ABB ACS380 Series, Hardware Manual

Page: 143 / 186

Safe torque off function 143

Cable types and lengths
• Double-shielded twisted-pair cable is recommended.
• Maximum cable length 100 m (328 ft) between activation switch (K) and drive
control unit.
Note: A short-circuit in the wiring between the switch and an STO terminal causes a
dangerous fault and therefore it is recommended to use a safety relay (including
wiring diagnostics), or a wiring method (shield grounding, channel separation) which
reduces or eliminates the risk caused by the short-circuit.
Note: The voltage at the S1 and S2 terminals of each drive must be at least 13 V DC
to be interpreted as “1”. Pulse tolerance of input channels is 1 ms.

Grounding of protective shields
• Ground the shield in the cabling between the activation switch and the control
board at the control board.
• Ground the shield in the cabling between two control boards at one control board
only.
Operation principle
1. The Safe torque off activates (the activation switch is opened, or safety relay
contacts open).
2. The STO inputs S1 and S2 on the drive control board de-energize.
3. The STO cuts off the control voltage from the output IGBTs.
4. The control program generates an indication as defined by parameter 31.22 STO
indication run/stop .
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. The 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
reset may be needed (depending on the setting of parameter
31.22). A new start
command is required to start the drive.