Advanced commissioning
7.9 Safe Torque Off (STO) safety function
Converter with control units CU250D-2
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Operating Instructions, 04/2015, FW V4.7.3, A5E34261542B AB
The STO safety function is standardized
The STO function is defined in IEC/EN 61800-5-2:
"[…] [The inverter] does not supply any energy to the motor which can generate a torque (or
for a linear motor, a force)."
The STO inverter function complies with what is defined in the standard.
The distinction between Emergency Off and Emergency Stop
"Emergency Off" and "Emergency Stop" are commands that minimize different risks in the
machine or plant.
The STO function is suitable for achieving an emergency stop but not an emergency off.
Risk:
Risk of electric shock:
Risk of unexpected motion:
Measure to minimize
risk:
Safe switch off
Switching off the electric power sup-
ply for the installation, either com-
pletely or partially.
Safely stop and safely prevent re-
starting
Stopping or preventing the dangerous
movement
Command:
Emergency Off
Emergency Stop
Classic solution:
Switch of the power supply:
Switch-of the drive power supply:
Solution with the STO
safety function inte-
grated in the drive:
STO is not suitable for safely switch-
ing of an electric voltage.
Select STO:
It is permissible that you switch of the
inverter supply voltage as well. How-
ever, switching off the voltage is not
required as a risk-reduction meas-
urement.
