DOC-0017-03-EN: AC15 Series - Hardware Installation Manual
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DOC-0017-03-EN-B 04.04.2023
8 Safe Torque Off (STO): SIL2, PLd
8.1 Overview
Introduction
The AC15 is an adjustable speed electrical power drive system that is suitable for safety related
applications (PDS(SR)).
The drive is used in typical applications such as pump controls, packaging machines, textile machines,
printing machines, or material forming machines.
Safe Torque Off (STO) is an important and widely used safety function, deployed to prevent the unexpected
start-up of motors.
STO functionality enables an operator to disable the torque at motor shafts or deactivate forces at linear
motors and actuators via digital inputs, before commencing work in a potentially hazardous area.
Application block diagram
STO
M
3
Safety
control
unit
CPU
STO
This section provides general information about Safe Torque Off (STO).
Two safety functions can be implemented with the inverter:
1. Safe Torque Off (STO)
2. Safe Stop 1 (SS1).
In order to meet all aspects of STO and SS1, an external safety control unit should be used.
To implement Safe Stop 1 (SS1), the external safety control unit causes the drive to decelerate to rest.
Once at rest, it invokes STO in the inverter. Please refer to EN61800-5-2:2017 para 4.2.3.3 for the formal
definitions.
It is the user’s responsibility to:
1. Risk assess the machine.
2. Design, implement and assess an appropriate solution for each application to meet all relevant
safety requirements.
In accordance with the machine standards 2006/42/EG, EN ISO 12100, EN ISO 13849-1 and
EN ISO 14121-1, it is the machine manufacturer who must project the safety system for the entire machine,
including all integrated components. This includes the electrical drives.
Note: STO is an electronic inhibit intended for use during normal operation of the machine, but It can also
be used in automatic, set-up and cleaning operation modes. However, it is not intended for use during
machine maintenance, repair, replacement or other similar activities. For these activities, recognised
electrical power isolation devices and lock-off procedures should be used.
The inverter STO function is a factory-fitted and factory-tested feature. See ‘Section 8.5: STO Safety
Warnings and Limitations’.
