Operation Principle - ABB ACS355 series User Manual

422 Appendix: Safe torque off (STO)

Operation principle

1. The Safe torque off activates (the activation switch is opened, or safety relay
contacts open).
2. The STO inputs IN1 and IN2 on the drive control board de-energize.
3. The STO cuts off the control voltage from the drive IGBTs.
4. The control program generates an indication as defined by parameter 3025 STO
OPERATION.
The parameter selects which indications are given when one or both Safe torque
off (STO) signals are switched off or lost. The indications also depend on whether
the drive 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. Motor coasts to a stop (if running). The drive cannot restart while the activation
switch or safety relay contacts are open. After the contacts close, a new start
command is required to start the drive.
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
You must store the signed acceptance test reports in the logbook of the machine. The
report must include documentation of start-up activities and test results, references to