single phase from 3-ph

On Thu, 29 Jul 2004 22:25:08 +0000 (UTC) Michael Moroney wrote: | "s falke" writes: | |>"In considering the marked advantage of the two-phase system for distribution |>and of the three-phase system for transmission, it occurred to me that that a |>combination of the two systems night secure the advantages of both, and I have |>worked out a simple and effective method of accomplishing this result.?" | | | What is this supposed "marked advantage" of two-phase for distribution?

3 wire system. But a corner grounded delta (open or closed) has this, too.

Simpler transformers. But an open delta has this, too.

It only takes 2 vectors to define 3 points in 2D space.

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On Thu, 29 Jul 2004 16:09:01 GMT, Bruce L. Bergman put forth the notion that...

Oh really? I've been licensed since 1974, and that's news to me.

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Checkmate

Okay, that was slightly overstated. ;-) Nothing outright, but there are a bunch of restrictions in Section 250 on using conduit for ground that you have to consider (and handymen never do) like the restrictions on using liquidtight and aluminum flexible conduit as the grounding conductor (conduit length and circuit ampacities).

But the more important part is that you really can NOT trust the conduit on old installations to be made up properly and a reliable grounding path, unless you want to crawl the attic and open the walls to inspect every inch of it... Fittings get loose, couplings get sprung inside walls where they can't be seen, "handyman" installers get creative when they don't have the right transition couplings (using plastic parts and big wads of tape)...

When in doubt, take a green grounding conductor back to the main with your other new conductors, and bond it to the boxes on your way - it's cheap insurance. And they'll rarely argue if you go a bit above and beyond the minimums that the codes call for.

Especially on very old houses that started out with an A-base meter,

30A fused disconnect and knob-and-tube through the attic, and have been upgraded piecemeal over the years with steel rigid, steel flex, alflex, Wiremold, XO breakers, Pushmatic breakers, Zinsco breakers... I've had lots of fun on those Electrical Archaeology projects.

Even on newer installations within the last 20 years I've had fun with ground faults and bad conduit grounds. Especially with cast zinc fittings that started a bit loose and burned themselves way loose - you wiggle the fitting and get a big spark across the gap...

(There's a /reason/ I listen to a portable AM radio at work, the static will let you pick up on stuff like that - If you wiggle the fitting and the radio goes staticky, start digging...)

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Bruce L. Bergman

Certainly sloppy work can compromise the integrity of a metal raceway system. You can screw up the copper too. I was a state inspector when we evaluated the raceways in a WWII era building that was being converted to a hospital. I was very surprised at how well a properly installed raceway and armored cable system lasts in the swamps of Florida for half a century.

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Greg

So why wasn't a corner grounded open delta three phase system proposed as having all the advantages, why did he suggest the awkward three phase for transmission and two phase for distribution?

(probably not really related: I notice that high voltage transmission line transformers are usually delta-connected while distribution systems are usually wye (although older ones are often delta). Why? I know one advantage of wye is you can run a single hot+neutral down residential streets rather than two hots, but why HV is wired delta?)

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Michael Moroney

On Wed, 4 Aug 2004 15:50:47 +0000 (UTC) Michael Moroney wrote: | snipped-for-privacy@ipal.net writes: | |>On Thu, 29 Jul 2004 22:25:08 +0000 (UTC) Michael Moroney wrote: |>| |>|>"In considering the marked advantage of the two-phase system for distribution |>|>and of the three-phase system for transmission, it occurred to me that that a |>|>combination of the two systems night secure the advantages of both, and I have |>|>worked out a simple and effective method of accomplishing this result.?" |>| |>| |>| What is this supposed "marked advantage" of two-phase for distribution? | |>3 wire system. But a corner grounded delta (open or closed) has this, too. | |>Simpler transformers. But an open delta has this, too. | | So why wasn't a corner grounded open delta three phase system proposed as | having all the advantages, why did he suggest the awkward three phase for | transmission and two phase for distribution?

I don't know why it wasn't proposed. Maybe because it has some efficiency issues for delta loads (the load winding across the open end drawing from the 2 ungrounded phases).

| (probably not really related: I notice that high voltage transmission | line transformers are usually delta-connected while distribution systems | are usually wye (although older ones are often delta). Why? I know one | advantage of wye is you can run a single hot+neutral down residential | streets rather than two hots, but why HV is wired delta?)

I'm not sure how these play out in transmission, but they could be issues. With delta, you have a 15% reduction in winding for a given power level. At transmission scales, this can be a cost savings they go for. Also, if the transformer is delta-delta, there's no phase shift (a wye-wye would do that as well). And why would a neutral be needed in transmission? As you point out, there's no side branch with one phase in transmission circuits (at least not that I know of).

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