154 The Safe torque off function
STO abbreviations
Abbreviation Reference Description
Cat. EN ISO 13849-1 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.
CCF EN ISO 13849-1 Common cause failure (%)
DC EN ISO 13849-1 Diagnostic coverage
FIT IEC 61508 Failure in time: 1E-9 hours
HFT IEC 61508 Hardware fault tolerance
MTTF
d
EN ISO 13849-1 Mean time to dangerous failure: (The total number of life
units) / (the number of dangerous, undetected failures)
during a particular measurement interval under stated
conditions
PFD IEC 61508 Probability of failure on demand
PFH
D
IEC 61508 Probability of dangerous failures per hour
PL EN ISO 13849-1 Performance level. Levels a…e correspond to SIL
SC IEC 61508 Systematic capability
SFF IEC 61508 Safe failure fraction (%)
SIL IEC 61508 Safety integrity level (1…3)
SILCL IEC/EN 62061 Maximum SIL (level 1…3) that can be claimed for a safety
function or subsystem
SS1 IEC/EN 61800-5-2 Safe stop 1
STO IEC/EN 61800-5-2 Safe torque off
T1 IEC 61508 Proof test interval. T1 is a parameter used to define the
probabilistic failure rate (PFH or PFD) for the safety
function or subsystem. Performing a proof test at a
maximum interval of T1 is required to keep the SIL
capability valid. The same interval must be followed to
keep the PL capability (EN ISO 13849) valid. Note that
any T1 values given cannot be regarded as a guarantee
or warranty.
See also section STO function maintenance (page 151).
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