Mike Dolt Enterprises
Mike Dolt Enterprises
User: lu#26, Location: Virginia
Asked on: Saturday, March 26, 2011 at 9:27 pm EDT
Answered on: Tuesday, May 14, 2024 at 3:20 am EDT
Question: I've been troubleshooting a VFD & 75 hp. cooling tower fan motor. The VFD produces a fault code of "Ground Fault" after the VFD and motor have run for three to four days. Reading the Allen-Bradley VFD manual tells me that this ground fault code only occurs after the ground fault is >25% of the rated load - pretty substantial. It's clear that the VFD is preventing this condition from reaching the MCC SWB where the VFD is housed because the MCC is connected to an HRG (high resistance ground) system and the Post Glover HRG controller does not see it.I've meggered & checked continuity of the fan motor windings with very acceptable results. While meggering the cables from the VFD load cables to the motor with the cables disconnected at both ends the results were A=432 Megohms, B=285 Megohms & C=285 Megohms - rather unbalanced readings.One note that could play a factor in my mind is that we discovered that the conduits that the cables are in were not duct sealed during installation and the conduit runs from inside the indoor MCC then underground to the outdoor cooling tower disconnect (we're duct sealing all similar conduits now). My question is this, with the cables being size 2-AWG and type XLP RHW-2 at 125 ft. long, and considering the odd megohm readings, just because the cables are rated for wet locations, would it affect the resistance readings if the conduit was full of water?Thanks for your consideration.Dan
Yes, it is possible that the presence of water in the conduit could affect the megohm readings of the cables. Water has a much lower resistance than air, so if the insulation of the cables is compromised in any way, water could penetrate and create a path for current to flow. This would result in lower megohm readings.In addition, the fact that the megohm readings are unbalanced suggests that there may be a problem with the insulation of one or more of the cables. This could also be caused by water damage.I would recommend having the cables inspected by a qualified electrician to determine if they are damaged. If they are damaged, they should be replaced.In the meantime, you could try to dry out the conduit by opening it up and allowing it to air out. You could also use a dehumidifier to help remove the moisture.Once the conduit is dry, you could re-test the cables to see if the megohm readings have improved. If they have not, then the cables should be replaced. Flag for review