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Ungrounded Delta/Corner grounded Delta. #201788
06/23/11 08:45 PM
06/23/11 08:45 PM
Yoopersup  Offline OP
Joined: Mar 2003
Posts: 840
Ungrounded 480 volt Delta & Grounding one corner of a ungrounded Delta. Just wondering how many of theses systems you guys see out there as the NEC is adding more info on them such as Grounded conductor (definitions), & 250.24C(3) Ect???
Thanks Yoopers

2017 / 2014 NEC & Related Books and Study Guides
Re: Ungrounded Delta/Corner grounded Delta. [Re: Yoopersup] #201798
06/24/11 02:05 PM
06/24/11 02:05 PM
gfretwell  Offline

Joined: Jul 2004
Posts: 9,165
I never saw ungrounded delta but I see corner grounded delta on pumping stations. It is pretty useful on a 3p only load situation.

Greg Fretwell
Re: Ungrounded Delta/Corner grounded Delta. [Re: Yoopersup] #201799
06/24/11 03:37 PM
06/24/11 03:37 PM
renosteinke  Offline
Cat Servant
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
I've got 40 acres of machinery that's primarily powered by ungrounded 480.

The place was built in 1992 - not that long ago. Alas, much of the equipment dated from WW2- but that's another story.

What I'm getting at is that the ungrounded delta is the devils' spawn of power distribution systems. Sure, it works fine, as long as ALL of your loads are PURE resistance or impedance loads. Heater coils and simple motors. Add just ONE fluorescent light to the mix and the gremlins come out to play.

With today's control circuits, VFD's, etc., you'll have some terrible power quality issues if you don't have that neutral; the transients have nowhere to go, and just keep circulating through the system. Let there be a accidental ground, and your voltages go to pieces; one bad light ballast can produce all manner of strange equipment faults throughout the plant.

Of course, the fact that some of our major loads are induction heating and welding doesn't help. For those who don't know, thats where we make the current alternate until it Hertz! From 60hz to 10,000hz, and at some serious amps. You just KNOW there are a lot of transients put back into the system.

We're just beginning a 'pilot project' to supply part of the plant with a grounded 'wye' system. I anticipate that a great deal of our daily problems will simply go away- at least in the area served by the new feed!

Re: Ungrounded Delta/Corner grounded Delta. [Re: Yoopersup] #201809
06/25/11 01:43 AM
06/25/11 01:43 AM
gfretwell  Offline

Joined: Jul 2004
Posts: 9,165
The advantage of hanging a corner ground on that delta is you don't need to add a conductor. Just put white tape on the grounded conductor. You needed an EGC anyway so that should be there. I would have had a problem approving an ungrounded delta in the first place unless the installer had a compelling reason. Usually this is going to be the heaters in a glass plant or something. Even then I would want to see non-critical supplies grounded. Like you say, spooky things can happen.

Greg Fretwell
Re: Ungrounded Delta/Corner grounded Delta. [Re: Yoopersup] #201815
06/25/11 09:55 AM
06/25/11 09:55 AM
Yoopersup  Offline OP
Joined: Mar 2003
Posts: 840
Soares grounding book has a lot of info on theses systems &
how they must be installed as well as grounded. I've worked with them over the years . Most Utilitys wwhere I'm located well Not give you a high leg delta but well give you a ungrounded Delta. Its them up to if you ground one leg or leave it as a ungrounded system. (ground detector lights are then required per 2011 NEC (250.21 (B)).if you leave it ungrounded.
I was just wondering if around the country more are being installed??

Re: Ungrounded Delta/Corner grounded Delta. [Re: Yoopersup] #201816
06/25/11 02:10 PM
06/25/11 02:10 PM
gfretwell  Offline

Joined: Jul 2004
Posts: 9,165
Lightning alone may make these ungrounded systems unattractive in Florida. It is just scary what kind of transients you can see on an ungrounded system. They would get grounded but it wouldn't be where you wanted it to be.

Soares has a good description of why we ground things right in the beginning of the book.

Greg Fretwell
Re: Ungrounded Delta/Corner grounded Delta. [Re: Yoopersup] #201828
06/25/11 10:36 PM
06/25/11 10:36 PM
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,708
Anaheim, CA. USA
Last Utility-Supplied Ungrounded Delta I worked with was back in 2001.
It was a 480V 3 Phase 3 Wire Ungrounded Delta, which supplied KVA to a 40K Sq. Ft. Commercial Office Building Complex, with individual Metered Gear Sections per Tenant Spaces.

We were performing Data & Security Systems Upgrades / Conversions for a well-known Financial Association Client (a "Mutual" Firm from the Northwest) at this Retail Branch.

Prior to 2001, used to come across Utility-Supplied Ungrounded Deltas infrequently in Commercial locations - most Services were 4 Wire Wyes - or 4 Wire Center-Tapped Deltas.
Utility-Supplied Systems on Industrial Projects were typically something Grounded - be it a Corner Grounded or Center-Tap Grounded 3 Wire Delta; not too many Ungrounded Deltas.

As for Separately Derived Systems (SDS), I have seen way too many Ungrounded Deltas - in the form of reverse connecting the typical 480V D x 208Y/120 Isolated Transformer. Very few of these had the required Ground Fault Indicator appurtenances.

Saw a few Grounded Delta SDS's installed.
These Projects had more Professional looking work done, as compared to the Ungrounded Delta counterparts (typically, but not unanimously!!!)

For me, the last Corner Grounded Delta I have dealt with was on a Design-Build Project we did back in October 2009.

A 480V SDS was needed for some Specific Equipment, per a T.I. on an older Industrial Building which only had a "Dual Metered System" - One 400 Amp, 120/240V 1ph. 3 Wire Service Section for "Lighting", One 400 Amp, 240V 3ph. 3 Wire Service Section for "Power"; both derived from a single 240/120V 3ph. 4 Wire Delta Transformer.

My Design was to Corner Ground the 480V Delta SDS, so as to derive a Stable Voltage-To-Ground.
The Plan checkers and Inspectors preferred this Corner-Grounded choice, along with the Client.
Field Installers were quite baffled by it, until explained thoroughly - after which it made sense!

Grounded Phase was "B" or Position X2, which oddly was the "Common Lead" between the ends of the outer Transformer Windings.

The issues with using an Ungrounded System come from the "Unstable" Voltage To Ground.
Higher Impedances between L-G result in Higher L-G Voltages - even though there is no "Solid L-G Voltage".
L-L Voltages are stable; L-G Voltages are unstable.

Issues include:
  • Premature Conductor Insulation Breakdown,
  • Premature Motor Winding Insulation Breakdown,
  • Failure to AC Filters,
  • VFD Component Failures,
  • Lack of complete understanding of operation by Installation and Maintenance Personnel.

By Grounding the System, the Voltage To Ground is stablized - however the Voltage To Ground _TYPICALLY_ becomes the System's Rated Voltage - and therefore will require fully rated devices.

Methods to Ground a System (via the "Output" of the Transformer):

  • Corner Grounded Delta:
    Ground One Output Phase, making that Phase a Grounded Conductor,
  • Center Tap Grounded Delta:
    Similar to the 4 Wire Delta, only not utilizing the Grounded Neutral.
    This results in a lower L-G Voltage, and no System Grounded Conductor,
  • 3 Wire Wye: Similar to the Center Tap Grounded Delta listed above.
    L-G Voltage is Phase Voltage;
    No System Grounded Conductor: just bond the Star Point to the Grounding Electrode System + Eq. Grounding Conductors.

Ungrounded Systems have their use and preferences - such as for Continuous Production Equipment; where stopping Equipment due to a Ground Fault will have disastrous results!

Back in the "Hot Chassis" days, a 1:1 Isolating Transformer would be used during Troubleshooting, so as to "Isolate" the "Live Chassis" from the Ground Reference.
The Internal Metallic Frame of these particular Sets were Bonded to the Ungrounded Conductor of the 120VAC Branch Circuit - via the Cord & Cap (Plug).

Others exist, but the above list is good for now... wink

-- Scott

Last edited by Scott35; 06/25/11 10:40 PM. Reason: found some spell-en air-ehrs

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!


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