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What is Safe Torque Off (STO)?
STO is a safety-related drive function intended to prevent torque generation. In the SEW-EURODRIVE implementation described in its official STO safety-concept documentation, activation interrupts pulse-width modulation (PWM) signals so they do not reach the insulated-gate bipolar transistors (IGBTs) in the output stage. The supply can remain present even though the drive can no longer produce torque through that output stage.
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SEW describes the distinction directly: “Instead of a galvanic isolation of the drive from the supply system by means of contactors or switches, the disconnection of the STO input described here safely prevents the activation of the power semiconductors in the output stage.” That is a description of this implementation, not a universal circuit design. ABB’s safety guide likewise characterizes STO as bringing a machine to a non-torque state and cautions that STO does not protect against electrical hazards.
Does STO stop a motor?
STO removes the drive’s ability to generate torque; it does not actively brake the motor. The motor generally coasts according to its inertia, load, friction and mechanics. SEW documentation maps its described STO behavior to stop category 0, an uncontrolled stop. Schneider Electric’s product documentation describes STO as supplying no power that can cause torque or force to the motor, but that wording should not be read as a promise that the shaft stops immediately.
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If a machine needs deceleration within a controlled time, has a load that could fall, or presents a hazard while the motor coasts, STO alone may not meet the application’s stopping need. The risk assessment may call for a different safety function, mechanical braking, or both. A brake must itself be selected and controlled appropriately for the safety function; the presence of a motor brake does not make STO an active braking function.
Does STO remove power or provide electrical isolation?
No. STO is a torque-inhibition function, not galvanic isolation and not an electrical safe-work disconnect. A drive may remain connected to its supply and retain hazardous voltage after STO is activated. Do not use the STO command as authorization to work on electrical equipment; use the isolation and verification procedures required for the equipment and task.
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The distinction is important when reviewing a safety design: the STO safety function addresses hazardous motion from torque generation, while electrical isolation addresses electrical exposure. ABB explicitly warns that STO does not protect against electrical hazards. Confirm the drive’s instructions for isolation and discharge before maintenance.
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Two-channel inputs and diagnostic behavior appear in the documented examples below, but they are not a universal wiring recipe. An architecture must match the exact drive’s safety inputs, voltage, channel arrangement, fault response, reset behavior and safety documentation.
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| Documented example | Input and internal approach | What the evidence establishes |
|---|---|---|
| SEW-EURODRIVE STO implementation | A two-pole 24 V signal from an external safety controller or relay connects to the drive’s safety-related STO inputs. The described internal dual-channel structure includes diagnostics. | The documentation says a channel fault or discrepancy locks PWM output and requires a reset. This is the stated behavior of that implementation, not a generic reset rule for other drives. |
| Texas Instruments TIDA-01599 reference design | A dual-channel 1oo2 architecture with hardware fault tolerance 1. When both STO inputs go active low, load switches cut primary- and secondary-side supplies to six isolated IGBT gate drivers. | The design guide says this prevents control and energization of the motor. TI reported that TÜV SÜD assessed this reference design as generally suitable for SIL 3 and PL e / Category 3. That assessment applies to TIDA-01599, not to every STO circuit or a complete machine. |
The TI description illustrates a different circuit-level approach from the SEW drive example. The table is not a performance ranking: the available descriptions do not establish that these are interchangeable products or equivalent system designs.
Does STO need a safety relay?
Sometimes an external safety relay or safety controller is part of the interface; whether it is needed depends on the selected drive and machine architecture. SEW documents a two-pole 24 V interface supplied by an external safety controller or relay. That example does not establish that a generic relay will work with another manufacturer’s drive.
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Before selecting an external device, verify the drive’s manual for the required input type and voltage, channel arrangement, diagnostic expectations, reset behavior and safety documentation. Do not infer compatibility from a relay’s channel count or nominal voltage alone.
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What is the difference between STO, SS1, SS2 and SLS?
These functions address different motion hazards. STO inhibits torque; it should not be used as a synonym for a controlled stop. The specific behavior and implementation are product-dependent.
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- STO: Prevents torque generation. The motor may coast rather than decelerate under drive control.
- SS1: Provides a controlled stopping phase before STO. ABB’s guide describes SS1 as stopping the motor safely and initiating STO below a specified speed or after a defined time. Schneider’s documentation describes SS1-r as monitored deceleration followed by STO once standstill is reached.
- SS2-r: Schneider’s documentation describes monitored deceleration followed by Safe Operating Stop (SOS). In SOS, the drive remains enabled to hold position rather than switching to STO.
- SLS: Safely Limited Speed monitors a speed limit; it is not itself the same as removing torque through STO.
Use the exact drive documentation to determine which of these functions is available and how it is implemented. A function name alone does not establish its timing, monitoring thresholds or safety performance for a particular product.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How should STO safety ratings be interpreted?
Safety figures belong to the product, reference design, variants and conditions named in the supporting documentation. A stated SIL, PL or category is not a general rating for all STO implementations, nor does one shared rating make two machine-level designs equivalent.
| Source and scope | Figures reported | Qualification |
|---|---|---|
| Texas Instruments, TIDA-01599 reference design; 2021 design guide | Generally suitable for SIL 3 and PL e / Category 3 | TI reports an assessment by TÜV SÜD for this reference design. Do not apply the rating to another circuit or to a complete machine without its own supporting safety case. |
| Danfoss, product families covered by its 2021 guide | SIL 2, SIL CL 2, PL d, Category 3; diagnostic coverage 90%; MTTFd 14,000 years; maximum input-to-output response time 20 ms, or 60 ms for specified variants | These are the guide’s stated figures for its covered products and variant exceptions, not universal STO characteristics. The guide also specifies that the STO signal must be SELV or PELV supplied. |
When comparing options, keep the claimed figures attached to the exact product or design and check the certificate and current manual. In particular, distinguish the drive’s component-level safety data from the safety performance required and established for the complete machine.
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Use the drive’s current manual and safety certificate alongside the machine-level risk assessment. Confirm each of these points rather than assuming that an STO label describes the full architecture:
- Torque-inhibition mechanism: What does activation disable in this drive, and what electrical energy or voltage can remain present?
- Input interface: Required voltage and signal type, channel count and arrangement, and any stated supply requirements such as SELV or PELV.
- Diagnostics and faults: How are channel discrepancies or other faults detected, what output state follows, and what reset or restart action is required?
- Stop behavior: Does the motor coast in the application, and is that acceptable for the load, stopping distance and hazard? If not, determine whether a controlled safety stop or additional braking measures are needed.
- Performance evidence: The exact product’s stated SIL, PL, category, diagnostic coverage, MTTFd and response time, including any variant-specific limits and applicable certificate.
- System boundary: Whether the evidence covers only the drive or the complete machine safety function, including the external controller, wiring, brake and reset logic.
These points are product- and application-specific. The vendor materials cited here do not establish clause-level requirements or the current edition and jurisdictional adoption of IEC 61800-5-2. Check the applicable standard and local requirements directly when determining compliance; do not infer them from a vendor summary.
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