Appendix: Safe torque off (STO) 421
Activation switch
In the wiring diagram above (page 419), the activation switch has the designation (K).
This represents a component such as a manually operated switch, an emergency
stop push button switch, or the contacts of a safety relay or safety PLC.
• If a manually operated activation switch is used, the switch must be of a type that
can be locked out to the open position.
• Inputs IN1 and IN2 must open/close within 200 ms of each other.
Cable types and lengths
• Double-shielded twisted-pair cable is recommended.
• Maximum cable length 300 m (984 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 INx terminals of each drive must be at least 13 V DC to be
interpreted as “1”.
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.
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