Machine safety: Safe Torque Off (STO) function

by Mar 7, 2018Machine Safety

Machine safety: Safe Torque Off (STO) function


The “Safe Torque Off” function, better known as STO function, is a basic safety function that can be integrated in motor drives. It mainly prevents the drives from generating torque on the motor as well as the unintentional on unexpected starting, as long as the STO function is enabled.

This function has many advantages either for machine standard functions or the safety functions.

Electromechanical contactors

The STO function could be used to avoid the use of several separate components (electromechanical contactors) which has a positive effect, that will be observed in the effort that would be required to install it (wiring, etc.). Moreover, this function has a much shorter switching time than the electromechanical components used for conventional solutions (electronic vs electromechanical).

Drive protection

This function allows to stop the generation of the torque while keeping the power supply to the drives, which are not well protected in sudden power removal stations. It also makes it possible to avoid the required drives set up effort.

For the safety concerns, several standards treat the STO function such as:

1-IEC 60204-1: Safety of machinery – Electrical equipment of machines – Part 1: General requirements, in this standard, the STO is considered as one of the functions to prevent unexpected start-up.

2-the standard IEC 61800-5-2-Adjustable speed electrical power drive systems – Part 5-2: Safety requirements – Functional, classifies this function as a means to a category 0 stop as explained in the previous blog.

The appropriation of the STO function to a category 0 stop limits its use in cases where the residual motion after the deactivation of the torque (inertia) is stopped in a short time unless additional measures to control the movement are jointly used.

The STO function also gives more alternatives to control hazardous energy control methods when conventional methods (lockout) are not applicable. However, an analysis and evaluation of the safety system in accordance with ISO 13849-1 and 2 must be completed to confirm that the STO provides a level of safety equivalent to the other methods.

Safety functions vs standard functions

As already mentioned at the beginning of this blog, STOs are generally integrated into variable speed power drive systems, and when they are used for, both safety and standard functions, all their components and software must comply with safety requirements, unless it is proven that the implementation of security functions and standard functions are sufficiently independent.

Electrical safety

Finally, it should be noted that Safe Torque Off does not provide an electrical isolation. The motor must be electrically isolated by disconnecting the power supply and locally isolating the motor before accessing the motor power connections.

For more information about that, contact us!

Tell us about your needs and an expert will answer you!

Intervention Prévention Inc.

Intervention Prévention concentrates its operations in the field of work safety, offering specialized services following Standards CSA Z462 – Workplace Electrical Safety, CSA Z460 – Control of Hazardous Energy: Lock-out and Other Methods, and CSA Z432 – Safeguarding of Machinery.


137-2020 rue André-Labadie
Beloeil (Québec)
J3G 0W6