This ensures at input voltage loss that no supply of the opto-couplers is possible. The supply of the opto-couplers, which are responsible for the commutation of the connected drive occurs via transformation coupling of the input voltage.
“The safety-related disconnection according to STO is reached by a two-channel opto-coupler blockage. The functional description for Safe Torque Off (STO) is: STO Function helps achieve safe operation of VFDs