Safe Torque Off vacon • 117
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9
9.5.1 Safety Capability Cat. 4 / PL e / SIL 3
For this safety capability, an external safety device must be installed. This must be used to dynam-
ically activate the STO inputs and to monitor the STO output feedback.
The STO inputs are dynamically used when they do not commute together (static use), but according
to the following picture (where the inputs are released with delay in turn). The dynamic use of the
STO inputs allows detecting faults that may otherwise accumulate.
Figure 92.
An emergency push button connected to the STO inputs does not assure the same
quality, because no fault detection is performed at a sufficient frequency (once a day
is recommended).
The external safety device, which forces the STO inputs and evaluates the STO output
feedback, has to be a safe device and it has to fulfil the requirements of the specific
application.
A simple switch cannot be used in this case!
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