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#74591 01/28/07 12:06 AM
Joined: Jan 2004
Posts: 171
Merlin Offline OP
I have got two cables running vertical shaft turbine pumps(480 VAC). Both identical 300 HP, one is fed by a 500 MCM type TC cable. The other a 350 MCM type TC cable, both 1200 foot in length. These systems were designed by an electrical engineer. Although I disagree with the application. They are in a stone quarry, subject to damage from falling rock. And they have been hit numerous times.

MY issue is the 500 MCM cable, I am always fighting shorts. The 350 MCM never gives me any problems. They both run parallel to and from the same location. The 500 MCM keeps blowing out. The last two have been within 2 feet of another splice.

Now, I don't understand what is causing the problem. When I expose the area, it has always been arcing between phases or phase to ground(which is uninsulated). Sometimes it looks like the insulation has just been "chewed off". Is it possible that this cable has been exposed to some sort of a surge and weakened it. Or is this a "bad" cable. (both cables installed at same time-August 2004)

I am looking for any suggestions on what might be causing this(before I pull all my hair out).

Thanks, Merlin

#74592 01/28/07 05:23 AM
Joined: Feb 2004
Posts: 1,438
Sometimes it looks like the insulation has just been "chewed off"

No chance of rodents getting in your raceway/on your cable tray is there? I'm thinking these cables might jump a little on motor startup, maybe check for some rough edges that might've been missed on your cable tray.

#74593 01/28/07 05:53 AM
Joined: Jan 2003
Posts: 4,391
I would contact the cable manufacturer.

It is possible that it has a manufacturing defect

Bob Badger
Construction & Maintenance Electrician
#74594 01/28/07 11:32 AM
Joined: Jan 2004
Posts: 171
Merlin Offline OP
This is tray cable but, it is not in a cable tray. It hangs on the quarry wall part of the distance. The rest of the way it either lays on the ground or hangs on the water dicharge pipe. Once again, in my opinion, this is totally the wrong application. I argued this at installation. My engineer told me that this was a good choice. He said its both "sunlight resistant and suitable for wet locations". However, it is also a very harsh environment. I guess he couldn't see that from his office.

When I say "chewed on" look, I am referring to the cable insulation. The jacket is usually still in tact, other than if it blows out, then it is a little burnt. It has appeared that water is getting inside the cable jacket. The last two occurences have been in a low point of the cable. When I cut it open, it was full of water and that is where it is shorted. It looks like it is shorting to the water and burning away the insulation causing the "chewed on" look.

As far as the rodents, that is not the case because the jacket is still in tact. Both of these cables originate at Benshaw softstarts so they shouldn't be jumping on start-up or under any heavy load. The only thing I know to do is contact the manufacturer.

#74595 01/29/07 02:15 AM
Joined: Dec 2005
Posts: 866
Likes: 4
Have you done a dc insulation test on all cores of the cable ?
So you can compare the readings if a certain phase is in worse condition.
It could be a poorly manufacturered cable or as already said before, high startcurrents cause the cables to move and wear slowly through the insulation.
Ideally under these conditions a steel armoured cable has to be used, perhaps flying debris damages the outer insulation ?

The product of rotation, excitation and flux produces electricty.
#74596 01/29/07 06:09 PM
Joined: Jan 2004
Posts: 171
Merlin Offline OP
That is exactly what is happening. The falling debris is damaging the outer jacket of the cable. That is usually where the faults begin. Even though the outer jacket is damaged, I don't believe that should be any reason for the conductors to cause a problem. As long as the insulation on the conductors isn't damaged. This is direct burial cable. It's rated XHHW-2, at 600 VAC, 90 degree C. It should withstand the environment.

I opened another area today with a fault. I found 3 spots that are blown out within six feet. One of which is a splice I made last week. A very good splice to boot, and 2 conductors were melted together at the connections. This was in between two other areas about 18-24 inches on either side.

Thanks for the input, Merlin

#74597 01/31/07 10:50 AM
Joined: Nov 2004
Posts: 138
just a shot in the dark here...this environment is very wet, so if the cable is exposed in a couple of spots within a few feet or inches or whatever, maybe it's arcing out because of the water content in the air.

#74598 02/01/07 03:46 AM
Joined: Sep 2004
Posts: 68
Could you have a D.C. voltage present because of the length of the cables? This is very common on drives and soft start starters. During the soft start period, or if drives are being used, a high D.C. voltage can appear on the lines to the motors. This can exceed the voltage rating for your cables, then kaboom! Check at the terminals of the starter during the soft start time period, (before the contactor closes, if there is one). The voltage does not have to be there long to pulse through the insulation. This has been a common problem for years on motor drives feeding circuits with moderate to long cables. Good luck!

#74599 02/01/07 08:06 AM
Joined: Jan 2004
Posts: 171
Merlin Offline OP
Thanks for the responses. Homer, you have an interesting point. I am a little unclear on how to check for this situation. Can you please explain to me a little more.

Is it possible something on the primary side could be causing this? The reason I ask is because yesterday the utility had a primary phase break feeding this area. This in result shut these units down on fault of a current imbalance. Could something been happening over the past month or so causing problem with my secondary?

Thanks again, Merlin

#74600 02/02/07 08:04 AM
Joined: Sep 2004
Posts: 68
Set your multimeter to DC volts. Clip on to the output terminals of the soft-start starter. (Phase A and Phase B for instance, or B and C) Start the pump and watch for a DC voltage. If there is one, it could be there just for the time of the start-up ramp up until full operating 480 or 600 VAC is applied through the starter. Many meters have a min/max function which can be helpful with voltage readings that are there for only a short time.
I assume that the cable is rated for 600 volts, since you have not indicated that it had a shield. A normal multimeter obviously cannot be used on circuits exceeding 600VAC.
When variable frequency drives or soft start starters are used, the normal AC is converted to DC and then back to a voltage that is not a true 60 cycle wave form. The motor acts like the wave is a standard sine wave, but there is also a DC component, and the cable acts like a capacitor, especially if the cable is long. If you find a high DC voltage, you would have to use a cable that is rated above that voltage. Even a brief pulse of DC can destroy mining cable or tray cable. Many drive manufacturers offer inductors which connect in series with the output of the drives to solve this problem. They are not cheap, but if the cable runs are long, the inductor solution might be cheaper. A call to the rep for the starter, or some internet time at their website might provide more specific information. I hope this gibberish is helpful!

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