Safety Functions; Safe Torque Off, Sto; Safe Stop 1 Time-Controlled, Ss1-T; Priorities And Response To Faults - Nord Drivesystems BU 0830 Functional Safety Manual

Functional Safety – Supplemental manual for series SK 300P

2.2 Safety functions

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WARNING
Mechanical brake failure
Control of a mechanical brake by means of the frequency inverter is not fail-safe. Triggering of the
STO and the fault response function causes the application of a mechanical brake which is controlled
by the frequency inverter. The brake takes the entire load of the drive units and all of its rotating
masses and attempts to stop it.
A brake which is not designed to take the full load (e.g. a holding brake) may be damaged and may
fail. This can cause severe or fatal injuries or damage to the equipment, e.g. due to falling loads.
• Design the brake as an operating brake
• Ensure that the drive unit is stopped before the STO function is activated.
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2.2.1 Safe Torque Off, STO

The STO function prevents the supply of motive power to the motor. According to ISO 1411, this
function can be used to prevent unexpected starting and/or switch off the driving torque as quickly as
possible (see technical data  response time) and allow the drive run down to a standstill (motor with
work machine).
This behaviour corresponds to stop category 0 (uncontrolled stopping) according to EN 60204-1.
Depending on the application the movement while running down to a standstill can cause a hazard for
an undefined time. Detection of whether or when the drive unit has achieved a safe state is not
integrated into the frequency inverter.
Depending on the switching equipment and use of a safe shut-down method, an STO function with
safety category 4 as per DIN EN ISO 13849-1 can be implemented.
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Bei einer Synchronmaschine kann es beim Versagen des Leistungsteils des Frequenzumrichters trotz
aktivierter STO-Funktion zu einer Ausrichtbewegung um eine Polteilung kommen.
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2.2.2 Safe Stop 1 time-controlled, SS1-t

The SK 3x1P series frequency inverter supports the implementation of the SS1 function in the SS1-t
(Safe Stop 1 time-controlled) version.
To stop the motor, the frequency inverter triggers a quick stop. After an application-specific time, the
STO function is 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).
Information
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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2.2.3 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
14 BU 0830 en-4121