240V from a 3 phase main ?

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Good information, Don. Thanks!
Bob Swinney

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Don Murray wrote:

At the transformer, as per the diagram in that excellent link. When you do the actual service lateral, you pull three primary cables and each one has an aluminum (usually) center conductor and a copper concentric grounded conductor. The grounded conductors are joined at the transformer and also bonded to the transformer case and ground rods.
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ATP wrote:

The concentric on the primary cable along with the transformer grounds are not always a metallic return to the substation. Primary neutral and common neutral (common to primary and secondary, run in the secondary position) are metallic returns to the substation. You find primary neutrals in the older 4KV where they've used 2400V transformers phase to neutral (star or wye). We also run a common neutral system in our 21KV that we use 12000V transformers on.
A side note to the underground cable, is they are not getting the life expectancy out of it that they thought when they put it in. One of the problems that we've run into is some soil conditions dissolve the copper neutral on the older installations where it was direct buried. We've since gone to a jacketted cable and require everything to be in conduit now. Another problem they find is when the cable is faulting it's through hairline cracks through the poly. We've replaced a considerable amount of this at great expense, and have recently been working with a company that injects silicone in the end of the strands that fills these hairline cracks. At 5 dollars a foot, it's about half the price of cable replacement. This year we have some 40000' coming up for cable replacement and another 20000' that we are going to do the silicone injection on.
Don
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Don Murray wrote:

You're way above my head, are you talking about upstream of the customer-owned cable? In the typical case, the concentric grounds are the only return, right?

That's probably the way to go- I once pulled an old lead-jacketed primary run through about 400' of conduit from a man-hole cover to an underground vault. It was a real PITA but pulling the new poly-jacketed stuff in was easy and saved a lot of trenching and restoration. Still, the majority of installations in this area are direct-bury. The only advantage is that sometimes you can Biddle the cable and make a quick repair without pulling the whole line out.
Another problem they find is when the cable is faulting

They inject silicone in the center strand which fills the gaps in the white poly insulation between the center strand and the semiconducting layer? That's a pretty neat trick. Is it only practical for utilities? Do they have to redo the potheads/elbows?
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ATP wrote:

I can't speak to what direction you are referring to about customer cable. Around here, it varies so much. We have some customer owned primary and secondary. But when I'm talking about the grounds not being a return to the sub, say you have a 3-phase primary (just 3 wires, no neutral) going out from the sub a couple of miles and now you install an pad-mount transformer (or string of transformers) being fed from a riser off of this 3-phase primary. Your grounds on all these transformers are tied together, like you say. But there is no metallic return to the sub on them.

Yes
I can't say if it's practical for other than utilities, you'd have to weigh the costs. And yes, they have to redo the potheads, and elbows. Around here we don't use that many potheads, we use 3M termination kits.
Don
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Don Murray wrote:

OK.
I'm referring to the legacy potheads in the underground transformer vaults which the utility desperately wants to phase out in favor of pad mounts.
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. .
. .
A note on DC- As of teo years ago, New York transit was still running DC rotarary converters that are about 100 years old-AC in, DC out, but not quite a motor generator like a welder. I believe the last went out of service recently, but I may be wrong. The hookups use have been thyristor converters (all electronic) since the 50's.
I read recently that the last of the Edison DC service was finally discontinued.
e
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enl snipped-for-privacy@yahoo.com (e) wrote:

If anybody's interested in this stuff, take a look at
http://www.nycsubway.org/tech/power/rotary.html
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Roy Smith wrote:

Also, "Networks of Power"
(Amazon.com product link shortened)
"If you're a history buff, and appreciate the technology that surrounds us all, you'll love reading "Networks of Power: Electrification in Western Society, 1880-1930" by Tom Hughes. Hughes takes us back to the days of fierce rivalry between Edison and Westinghouse; the early era of electric power generation and consumption where the battle of DC vs. AC consumer power was born and decided.
Hughes doesn't stop there. Also included in this well-footnoted edition are in-depth narratives of the evolution of commercial power systems in England and Germany through 1930. A well written, readable snapshot in time.
Compelling historical reading for the non-technologist as well as the student of electrical power commercialization."
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ATP wrote:

(Amazon.com product link shortened)
Do they mention anything in the book, "Networks of Power" about the Folsom Powerhouse? I was privileged enough in my career to work with a man that operated it. He operated it from 1948 til they closed it in 1955. I worked with him in the '70's when he was Chief Dispatcher with the company I work for. I took a tour of the powerhouse with him and I took my video camera. I have him telling a lot of wonderful stories of things like the collectors flying off the generators, sparks everywhere and him diving out the window.
http://www.parks.ca.gov/default.asp?page_id 35
Today when you talk about a network in the electrical utility industry it usually refers to a system in the downtown area of large cities. This is different from the transformers in the suburbs, in that, the secondaries are all in parallel. So if you lose a transformer you don't drop any customers, you just lose KVA. But you have to run your network with more reserve than the largest transformer.
Don
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Don Murray wrote:

(Amazon.com product link shortened)
It's been a few years since I read it, but I don't remember that. A lot of the book dealed with rationalization (standardization and the rationalization movement) of the power industry, the balance between competition and the need for a unified grid. On the technical side, much of what we take for granted, such as the concept of a feed with two hots and a neutral, (vs. one hot and one neutral) was once a bright new idea. With your background, I think you would enjoy at least some of the book.
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And anyone in Southern California that wants to see a working 2400 VAC 60Hz 6-phase to 600 VDC Rotary Converter station only has to go to the Orange Empire Railway Museum on an operating weekend. http://www.oerm.org There's a nice little GE station there, with a completely restored and re-wound rotary converter (ask about how that restoration came about, it's interesting). And it's all self-starting and self-shutdown, done with a big drum controller - just hit the button and watch it come up.
Looks like something Rube Goldberg designed, complete with a ball-and-worm armature shaft wiggler to keep the brushes wearing evenly, and big live-front contactors with open arc chutes...
They now have a solid-state converter for everyday use, but they can still fire up the rotary - for demonstrations, if they're working on the other power plant, or if they're going to be running a lot of rolling stock at once.
--<< Bruce >>--
--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
  Click to see the full signature.
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Hey Bob,
His suggestion for ASCII art made me cringe too, but it actually came out perfect on my computer.
Bit I also had some problem "following" along.
Take care.
Brian Lawson, Bothwell, Ontario. XXXXXXXXXXXXXXXXXXXXX On Fri, 09 Jan 2004 19:43:45 GMT, "Bob Davis"

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Clarke, you must have never heard of Delta Power. In that system, Two of the three phase are 120 to ground. The third phase is 208 to ground. between any two phases is 240 volts. Really.
With Delta Power, you have 120 volts from two of the three phases, and you have 240 volts three phase.
Pete
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snipped-for-privacy@please.reply.to.newsgroup wrote:

Wrong.
NO YOU WON'T.
In 208V 3-phase service, the phase-to-phase voltage is 208V, and the phase-to-neutral voltage is 120V.

Delta or wye, it doesn't matter. You won't get 240V anywhere in a 208V 3-phase service.
-- Doug Miller (alphageek at milmac dot com)
How come we choose from just two people to run for president and 50 for Miss America?
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3-phase
I don't know about everywhere else, but I rented a shop that had 208-3ph. It was just as Doug described. Phase to phase = 208 volt, any phase to neutral was 120 volt. Greg
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It was probably set up as a warehouse originally. This was often done because all that was needed was power for lights. The 208 wye service has the added advantage that your load is more balanced since you have three 120 circuits to use not just two. Plus you can run just about any 3-phase power machinery.
The only drawback to the 208 wye service is that any 240 2-phase stuff that you use won't get the power for which it was designed. But then, for just about anything that you find in a shop, that's not a problem anyhow. For instance, if you've got a 240 2-phase air compressor it will run happily on 2 legs of the 208 service. The power output might be a bit less, but you'll never notice it - plus the manufacturer overstated it in the first place anyhow.
Oh, and one more thing: Where Doug was saying "Wrong" and "NO.." up there. You now know that the correct answers are "Right" and "YES.." don't you?
George.
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What kind of "240V" motor won't run on 208V?

Not very likely. What center is there to tap?

On a delta-connected transformer, you'll get random values WRT ground. Line-to-line you'll get the same reading on all three pairs.

You are not likely to see anything but Y-connected trans- formers at the load end of the line. The supply end may well be delta-connected.
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He is talking about "red leg" delta. That is 240 between phases though. What they do is put 2 regular pole pigs up (delta primary), tap the first one like the normal 120/240 and connect the end of the secondaries together, leaving the 3d side of the delta open (also called "delta Vee" or "open leg delta"). It is just a cheap way for the utility to give someone 120/240 and also 3P 240. The L/N will be 120/120 and 208 on the "red leg", actually orange conductors in the code. You see this in light industrial areas. It works fine as long as the 3p loads are balanced across the phases. I have had problems in computer rooms connected this way since the load will be a combination of 3p and 1p loads inside the computer. We can usually fix it by rolling the phases around so all the single phase loads are on the center tapped transformer and the open leg only sees 3p loads. The utility generally upsizes the center tapped pig. You can identify this on the street by looking for 2 transformers together on a pole and a 3P drop going to the building. If you see 3 transformers it will be 3P wye (208) about 99% of the time. I suppose you could spec 3P delta if you really wanted 240v 3P but it might cost more and you won't have any 120v without the center tap and red leg service. You are also left with how you ground this service. Corner grounded kicks open another can of worms but you could use single phase service equipment. Usually these are impedance grounded or even floating for certain rare applications.
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We have 440v and 220v 3 phase in our building. When we need 220v single phase we just take any two lines from the 220v 3 phase. When using plugs, we use the 3 phase plugs, but only use two of the taps going to the load. Works on motors, welders, everything, everytime. I never knew there was a problem until I read this thread. (this ain't theory, we been doing this for years). Oh yes, when we need 110v we use one leg from the 220v 3 phase. Paul
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