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#15499 - 10/19/02 02:30 PM 480 volts to ground
The Watt Doctor Offline
Member

Registered: 12/23/01
Posts: 443
Loc: Mont Belvieu, TX
Friends....as some of you may know, the company that I recently took employment with does roadway signing and lighting. I was discussing electrical systems with someone the other day, and the subject of a 480 volt to ground system came up. This type of system is all, but history. The remaining 480 volt to ground systems are being phased out. The person that I was talking with didn't know anything about how it works. The only thing that I could imagine was a 480 volt, open delta system with the two joined legs grounded.
Is there some type of wye configuration that will yield 3 phase 480 to ground? Does anyone have any knowledge of these systems? I was thinking maybe Scott35 might have, or know of a diagram for this system.
Scott, if you're out there, give me a ring.

Best Regards,
Doc
_________________________
The Watt Doctor
Altura Cogen
Channelview, TX

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#15500 - 10/19/02 04:51 PM Re: 480 volts to ground
Nick Offline
Member

Registered: 08/13/01
Posts: 603
Loc: Riverside, CA
Doc,
A corner grounded Delta will give you 480V to ground.


[This message has been edited by Nick (edited 10-19-2002).]

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#15501 - 10/19/02 06:44 PM Re: 480 volts to ground
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
WD, if it's a remote/isolated system for roadways, it may very well be 2-wire 1ø service. It might be fed from a single overhead transformer, as (unmetered) "flat-rate" from the utility.

It is probably more common [per local customs, again] to have an ungrounded 480V 2-wire service—and measure roughly 240V-to-ground on the two conductors with good insulation to everything served. If insulation fails on any part of the circuit, voltage-to-ground will shift accordingly. It is also possible to have the serving transformer’s secondary center tap solidly grounded, giving a “firm” 240V-to-ground on both legs until a fuse or breaker operates.

Be careful though—once came across a 480V 2-wire lighting loop that had repeatedly wasted a bunch of ballasts and lighting-contactor control transformers. It measured 480V line-to-line, but 1800V+ to ground(!) Primary-to-secondary insulation in the local 12,000-480V transformer had failed. \:\/ Lucky to have a stout 1000V meter.

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#15502 - 10/19/02 07:05 PM Re: 480 volts to ground
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Just goes to show, it pays to buy good test equipment.
Bjarney,what sort of meter were you using?.
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#15503 - 10/19/02 07:09 PM Re: 480 volts to ground
The Watt Doctor Offline
Member

Registered: 12/23/01
Posts: 443
Loc: Mont Belvieu, TX
Nick,
I can dig the diagram that you posted. Part of my problem is that I don't have all the information, so I don't know if it is a 3 phase system or not. I'm familiar with a corner grounded delta. For some reason I get the feeling that the system that "they" (the person I was talking to about this system) weren't talking about a delta configuration. I'm supposed to talk with an inspector from the state who is "in the know" when it comes to these systems. I will post the results of my conversation with him.
Bjarney,
Thanks for the "heads up". Thats the kind of info that will keep a guy's backside out of a pine box. Believe me, as much as "The Watt Mama" chews on my backdoor, I need all the help I can get.


Both cheeks still intact,
Doc
_________________________
The Watt Doctor
Altura Cogen
Channelview, TX

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#15504 - 10/19/02 07:48 PM Re: 480 volts to ground
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Trumpy, it was a battle-worn Fluke 27. Checking afterwards, the calibrator topped out at 1100V, but the meter seemed unfazed. [I learned under “less-puckering” circumstances to toss the lead set if they are not in top-notch condition. They’re cheaper than fingers.]

WD, for the “normal” 480V or less, to be effective at troubleshooting, it’s important to be able to sort out readings that indicate a true problem from those that are nothing to worry about.

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#15505 - 10/20/02 02:47 AM Re: 480 volts to ground
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
WD,

Sounds like either a Corner Grounded Delta (like the one shown in the Schematic),
or a 1 Phase 2 Wire system with one Line Grounded (As Bjarney mentioned).

Could also be an Ungrounded system with the "Normal L-G Voltage Readings" that come from using Hi Z Volt Meters.

If you have the chance to check Voltage yourself, see what readings a Wiggy gives.
Test L-L, then L-G (Eq. Gnd) on both Conductors. See what readings pop up.

About the only way to 100% verify the system type (1ph or 3ph) would be to find the Transformer bank feeding the system, or to check the main switch gear.

Chances are if there are many Luminares and a few pumps, the system will be 3 phase.

I have seen a setup on the 405 Fwy at the 605 Jct., with both a 1 phase and 3 phase delta services for highway equipment.
The Pots are pole mounted 480 Volt 2 wire Animals, and appear to be at least 100 KVA from how large they are.

Scott S.E.T.
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#15506 - 10/20/02 12:24 PM Re: 480 volts to ground
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
On grounded versus ungrounded 480V electric service—if one conductor is intentionally grounded {in the US} that wire needs to be white, identifying a grounded circuit conductor. If solidly grounded, one knows voltage-to-ground expectations in troubleshooting. If ungrounded, there is not automatically a problem with varying voltage to ground—that is essentially determined by the condition of insulation on the energized parts of the local system. Knowing what to anticipate can save time and frustration at startup or with later problem hunting.

Scott, you raise a good point—if there are any storm pumps, 3ø service would be handy. For 480V ungrounded service, 1ø or 3ø, it is worthwhile to understand what voltages-to-ground one can expect.

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#15507 - 10/20/02 12:43 PM Re: 480 volts to ground
The Watt Doctor Offline
Member

Registered: 12/23/01
Posts: 443
Loc: Mont Belvieu, TX
Thanks for the info, guys. Within the next few days, I should be talking to the guy that I mentioned above. So far, I haven't been very impressed with the amount of electrical knowledge that many of these state inspectors have shown. Most of them do not come from an electrical background, and have to learn as they go. From what I understand, this particular inspector knows his stuff. You can bet, that I will put him to the test, and if he passes, I will be placing many more calls to him in the future.

Testing 1, 2, 3,
Doc
_________________________
The Watt Doctor
Altura Cogen
Channelview, TX

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#15508 - 10/20/02 01:11 PM Re: 480 volts to ground
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
If you took a 277/480V wye xfmr and grounded what would normally be a hot phase instead of grounding the star-point, then you'd also have 480V to ground on the other two legs.

I can't think why this arrangement would be used, and with what I now know about the development of U.S. power systems (thanks guys!) is seems unlikely, but it's just a thought.

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