Rockwell Automation 7000 PowerFlex Medium Voltage Air-Cooled (B Frame) Installation - ForGe Control - Manual (Page 79)

Rockwell Automation Publication 7000-IN007E-EN-P - June 2014 79
Appendix
B
Meggering
Drive Meggering When a ground fault occurs, there are three zones in which the problem may
appear: input to the drive, the drive, output to the motor. The ground fault
condition indicates a phase conductor has found a path to ground. Depending on
the resistance of the path to ground, a current with magnitude ranging from
leakage to fault level exists. Based on our experiences in drive systems, the highest
probability for the source of the fault exists in either the input or output zones.
The drive itself rarely has been a source of a ground fault when it is properly
installed. This is not to say there will never be any ground fault problems
associated with the drive, but the chances are the fault is outside of the drive.
Also, the procedure for meggering the drive is more complex than meggering
outside the drive.
With these two factors, it is recommended to first megger the input and output
zones when encountering a ground fault. If the location of the ground fault can
not be located outside the drive, the drive will need to be meggered. This
procedure must be performed with due care as the hazards to drive exist if the
safety precautions in the procedure are not followed. This is due to the fact the
megger procedure applies high voltage to ground: all the control boards in the
drive have been grounded and if not isolated, they will have high potential
applied to them causing immediate damage.
Meggering the PowerFlex 7000
Ensure you check the insulation levels before energizing power equipment.
Megger tests provide a resistance measurement from the phase-to-phase and
phase-to-ground by applying a high voltage to the power circuitry. Perform this
test to detect ground faults without damaging any drive equipment.
This test involves “floating” the drive and all connected equipment to a high
potential while measuring the leakage current to ground. Floating the drive
means to temporarily remove any existing paths to ground necessary for normal
operation of the drive.
ATTENTION: Use caution when performing a Megger test. High voltage testing
is potentially hazardous and may cause severe burns, injury or death. Where
appropriate, connect the test equipment to ground.
ATTENTION: There are risks of serious or fatal injury to personnel if you do not
follow safety guidelines.