Wires shorted in conduit several times, have I found the problem? Long post, sorry

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hockeyoligist2

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Two identical 460v sewer grit pumps. They both have had the wire short to ground and/or burn one wire in two inside the concrete enclosed conduit several times since they went online a year ago. The original contractor has replaced it under warranty and it is good for a while, then we get a call that it is tripping.

Every time I or one of the other electricians get there we find that it is shorted or lost a leg. Hook up temp wires, everything is good. Last week I got the dreaded call, checked no power on one wire. Hook up our temps again, everything good. Next day the other one lost a leg. Temped it.

Got the call again today, temps still connected, I checked for shorts, power, and motor, everything good. Turned it on and it tripped immediately. Went downstairs to check the motor again and it was running without power and was cold! Pulled the disconnect and it was still turning. I could hear water coming through the pipes. It throttled down and stopped.

I traced out the pipes and found a backwash valve that clears the lines after it shuts down to prevent the lines and pump from stopping up with grit. It is controlled separately by the plants computer system. The computer system gets a signal from another valve telling it that it is closed, then it purges the pipes. We don't have very good control drawings, so we never knew it was there.

The pumps cycle every 4 1/2 minutes, the PC was telling it to purge for 3 minutes. The valve takes about 1 1/2 minutes to close, so the pump is still spinning backwards when it tries to restart! I changed the PC timer to 2 minutes which is more than needed to purge.

I'm guessing that the wire is the weakest link and it eventually breaks down. Sound reasonable?
 

iwire

Moderator
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Location
Massachusetts
Any idea if they have been shorting in the same spots?

Just a guess that when the motor has been pulling high start current the conductors had been vibrating a lot and there is a sharp spot in the conduit.
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
You've made a post about this before, right? I seem to remember someone mentioning a recurring problem with wires shorting underground to a motor?

If this conduit only contains one set of motor leads I'd probably look into pulling VFD cable back into it just to add some abrasion resistance to the wiring. You'll have to definitely check the conduit fill though. Like Bob said, it sounds like someone forgot to ream the conduit somewhere...

We use the Belden VFD cable, it has a pretty tough outer jacket.
 

Jraef

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Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Any idea if they have been shorting in the same spots?

Just a guess that when the motor has been pulling high start current the conductors had been vibrating a lot and there is a sharp spot in the conduit.
Vote #3 from me, that's the first thing I thought of when reading it.

The wires should NOT be the weakest link, the protection devices should*. If the motor were pulling Locked Rotor Current because of it spinning backwards, it would only do that for a second or two longer than normal (assuming here that you are starting across-the-line). That in and of itself should be OK. But the one factor that presents a danger is that the conductors inside the conduit are going to be jumping around quite a bit as they try to move away from each other under the magnetic fields created by the high current. A burr left on a conduit cut would be like resting the conductors on a hack saw blade and although the blade doesn't move, the conductors moving across it has the same effect. This failure would likely have happened anyway, the back spin issue would have probably just cut the failure time in half, so everyone notices more often.

The tricky part is going to be finding and cleaning it up, depending on the length of the run.

* Assuming the wire is properly sized and protected.
 

hockeyoligist2

Senior Member
Vote #3 from me, that's the first thing I thought of when reading it.

The wires should NOT be the weakest link, the protection devices should*. If the motor were pulling Locked Rotor Current because of it spinning backwards, it would only do that for a second or two longer than normal (assuming here that you are starting across-the-line). That in and of itself should be OK. But the one factor that presents a danger is that the conductors inside the conduit are going to be jumping around quite a bit as they try to move away from each other under the magnetic fields created by the high current. A burr left on a conduit cut would be like resting the conductors on a hack saw blade and although the blade doesn't move, the conductors moving across it has the same effect. This failure would likely have happened anyway, the back spin issue would have probably just cut the failure time in half, so everyone notices more often.

The tricky part is going to be finding and cleaning it up, depending on the length of the run.

* Assuming the wire is properly sized and protected.

By weakest link I meant easiest damaged. I'm guessing there is probably some burs in the conduit too.

I'm hoping they won't be moving around so much now.
 

hockeyoligist2

Senior Member
You've made a post about this before, right? I seem to remember someone mentioning a recurring problem with wires shorting underground to a motor?

If this conduit only contains one set of motor leads I'd probably look into pulling VFD cable back into it just to add some abrasion resistance to the wiring. You'll have to definitely check the conduit fill though. Like Bob said, it sounds like someone forgot to ream the conduit somewhere...

We use the Belden VFD cable, it has a pretty tough outer jacket.


I have made a post similar before. Different plant different machine. We ended up installing new conduit on that one. If it goes out again, after the warranty expires, I'll mention the VFD cable to the boss.
 

ceb58

Senior Member
Location
Raeford, NC
I will make a wild suggestion. I like the others thought of something in the conduit cutting the cable. When you pull the cable out of the conduit and before pulling in new run a sewer camera through the conduit,if it is big enough. I am sure the city has one that can be used. Then if you see something that would be cutting the cable you will know where the problem area is.
 
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