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Why DC bus discharge time is calculated from DC bus fall from 825V DC to 50V DC in case of ATV6171...N4 drives? Why shorter measured time is OK?

The tables with the values of discharge time publisehd in maintenence manual are not for purely capacitor diagnostics. These values will not tell you whether capacitors has rated capacity or not.
The values are important to tell you how much time you have to wait since disconnecting power to the moment when you can touch living parts ! It is about safety of service personel.
It meas if DC bus is discharged earlier than 760 seconds for ATV71HD15N4, then you can be sure that no risk of electric shock exists for personel providing repair.
Voltage lower than 50V DC is considered as safe (extra low voltage), so that is why we refer to limit 50V DC.
And do not forget that waiting time must be calculated for the worst situation: Drive is braking (DC bus voltage is rising) , BRA=no, braking resistor is missing and the drive DC bus will achieve 825V DC which is max DC bus voltage. Then if service personel switches off the drive (or the drive trips in overbraking) and starts counting discharge time, we have to guarantee that DC bus voltage will fall below 50V DC within 760seconds for HD15N4.

So now you know the answer why 50V is the treshold (ELV), why discharge time is counted from 825V DC (the worst case) and why any shorter measured discharge time is OK (Safety).
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