Aluminum Corrosion

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hillbilly

Senior Member
I gat a call from a new customer last week who had a electrical problem with a outbuilding.

This building is fed from a Main Breaker panel located at his house house, approx. 75 feet away.
The building is fed with 4/0 al URD Quarduplex cable, direct buried.
240V single phase.
The cable was buried about 2 months ago.

The customer turned on the lights to his building and noticed that the lights were very dim.
Also, none of his 230V equipment would work.

I checked voltage at the Main....240V OK.

Checked voltage at the out building panel.... 200V.
120V one leg to ground, 80V other leg to ground.

I disconnected the cables at each end, and there was no continuity between any of the conductors.
I plugged one of my meter leads into the earth and checked each leg to to earth.
Three legs read infinity...good.
The other (one of the hots) read 1200 Ohms to earth....Uh oh.

I Told the owner that we had a damaged underground cable.

The owner has a backhoe, deep pockets and plenty of time, so he starts digging the cable up.
What we found really suprised me.

At one location the cable insulation was split about 3 inches, and the aluminum was completely corroded into aluminum oxide, or what ever it turns into when it turns to powder.
It was buried in damp, red clay, and there was nothing special about the dirt that I'm aware of.

The "installer", whom I replaced, must have damaged the insulation during burial.:roll:
I'm suprised that the aluminum failed so fast, plus, the owner had been using the shop the day before, and didn't notice any problem.
It finally failed when the last strand opened.
It had only been conducting electricity about 3 weeks, and that was part time at best.

I replaced the cable (in PVC conduit this time) and covered it up...problem solved.

I'm still wondering how that aluminum failed so fast.

steve
 
charlie

charlie

Senior Member
Location
Indianapolis
hillbilly said:
. . . I'm still wondering how that aluminum failed so fast. . .
Click to expand...
Steve, this is a dumb answer but aluminum will do that when it is exposed to moisture and current. It is also interesting to note that the URD cable (basically XHHW) will be eaten through in a septic field or where installed in contact with fresh concrete alike a poured footer. :)
 
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nakulak

Senior Member
it may also be worth noting that the red clay is red generally due to iron oxide in the soil. if the soil was damp, then an electrolytic cell was potentially formed and accelerated deterioration of the Al would have generally resulted.
 
zappy

zappy

Senior Member
Location
CA.
Hillbilly, when you say you plugged one of your leads to earth, do you mean actually sticking your lead in the dirt, or touching ground thru the grounding electrode? Just curious.
 
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hillbilly

Senior Member
zappy said:
Hillbilly, when you say you plugged one of your leads to earth, do you mean actually sticking your lead in the dirt, or touching ground thru the grounding electrode? Just curious.
Click to expand...

Yes, I actually stuck one of the leads into the earth.
I picked a spot where the earth hadn't been disturbed.
I got a high resistence reading, but a reading nonetheless.
Enough to tell me that I had a break in the underground conductor insulation.

steve
 
480sparky

480sparky

Senior Member
Location
Iowegia
Prime candidate for a fault locator.

PE2003.jpg
 
weressl

weressl

Esteemed Member
Location
Texas by the Gulf, geat crab, great shrimp
nakulak said:
it may also be worth noting that the red clay is red generally due to iron oxide in the soil. if the soil was damp, then an electrolytic cell was potentially formed and accelerated deterioration of the Al would have generally resulted.
Click to expand...

Iron and aluminum are fairly close to each other on the EP table so they don't 'build' a cell worth nuthin'. That is why the code allows aluminum and steel conduit to be installed together.

There is got to be something more to this that happens in 3 weeks.
 
480sparky

480sparky

Senior Member
Location
Iowegia
ItsHot said:
Ken, is this not a "thumper"?:-?
Click to expand...

No, it's not a 'thumper'. Works on a very similar principle, though. You create a circuit by attaching one lead to the faulted conductor, and the other to a small 'ground rod', which is nothing more than a long scratch awl. The transmitter, which looks like the toolbox, sends a high-voltage DC pulse (1200VDC, I think) into the faulted conductor. As the voltage enters the ground and finds a path through the ground, it starts to go dissapate. So a DC voltmeter can be used to locate the fault

The "A-Frame" has a basic DC voltmeter on it. Each leg has a spike on it, that are connected to the meter. Center is 0 volts. If the fault is to the left of the frame, the meter will deflect to the left when the transmitter sends out a pulse (This is why it's called the Pulsar!). It beeps when it does this so you can watch the meter. If the fault is to the right, the meter will deflect to the right.

The meter is adjustable so you can change the setting depending on how far you are from the fault. As you get closer, the needle deflects more and more until it hits the far stop. Then you 'dial down' the meter to make it less sensitive. Once you are in line with the fault, it will deflect less. When you (or I should say, the frame) are directly 90? to the fault, there will be no movement.

You then simply turn 90?, and 'follow the needle' to the fault.

To verify I've found the correct spot, I go around the suspected location in a circle, and the frame (and meter) should always 'point' to the fault. Another double-check I do is to center the frame over the location and then turn in, always keeping it centered. If the fault is there, the meter will not deflect.
 
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2

220/221

Senior Member
Location
AZ
Two months is pretty quick.

I see underground AL failures all the time. That's why I always use copper and always use conduit.

POCO here hasn't direct buried any cable in decades.
 
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