358 The Safe torque off function
Operation principle
1. The Safe torque off activates (the activation switch is opened, or safety relay contacts
open).
2. The STO inputs of the inverter control unit de-energize.
3. The control unit cuts off the control voltage from the output IGBTs.
4. The control program generates an indication as defined by parameter 31.22 (refer to
the firmware manual of the inverter).
The parameter selects which indications are given when one or both STO signals are
switched off or lost. The indications also depend on whether the inverter is running or
stopped when this occurs.
Note: This parameter does not affect the operation of the STO function itself. The
STO function will operate regardless of the setting of this parameter: a running drive
will stop upon removal of one or both STO signals, and will not start until both STO
signals are restored and all faults reset.
Note: The loss of only one STO signal always generates a fault as it is interpreted as
a malfunction of STO hardware or wiring.
5. The motor coasts to a stop (if running). The inverter cannot restart while the activation
switch or safety relay contacts are open. After the contacts close, a reset may be
needed (depending on the setting of parameter 31.22). A new start command is
required to start the inverter.
Start-up including acceptance test
To ensure the safe operation of a safety function, validation is required. The final
assembler of the machine must validate the function by performing an acceptance test.
The acceptance test must be performed
• at initial start-up of the safety function
• after any changes related to the safety function (circuit boards, wiring, components,
settings, etc.)
• after any maintenance work related to the safety function.
Competence
The acceptance test 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. The test procedures and report must be documented
and signed by this person.
Acceptance test reports
Signed acceptance test reports must be stored in the logbook of the machine. The report
shall include documentation of start-up activities and test results, references to failure
reports and resolution of failures. Any new acceptance tests performed due to changes or
maintenance shall be logged into the logbook.
Acceptance test procedure
After wiring the Safe torque off function, validate its operation as follows.
Note: If the inverter is equipped with safety option +L536, +L537, +Q950, +Q951, +Q952,
+Q957, +Q963, +Q964, +Q978 or +Q979, do the procedure shown in the documentation
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