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What are the safe stop methods in EN 61800-5-2?

The EN 61800-5-2 standard defines methods for safe stop, safe motion and safe braking in variable-speed drive systems. In this blog we’re going to take a brief look at the safe stop functions included in the standard.

Safe Torque Off (STO)

STO safely removes the power to an electric motor and makes further movement impossible. The variable-speed drive is placed in a no-torque state which averts accidental start-up. STO is most commonly used for conveyor applications.

Safe Stop 1 (SS1)

SS1 ensures that while the motor is not moving, the STO function is applied, corresponding with a controlled Category 1 stop in accordance with IEC 60204-1. This allows for prompt braking which is essential in many processes such as press applications and others with a large load inertia.

Safe Stop 2 (SS2)

This method differs from SS1 in that the motor is in closed-loop operation during standstill, meaning that its position is held precisely. This helps maintain synchronisation between axes and allow a process to be restarted. SS2 is normally used in applications where maintaining position synchronisation is vital.

For more information on the safety functions of ABB drives, call Alan Roberts on 07966 468430 or email

Variable-Speed Drives

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