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.
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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...
-- Scott
