1.5 Symbols, Abbreviations and Conventions
Abbreviation Reference Description
Cat. EN ISO 13849-1 Category, level “B, 1-4”
DC Diagnostic Coverage
FIT Failure In Time: 1E-9/hour
HFT EN IEC 61508 Hardware Fault Tolerance: HFT=n means that n+1 faults could cause a loss of the safety
function
MTTFd EN ISO 13849-1 Mean Time To Failure - dangerous. Unit: years
PFH EN IEC 61508 Probability of Dangerous Failures per Hour. Consider this value if the safety device is
operated in high demand or continuous mode of operation, where the frequency of
demands for operation made on a safety-related system is greater than one per year.
PFD EN IEC 61508 Average probability of failure on demand, value used for low demand operation.
PL EN ISO 13849-1 Discrete level used to specify the ability of safety related parts of control systems to
perform a safety function under foreseeable conditions. Levels a-e.
SFF EN IEC 61508 Safe Failure Fraction [%]; percentage of safe failures and dangerous detected failures of
a safety function or a subsystem related to all failures.
SIL EN IEC 61508
EN IEC 62061
Safety Integrity Level
STO EN IEC 61800-5-2 Safe Torque Off
SS1 EN IEC 61800-5-2 Safe Stop 1
SRECS EN IEC 62061 Safety Related Electrical Control System
SRP/CS EN ISO 13849-1 Safety Related Parts of Control Systems
PDS/SR EN IEC 61800-5-2 Power Drive System (Safety-related)
Table 1.2 Abbreviations Related to Functional Safety
Conventions
Numbered lists indicate procedures.
Bullet lists indicate other information and description of illustrations.
Italicised text indicates
•
cross reference
•
link
•
parameter name
Introduction Operating Instructions
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