Industrial Engineer · phase 14 of 16

STO

Safe Torque Off, a certified drive safety function that prevents torque-producing energy without serving as electrical isolation.

The Explain card

Plain English
Safe torque off is a certified safety input on the drive that removes the gate signals from the IGBTs. With STO active the drive cannot make torque, whatever the control board is told.
Example
The emergency stop circuit opens the two STO inputs on the drive. The motor coasts, the drive reports it, and no contactor was needed to guarantee the motor could not be driven.
Why it matters
STO lets a drive be part of a safety circuit without a contactor in the motor leads, which is cheaper and more reliable. It is safe torque, and nothing more, so the motor terminals can still be live and a hoist still needs a brake.
Hook
The drive is disarmed, and the wires are still hot.

Word knowledge

How the word is built, where it came from, and what it sits beside in memory.

In a sentence

During commissioning, the technician opens both safety relay channels and verifies that the drive reports STO before entering the guarded conveyor area.

Letter by letter
  • S Safe marks the term as a certified safety-function label
  • T Torque names the controlled mechanical quantity
  • O Off indicates the function's commanded state
Where it came from
Origin
English initialism formed from Safe Torque Off
Entered the language
2000s
What changed
became the standardized drive-safety label in IEC 61800-5-2, replacing earlier terminology such as safe standstill
How it is spelled
Capitalisation
written STO in all capitals, including mid-sentence
Plural and possessive
plural STOs; singular possessive STO's; plural possessive STOs'
Out loud
Spelled out letter by letter.

Same family

  • SLS
  • SOS
  • SBC
  • SS1
Broken into chunks
  • S
    • SLS
    • SOS
    • SBC
  • T
    • torque control
    • torque limit
  • O
    • switch off
    • power off
What it sits beside

Same subject

Same shape

IEC 61800-5-2 drive safety functions

  • STO
  • SLS
  • SOS
  • SBC

safety stop functions

  • STO
  • SS1
  • SS2

Where it sits in the deck

Phase 14: VFD Safety, Faults, and Diagnostics

What a drive does when it trips, what the fault codes are telling you, and how safe torque off stops the motor without killing the panel.

Neighbours in this phase