Safely switching off frequency inverters — Safe Torque Off (IEC 61800-5-2)

The emergency stop categories guide describes stop categories 0, 1, and 2 from NEN-EN-IEC 60204-1. For a motor controlled via a frequency inverter (VFD), this concept runs through a separate product standard: IEC 61800-5-2, with the function Safe Torque Off (STO).

What STO does — and what it does not

When STO is activated, the frequency inverter no longer supplies energy to the motor to generate torque. Functionally this corresponds to an uncontrolled stop, stop category 0 according to NEN-EN 60204-1: the motor coasts to a stop freely, without active braking via the inverter.

STO is not the same as switching off the supply voltage of the inverter itself: the power electronics of the inverter can remain energized, only the control signal to the motor is interrupted. For work on the motor or drive train, STO may be sufficient; for work on the inverter itself, a separate, visible isolation of the supply is required (see also the LOTO guide).

Relationship between the standards

IEC 61800-5-2 defines three stop functions that correspond to the three stop categories 0, 1, and 2 from NEN-EN 60204-1 and ISO 13849: Safe Torque Off (STO) corresponds to stop category 0, Safe Stop 1 (SS1) to stop category 1, and Safe Stop 2 (SS2) to stop category 2. IEC 61800-5-2 is thus a product standard for electrical drive systems with integrated safety functions, translating the generic machine safety requirements from 60204-1 into the specific situation of a frequency inverter.

Note: which stop function (STO, SS1, or SS2) is required for a specific application follows from the machine's risk assessment in accordance with machine safety 60204 — this article provides the standards framework, not a universal application instruction.

Practical relevance

For pumps, fans, and other drives controlled by frequency inverters in a horticultural business or company hall, it is important for a technician to know whether an activated STO function is sufficient for the planned work, or whether a complete, visible isolation of the inverter itself is required — an incorrect assumption here can result in power electronics that are still energized being treated as "safe".

Common mistakes

1. Confusing STO with a complete isolation of the supply — the power electronics of the inverter can still be energized after STO activation.
2. Allowing work on the inverter itself based on STO alone, without visibly and verifiably interrupting the inverter's supply (see the LOTO guide).
3. Not distinguishing between STO, SS1, and SS2 when assessing which stop function is required for a specific task — this follows from the machine's risk assessment, not from a default choice.

Related

• Emergency stop categories (60204)
• Machine safety (60204)
• LOTO — 5 steps