VFD Safety Explained: STO, Safe Torque Off, and Functional Safety Basics
VFD safety is often misunderstood. Many installers assume that stopping a motor via a drive command is “safe” — but in many applications, this is not enough.
Modern inverter drives include safety functions such as STO (Safe Torque Off), but knowing when and how to use them correctly is critical for both safety and legal compliance.
This guide explains VFD safety functions, how STO works, when it is required, and how it fits into functional safety and machinery regulations.
Quick Links
- What VFD safety really means
- What is STO (Safe Torque Off)?
- STO vs normal stop commands
- When STO is required
- Functional safety (SIL & PL) explained
- How STO is implemented
- Common VFD safety mistakes
- Relevant categories & products
- FAQ
What VFD Safety Really Means
Stopping a motor is not the same as making it safe.
A standard VFD stop command:
- Relies on software
- Does not remove torque-producing energy
- Can fail if the drive electronics malfunction
For many machines, regulations require a hardware-based safety function that prevents torque from being generated — regardless of software state.
This is especially relevant when VFDs are installed in machinery, as discussed in: VFD Compliance Explained
What Is STO (Safe Torque Off)?
STO is a safety function that removes power from the motor’s torque-producing circuits.
When STO is activated:
- The motor cannot generate torque
- The drive remains powered
- No restart is possible until STO is reset
STO is designed to meet functional safety standards and is implemented in hardware, not software.
STO vs Normal Stop Commands
| Feature | Normal VFD Stop | STO |
|---|---|---|
| Torque Removed | No (controlled) | Yes (guaranteed) |
| Software Dependent | Yes | No |
| Meets Safety Standards | No | Yes |
STO does not provide controlled stopping — it prevents torque generation. Braking and stopping behaviour must be handled separately.
When Is STO Required?
STO is commonly required when:
- Personnel can access moving machinery
- Emergency stop functions are required
- Maintenance is performed with power present
- Machinery must meet safety standards
STO is not optional in many industrial applications — it is a regulatory requirement.
Functional Safety: SIL & PL Explained
Functional safety defines how reliably a safety function performs.
- SIL (Safety Integrity Level): IEC-based safety rating
- PL (Performance Level): ISO-based safety rating
Most modern VFD STO functions are rated to:
- SIL 2 or SIL 3
- PL d or PL e
The required level depends on a risk assessment — not personal preference.
How STO Is Implemented in Practice
STO is usually implemented using:
- Dual-channel safety inputs on the VFD
- Safety relays or safety PLCs
- Emergency stop circuits
Incorrect wiring or bypassing safety channels invalidates the safety function.
Related safety components: Industrial Safety Equipment
Common VFD Safety Mistakes
- Using a normal stop instead of STO for safety
- Assuming STO provides braking
- Single-channel wiring of dual-channel STO
- No risk assessment documentation
These mistakes often appear alongside installation errors: VFD Installation Mistakes That Cause Drive Failures
Recommended Categories for VFD Safety
Example Drives with STO
- ABB ACS355 – built-in STO: ABB ACS355-03E-04A1-4
- Allen Bradley PowerFlex 525 – integrated safety inputs: Allen Bradley 25B-D017N114
FAQ
Does STO replace an isolator?
No. STO prevents torque but does not isolate electrical power.
Does STO stop the motor instantly?
No. STO removes torque; stopping depends on inertia and braking.
Is STO required on all VFD installations?
No — only where risk assessment determines it is necessary.
Need Help With VFD Safety?
If you’re unsure whether STO or additional safety functions are required for your application, we can help review your setup.
Contact us: https://driveoutletmegastore.com/contact-us/
