31.22) delay: < 500 ms
• STO warning indication (parameter 31.22) delay: < 1000 ms.
￭
Terms and abbreviations
DescriptionReferenceTerm or
abbreviation
Classification of the safety-related parts of a control
system in respect of their resistance to faults and their
subsequent behavior in the fault condition, and which is
achieved by the structural arrangement of the parts, fault
detection and/or by their reliability. The categories are:
B, 1, 2, 3 and 4.
EN ISO 13849-1Cat.
Common cause failure (%)EN ISO 13849-1CCF
Diagnostic coverage (%)EN ISO 13849-1DC
Hardware fault toleranceIEC 61508HFT
Mean time to dangerous failure: (Total number of life
units) / (Number of dangerous, undetected failures)
during a particular measurement interval under stated
conditions
EN ISO 13849-1MTTF
D
Average probability of dangerous failure on demand,
that is, mean unavailability of a safety-related system to
perform the specified safety function when a demand
occurs
IEC 61508PFD
avg
178 The Safe torque off function
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