Rockwell Automation 1336T Wiring and Grounding Guide, (PWM) AC Drives - Manual (Page 27)

Rockwell Automation Publication DRIVES-IN001M-EN-P - March 2014 27
Power Distribution Chapter 2
Ungrounded Secondary
If the system is ungrounded, other general precautions, such as a system level
ground fault detector or system level line to ground suppressor, can be necessary.
Or consider an isolation transformer with the secondary of the transformer
grounded.
Refer to local codes regarding safety requirements. Also refer to Surge Protection
MOVs and Common Mode Capacitors on page 45 .
High Resistance Ground
Grounding the Wye Secondary Neutral through a resistor is an acceptable
method of grounding. Under a short circuit secondary condition, any of the
output phases to ground do not exceed the normal line-to-line voltage. This is
within the rating of the MOV input protection devices on the drive. The resistor
is often used to detect ground current by monitoring the associated voltage drop.
Because high frequency ground current can flow through this resistor, be sure to
properly connect the drive motor leads by using the recommended cables and
methods. In some cases, multiple drives (that can have one or more internal
references to ground) on one transformer can produce a cumulative ground
current that can trigger the ground fault interrupt circuit. Refer to
Surge
Protection MOVs and Common Mode Capacitors on page 45 .
ATTENTION: Grounding the transformer secondary is essential to the safety of
personnel and safe operation of the drive. Leaving the secondary floating
causes dangerous high voltages between the chassis of the drive and the
internal power structure components. Exceeding the voltage rating of the
drive’s input metal oxide varistor (MOV) protection devices can cause a
catastrophic failure. In all cases, the input power to the drive is referenced to
ground.