242 Safe torque off function
Maintenance
After the operation of the circuit is validated at start-up, the STO function shall be
maintained by periodic proof testing. In high demand mode of operation, the
maximum proof test interval is 20 years. In low demand mode of operation, the
maximum proof test interval is 5 or 2 years; see section Safety data on page 244. It is
assumed that all dangerous failures of the STO circuit are detected by the proof test.
To perform the proof test, do the Acceptance test procedure (page 239).
Note: See also the Recommendation of Use CNB/M/11.050 (published by the
European co-ordination of Notified Bodies) concerning dual-channel safety-related
systems with electromechanical outputs:
• When the safety integrity requirement for the safety function is SIL 3 or PL e (cat.
3 or 4), the proof test for the function must be performed at least every month.
• When the safety integrity requirement for the safety function is SIL 2 (HFT = 1) or
PL d (cat. 3), the proof test for the function must be performed at least every 12
months.
The STO function does not contain any electromechanical components.
In addition to proof testing, it is a good practice to check the operation of the function
when other maintenance procedures are carried out on the machinery.
Include the Safe torque off operation test described above in the routine maintenance
program of the machinery that the drive runs.
If any wiring or component change is needed after start up, or the parameters are
restored, follow the test given in section Acceptance test procedure (page 239).
Use only spare parts approved by ABB.
Record all maintenance and proof test activities in the machine logbook.
Competence
The maintenance and proof test activities of the safety function must be carried out by
a competent person with adequate expertise and knowledge of the safety function as
well as functional safety, as required by IEC 61508-1 clause 6.
Fault tracing
The indications given during the normal operation of the Safe torque off function are
selected by drive parameter 31.22 STO indication run/stop (see the drive firmware
manual).
The diagnostics of the Safe torque off function cross-compare the status of the two
STO channels. In case the channels are not in the same state, a fault reaction
function is performed and the drive trips on an “STO hardware failure” fault. An
attempt to use the STO in a non-redundant manner, for example activating only one
channel, will trigger the same reaction.
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