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3 Safety Function
MITSUBISHI CNC
3.3 STO (Safe Torque Off) Function
CAUTION
Device manufacturer accepts responsibility for all the risk assessments and related residual risks. The followings are the
residual risks relevant to the STO function. Mitsubishi Electric will not accept liability for any accidents such as damage or
injury caused by such residual risks.
1. STO function is a function to remove the energy of the motor electronically and not the function to shut off the input
power to the drive unit or the connection between the drive unit and the motor physically. Therefore, the risk for electric
shocks cannot be eliminated with the STO function. To prevent electric shocks, use the EMG function.
2. STO function is a function to disable the energy to the motor electronically. It does not guarantee the shutoff or the
procedure for the deceleration control of the motor.
Read carefully the manual of each safety-related device for the correct installation, wiring, and adjustment.
For all the safety-related relay, sensor, etc., use the one which satisfies the safety standards. TUV SUD has confirmed
that the safety-related parts by Mitsubishi Electric described in this manual satisfy EN ISO13849-1 Category 3, PL d.
3. Even though the STO function is enabled by the STO switch, voltage may still be residual at the servo motor for the delay
specific to the device.
4. Safety is not guaranteed until the installation or adjustment for the safety-related parts in the system has been
completed.
5. When replacing the drive unit, make sure that the new product is the same as the one before the replacement. After the
installation, be sure to confirm the performance of the safety function before operating the system.
6. Perform all the risk assessments and safety level certifications for the whole device and system. The use of a third-party
certifier such as TUV SUD is recommended as a final safety certification of the system.
7. To prevent the accumulation of failure, perform an appropriate safety confirmation check at regular intervals as required
by the safety standards. The safety confirmation check must be performed at least once a day regardless of the safety
level of the system.
8. When up-and-down short circuit is occurred to the power module of the drive unit, the servo motor shaft rotates for up to
0.5 revolution.
9. Be sure to supply the STO input signal (STO1, STO2) from one power supply. If the power is divided, STO shutoff state
may not be realized due to a malfunction of the STO function caused by a sneak current.
DANGER
Improper installation of a safety-related device or a system could cause a operation state without safety guaranteed and
may lead to a serious or fatal accident.
Preventive measure against the above danger
As described in IEC 61800-5-2, the STO (Safe Torque Off) function is a function not to supply a energy from a drive unit to
a motor and does not guarantee that a motor is not moved by an external force and other influence.
Take safety measure such as brake or counter balance additionally when the external force is acted on by the motor itself.