What's STO doing in VFD?

In my experience at Vacon Drives, the STO card has a double contact on their to be sure that the power stage is disabled. Much like having two contacts on e-stop, but now two relays to disengage the power, just in case one gets stuck. The Safe Stop 1 (SS1) safety function consists of two safety related subsystems, an external time delayed safety relay and the STO safety function. These two subsystems combined compose the Safe Stop 1 safety function. Contrary to popular belief this is a programmable input in the manor that the action of the STO is programmable.

Safe Torque Off (STO)
Hardware based 'Safe Torque Off' safety function to prevent the drive from generating torque
on the motor shaft. STO safety function has been designed for use in accordance with the
following standards:
• EN 61800-5-2 Safe Torque Off (STO) SIL2
• EN ISO 13849-1: 2006 PL "d" Category 3
• EN 62061: 2005 SILCL2
• IEC 61508: 2000 SIL2
• The function also corresponds to an uncontrolled stop in accordance with stop category 0,
EN 60204-1: 2006.
• EN 954-1, Category 3
The STO safety function has been certified by IFA*

Safe Stop 1 (SS1)
SS1 safety function is realized in compliance with type C of the drives safety standard EN
* (Type C: "The PDS(SR) initiates the motor deceleration and initiates the STO function after an application specific time delay"). SS1 safety function has been designed for
use in accordance with the following standards:
• EN 61800-5-2 Safe Stop 1 (SS1) SIL2
• EN ISO 13849-1: 2006 PL"d" Category 3
• EN 62061: 2005 SILCL2
• IEC 61508: 2000 SIL2
• The function also corresponds to a controlled stop in accordance with stop category 1, EN
* 2006.

The SS1 safety function has been certified by IFA *