RHW-2, USE-2 Cable: Does Wet Location = Submersible

[lu#26]

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

 

[ptonsparky]

The cables are bad. We test USE cable in the 1300' range that test at better than 2-4G. Some of it in water and sand for most of its length. Since younwill be replacing the cable, any chance of moving the drive closer to the motor?

You can turn off the ground fault feature but you may want to think a.bit before doing it.

 

[ptonsparky]

I would think, just based on my experince, most buried conduit has water in it.

 

[lu#26]

The cables are bad. We test USE cable in the 1300' range that test at better than 2-4G. Some of it in water and sand for most of its length. Since younwill be replacing the cable, any chance of moving the drive closer to the motor?

You can turn off the ground fault feature but you may want to think a.bit before doing it.

Thanks. There's very little chance of moving the VFD any closer due to the layout of the mechanical plant. It's hard enough trying to convince the mechanical plant manager that the cables are bad. It looks as though I'll have to test the cables of a VFD/motor that aren't having any problems in order to help him make a decision.
Thanks again.

Dan

 

[lu#26]

I would think, just based on my experince, most buried conduit has water in it.

That's been my experience also, but it was suggested to me that if the conduit was full of water that it could affect the resistance values I recorded.
If water had gotten between the copper & the insulation, I could understand that argument. But considering that the cables carry approximately 65 amps. while running for days at a stretch, I'd think that any moisture that had intruded during installation would have been taken care of but I'm not real sure of that either.
Thanks.

Dan

 

[kwired]

Do you have a line reactor installed on the drive output? This will help bring down the peak voltage.

Think about what you have if there is water in the conduit. You have a conductor/ insulation/ water that is likely conductive. That makes a capacitor. VFD do not like capacitors in the output circuit.

 

[Dennis Alwon]

The W is rated for wet but is it really rated for direct submersion? I am not sure, however if the duct is filled with water could that be the issue???? Just curious and I thought that was what the OP was asking.

 

[kwired]

The W is rated for wet but is it really rated for direct submersion? I am not sure, however if the duct is filled with water could that be the issue???? Just curious and I thought that was what the OP was asking.

Underground raceways often are filled with water with standard conductors containing the W in the name and there is generally no problem. Another issue here that can not be ignored is the high voltage spikes introduced by the VFD. If it is a 480 volt drive the peak voltage is over 750 volts IIRC.

 

[Dennis Alwon]

Underground raceways often are filled with water with standard conductors containing the W in the name and there is generally no problem. Another issue here that can not be ignored is the high voltage spikes introduced by the VFD. If it is a 480 volt drive the peak voltage is over 750 volts IIRC.

That may be true but they do make marine wire, I believe.

 

[ptonsparky]

That's been my experience also, but it was suggested to me that if the conduit was full of water that it could affect the resistance values I recorded.
If water had gotten between the copper & the insulation, I could understand that argument. But considering that the cables carry approximately 65 amps. while running for days at a stretch, I'd think that any moisture that had intruded during installation would have been taken care of but I'm not real sure of that either.
Thanks.

Dan

The only way water can get between the cu, or al, and the insulation is if it comes from either end or you have a hole in the insulation between the ends. No hole, no water. Your meg test and the drive have indicated a break in the insulation. 65 amp current flow is not enough to dry out a wire once the water gets in. Well maybe 65 amp on a #14.:roll:

One of the problems with water in the conduit is when it freezes. This does cause damage to the insulation.

 

[lu#26]

Do you have a line reactor installed on the drive output? This will help bring down the peak voltage.

Think about what you have if there is water in the conduit. You have a conductor/ insulation/ water that is likely conductive. That makes a capacitor. VFD do not like capacitors in the output circuit.

Yes, there is a line reactor on the input & the output. You hit on what I was thinking with creating unwanted capacitance. I brought this up to my plant manager and his reaction is basically that he cannot believe that this would be an issue because the electrical engineers would have predicted this and compensated for it since having water in a conduit is not unusual.
What's more, although reactors are not true RFI filters, the Allen-Bradley VFD manual specifically states that RFI filters on the output should not be installed on systems that are not solidly grounded due to their inclination to bleed off current to ground, and as I mentioned earlier, this is connected to an HRG system.

 

[lu#26]

Underground raceways often are filled with water with standard conductors containing the W in the name and there is generally no problem. Another issue here that can not be ignored is the high voltage spikes introduced by the VFD. If it is a 480 volt drive the peak voltage is over 750 volts IIRC.

Understood. Thanks for reminding me that I didn't mention the system voltage which is 347/600.

 

[lu#26]

The only way water can get between the cu, or al, and the insulation is if it comes from either end or you have a hole in the insulation between the ends. No hole, no water. Your meg test and the drive have indicated a break in the insulation. 65 amp current flow is not enough to dry out a wire once the water gets in. Well maybe 65 amp on a #14.:roll:

One of the problems with water in the conduit is when it freezes. This does cause damage to the insulation.

I was on & off the job site during construction for QC purposes, and one of the issues that I pointed out to the contractor was their habit of cutting cables to length and then leaving them out in the rain. That's where I was thinking that water intrusion could have occured. As far as freezing goes, that's a good point - we just finished a fairly harsh winter here in Virginia.