Underground Feed Replacement

[Eddie_T]

The electrical utility has begun work on replacing some 600-1000 ft of underground 7200V feeder. They are not digging an open trench. The guy that marked the path said their contractor would dig holes every 25 ft, shoot missiles from hole-to-hole, insert pipes, then the electrical team will come and run new cable. The old cable has an exposed concentric braided ground. I hope their missiles do not hit existing electrical or telephone cables.

edit: I just spoke with the crew chief, the missile is a pneumatic drill driven by a compressor. He said his crew will be finished with a 2" pipe installed today. Then it's up to electrical team scheduling.

Any future faults will not require digging, just cable pulling.

 

[Guzzle]

Thick insulation necessary on that wire. Projected cable lifetime might be >50 yrs.
Our 7kV line is overhead.

That drill might be autonomous & maybe they can see what the drill sees.

What will they do if the drill bumps up against a boulder as large as a VW? Then it will be Miller Time!

 

[Eddie_T]

I think the original line was installed almost 50 yrs ago. I saw a piece that was removed when I had my last feed failure. The ground was corroded but intact. That was a long continuously grounded conductor but it was also grounded to a rod at my transformer primary.

 

[Guzzle]

Our transformer woven ground cable has less than 20 mA thru it, same with our house ground lead & there was a few mV between the pole ground cable & our hose bibb.

I still don't have the nerve to open our house ground & check the voltage across the gap but I imagine it is 120vac or less.
I may compromise & put a 1 ohm, 1A, 1w resistor in the gap so the ground is not fully opened. The 25 ohm or less ground resistance shouldn't notice.

 

[Eddie_T]

Our transformer woven ground cable has less than 20 mA thru it, same with our house ground lead & there was a few mV between the pole ground cable & our hose bibb.

I still don't have the nerve to open our house ground & check the voltage across the gap but I imagine it is 120vac or less.
I may compromise & put a 1 ohm, 1A, 1w resistor in the gap so the ground is not fully opened. The 25 ohm or less ground resistance shouldn't notice.

Mine on the feed has full current as it is the return grounded conductor for the 7.2 kV.

 

[Guzzle]

& for your neighborhood current draw.

Our overhead line might be 1/4th inch diameter. There's no messenger cable so it might be something stronger than copper.

The less cable sag the higher the tensile strength. IIRC the cable shape is a catenary & the PoCo needs to consult their design formulas for this.
"Zero" sag = "infinite" tension on the wire.

Our cable set our tree branch on fire. I don't guess the PoCo noticed this on their gauges & the neighbor called it in.

 

[Eddie_T]

I have the only yard transformer in the neighborhood, everyone else gets 240V from a pole transformer.

I think I am safe now. They are working down the road and up another property line now and both my electric and internet are still working. I can't hear the compressor so they may be getting ready to install pipe.

 

[dsteinhorn]

I had a water supply replaced that way in St. Paul 30 years ago. The company called it a pneumo-auger and the guy shot it straight from my basement to a hole near the street. Fantastic method to avoid trenching the entire front yard, but woe unto the device that hits a rock or gets wedged in. They have a steel cable attached to a winch to retrieve it, if necessary.

 

[Guzzle]

And, in other boring news [pun]
"When reassembled, the machine will be 46 feet tall and more than 430 feet long, weighing more than 9 million pounds. Will move roughly 50 feet per day. Powered by 12 electric motors totaling 12,000 horsepower. Maximum torque in 36 million foot-pounds."
Which comes out to 2 RPM.

 

[Eddie_T]

The electrical team is here today. I got a look at the cable where they trimmed the end off. It's an aluminum center conductor and the concentric return is copper ( maybe extruded not braided). The secondary lugs of my transformer are long with three empty holes. IOW my transformer is sized to supply four houses but I am the only one in the neighborhood with underground service. My 240V feed is only about 10 ft so minimal voltage drop for sure. The tech said normally the transformer would have been near the street supplying more houses but this is a rural unplanned community.

 

[Guzzle]

Well, that cable doesn't need tensile strength.
And with no neighbors pulling 240v your house voltage might be relatively constant. Our 120 varies randomly one or two volts.

Your panel breaker "AIC rating" is somehow connected to how much short circuit current your new 'former can deliver.

I measured our main panel at 5000A, I assume we have 200A service & I have no idea what our breakers are rated at.
https://www.lutron.com/TechnicalDocumentLibrary/048176.pdf
Seeing two fuses in series on a schematic has something to do with this, like RCA's VTVM wiring.

 

[Eddie_T]

All is complete with no surprises. The techs estimated 2:30 hrs outage but had me back online in 1:45 hrs. I enjoyed chatting with the crew (three nice savvy guys), turns out they were also contactors. I suppose the utility saves by not having to supply benefits.

 

[Guzzle]

If they screwed up you'll probably know within 24 hrs, certainly within a week.

 

[Eddie_T]

If they screwed up you'll probably know within 24 hrs, certainly within a week.

I was just thinking, I now have a 2" pipe coming downhill into my transformer enclosure. What could go wrong?

 

[Guzzle]

I was just thinking, I now have a 2" pipe coming downhill into my transformer enclosure. What could go wrong?

In that case maybe a few days of monsoon rain will reveal flaws. Flickering lights, I'd guess.

 

[Eddie_T]

Maybe some steam *** well!