I know...just not exactly how

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ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Spent the last two days repairing old copper underground feeders. Direct bury. After the first find and repairs yesterday we turned things back on and still no power on one leg. I wasn't involved in the ditch work. "Didn't you check for other faults?" I calmly asked. "yea, but the tester didn't show any" Greenly tester that is good for 1 meg at 9 volts. They have a 1507 Fluke on the truck. :mad:

I went back this morning and found two more right away, including one that they thought was just a fluke yesterday. They had hand dug down the 4' to the wire but had missed the locate by a few inches to many.

After digging up and isolating another section the help told me his Greenlee indicated another fault and proceeds with another test which showed everything just fine. His face just dropped. I had him reverse the leads and of course the wire failed again. Megger proved it, .03 M at 34 volts.

When copper oxidizes does it always act somewhat as a diode or just when it has the rubber type insulation that was used in the past?
 
cadpoint

cadpoint

Senior Member
Location
Durham, NC
ptonsparky said:
...old copper underground feeders. Direct bury.
Click to expand...

The insulation jacket has just broken down because there are continuals failures. The jackets is compromized, you'll be back!

Price out for re-work now... Tell the customer now. JMO

The EMF of the circuit pulls in other minerals that helps to attack the AL.
 
C

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
ptonsparky said:
Not AL. COPPER. We did all we could do for the money he wanted to spend. I told him up front he was wasting it on repairs of this extent. 15 butt splices and HTML shrink plus two 100 amp terminal boxes. Really did try.
Click to expand...

That's a lot of repairs!!

We have quite a few DB repairs in the spring irrigation season, one particular run to a circle probably has at least 6 repairs in it. The farmer keeps saying he'll replace it come winter....:roll: But it's job security I guess, because we're down there every season to fix it.

I don't hardly ever come across DB copper though...
 
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G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
101130-2219 EST

ptonsparky:

In prior days some low voltage rectifiers were copper oxide. The original Simpson 260 used a copper oxide rectifier. My oldest 260 has one and I believe it still works.

Just the right conditions will be required to create a copper oxide rectifier.

Here is a very good reference:
http://www.jmargolin.com/history/trans.htm
This brings back some memories of my early days. Crystal sets and vacuum tubes. In 1952 while in the Navy I was in a lab at the Brooklyn Naval Shipyard in a group that was developing test equipment to evaluate Western Electric transistors. So knowing of their existence and sort of how they were made I was able to construct one in my room at the Brooklyn YMCA. Although I was on active duty USNR I wore civilian clothes while at the shipyard and lived off base.

I still have that transistor and I might try and see if it still works sometime. Point contact transistors are quite different than junction transistors.

.
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
I was wondering under what conditions the oxidation of copper will become a rectifier.

When my help applied the Greenlee tester to the direct bury UF he had his test leads in one position. It showed continuity to earth. When I came back with the megger he wanted to show me how he confirmed that the cable was faulted. This time he had the leads reversed and the oxidation blocked the current flow giving him a "good" indication. When I had him reverse the leads, current would then flow showing a faulted conductor.

Sometimes a "good" conductor will charge just as you would when testing a capacitor and actually hold the charge until you reverse the leads and charge it again with the opposite polarity. A faulted conductor will not charge or you can repeatedly charge it with the same polarity.

We stopped searching for more faults when we got the cable to test @ 130V before failing. All of the problems we found were previous repairs and buried T taps. Reason for the terminal boxes.

As Gar pointed out it was just luck of the draw on this one.
 
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