Glossary · Motion control and drives
Safe Stop 2 (SS2)
Also known as: SS2
German: Sicherer Stopp 2 (SS2)
In IEC 61800-5-2, Safe Stop 2 (SS2) is a drive-integrated safety function that initiates and monitors the deceleration of the motor and, once the motor has stopped, activates Safe Operating Stop (SOS), keeping the drive energized to hold the position.
- Motion control
- Functional safety
- Standards
In one sentence
Safe Stop 2 (SS2) brakes a motor under monitoring and then holds it at standstill with Safe Operating Stop, power still on.
Example
When a guard door opens during a format change, the axes stop with SS2 and remain in position-controlled standstill, so they don't need re-homing.
How it applies
- Engineering: SS2 corresponds to Stop category 2 of IEC 60204-1: controlled stop with power left available. It suits axes that must hold position, for example vertical axes or coupled axes.
- Commissioning: Deceleration monitoring (ramp or time), standstill tolerance and the fault reaction are parameterized and validated. Stopping time is measured for safety distance calculation.
- Operation: Because the drive stays energized during SOS, standstill is maintained by control; a fault triggers the fault reaction, usually STO.
- Documentation: State clearly that during SS2/SOS the motor is still energized. People may enter only where the risk assessment allows it; SS2 is not a replacement for Energy isolation during maintenance.
SS2 vs. SS1
Safe Stop 1 (SS1) brakes the motor and then switches off torque with STO. SS2 brakes the motor and then holds it in position with SOS.