The Safe torque off function 361
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 (page 362). 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 358).
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 inverter 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 358).
Use only ABB approved spare parts.
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 inverter parameter 31.22. The indications can be read via fieldbus. The
indications are not safety-classified signals.
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 inverter 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.
See the inverter firmware manual for the indications generated by the inverter, and for
details on directing fault and warning indications to an output on the control unit for
external diagnostics.
Any failures of the Safe torque off function must be reported to ABB.
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