2 Function description
BU 0630 en-2020 15
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2.3.2 Safe stop 1 with time control, SS1-t
SK 500P series frequency inverters support implementation of the SS1 function in the variant SS1-t
(safe stop 1 with time control).
To stop the motor, the frequency inverter triggers a quick stop. After a time which is specific to the
application the STO function can be switched on. This behaviour corresponds to stop category 1
(controlled stopping) according to IEC 60204-1. The changeover to the STO function is time-controlled
via an external, fail-safe time relay (delayed output of an external safety device).
Controlled stopping
Controlled stopping is triggered via the Safety Digital Input and meets lower safety requirements than
the STO function, which is triggered via the Safe Pulse Block.
Controlled stopping with SS1-t can fail without detection and must not be used if this failure can cause
a hazardous situation in the application.
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2.3.3 Priorities and response to faults
As the Safe Pulse Block is based on switching off the supply voltage to the drivers of the inverter’s
semiconductor switches (T1 to T6), this shut-down method has the highest priority. I.e. if the safety
function STO is implemented with the Safe Pulse Block, the safety function STO also has the highest
priority. If the safety functions STO and SS1-t are implemented in parallel, it must be noted that
controlled stopping of the motor via the Safety Digital Input is interrupted by triggering the Safe Pulse
Block.
The Safety Digital Input has an internal fault diagnosis. If this diagnosis detects a fault, the control
signals to the drivers of the semiconductor switches (T1 to T6) of the inverter are switched off. This
fault response corresponds to the behaviour of the safety function STO with the special feature that
this fault state can only be acknowledged by resetting the mains voltage of the frequency inverter.
Acknowledgement via a bus system or the digital inputs is not possible.
NOTE: The SS1-t function must not be used if a hazardous situation can result in the application due
to the fault response.
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2.4 Examples / Implementation
The following shows some examples of solutions for the safety functions STO and SS1-t.
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2.4.1 STO function
Implementation of a safety function usually requires the use of a protective switching device. The
category of the function is determined by the component with the lowest category.
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