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#129210 - 11/01/04 11:55 AM Ungrounded Delta  
Yoopersup  Offline
Member
Joined: Mar 2003
Posts: 822
Michigan
I went to a Job site Checked the voltage on a 480 volt service No neutral. From Each phase to Ground I got 277 volts. I called the power company asked them Why I got Voltage to Ground on a ungrounded System ?? . Perhaps I had a Y with no netral brought in. They told me its stray Voltage and would collapes When a load was applied?? Has any one ever heard of that.? If its a Y I told them a Neutral is required. I should be able to put each phase to Ground one at a time IF its a true ungrounded Delta Right!


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#129211 - 11/01/04 02:26 PM Re: Ungrounded Delta  
Bjarney  Offline
Moderator
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
 
Assuming that ø-g votlages are always balanced {or “zero”} in ungrounded-delta systems can be hazardous to your health. Phases have inherent capacitance to ground formed across system insulation; can be quite variable based on insulation quality and affected by transient and resonant conditions. ‘Neutral shift’ with respect to ground can an does result in temporary overvoltages well above system ø-ø voltage, wreaking havoc to “600-volt” insulation.

The characteristics of a low-voltage ungrounded-delta {or sometimes ungrounded-wye} systems are discussed in chapter 6 of Beeman’s Industrial Power Systems Handbook or chapter 1 of IEEE standard 142. [green book]

See discussion in https://www.electrical-contractor.net/ubb/Forum9/HTML/000043.html and short reference at http://6L6.net/localuser/bjarn/ecn.beemaIPSH6z..htm


#129212 - 11/01/04 08:48 PM Re: Ungrounded Delta  
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
See if you get any readings from L-G with a "Wiggy".

If Ungrounded, you should get a transient "pulse" reading on the Solenoid and Neon Lamp of the wiggy, at the moment the probes make contact - and meter is placed between Line and Ground (the Enclosure of the Switchgear will be fine for the "Ground" side).

The wiggy should react like a Capacitor is discharging rapidly through it.
Only a very quick ( < 100 ms) reaction will be observed, which decays rapidly to a "Zero" reading on the Wiggy.

If you are able to sustain a low level reading, then you may be looking at an Impedance Grounded System.

If the Wiggy holds nice and steady - similar to L-L connections, then it may be Impedance Grounded or Solidly Grounded system.

*** Disclaimer: The items above, are simply examples of some "Common" occurences which I have come across over the Years.
These examples are definitely NOT 100% Fool-Proof, Absolute, Written In Stone or anyway Default to all scenarios!!!
[Linked Image]
The information is only suggestions to consider!!! Please take all precautionary measures when investigating this system!

In all cases, please take Bjarney's post as sound advice - do not under think the resultant Voltage Readings from Meters on Ungrounded systems!!!
The most effective methods to determine what's what are Visual - Non Contact, determination of the System's Connections schemes (at the Transformer and the Service Equipment), usage of - and understanding of correct Metering Devices, and the most effective - verification via the PoCo.

If important enough, ask the PoCo Design Engineering staff, or possibly the Building Department, for existing Service information.
They normally help out whenever possible, but don't expect instant results / answers when the Offices are swamped and paperwork is stacked 4 feet high.
[Linked Image]

Lastly, if I sounded coarse in this message, I appologize. I am not trying to sound like a Deee-Oooccck, just making it well known to other readers there are extreme hazards involved, and danger silently lurks behind the deadfront sections of these systems' panelboards and gear sections - waiting for the chance to ... well, for lack of a better term:

Fry Up Some Huevos Rancheros!
and the "Huevos" being attached to some person's midsection!
[Linked Image]

Scott35


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

#129213 - 11/02/04 11:11 AM Re: Ungrounded Delta  
JBD  Offline
Member
Joined: Jul 2001
Posts: 599
WI, USA
Yoopersup,

Yes, you can measure voltages from line to ground on ungrounded systems.

It is just by coincidence that you measured these at 277V. I have seen them measured as high as line to line during steady state conditions. Theoretically the line to ground voltage on an ungrounded system can go up-to 2x the line to line voltage.

Digital meters are not the best tool to use when working with ungrounded systems. Their high input impedance can "interact negatively" with the system charging capacitance causing easily mis-interpreted readings. Using a lower impedance analog or solenoid type meter may be a better choice.


#129214 - 11/04/04 11:10 AM Re: Ungrounded Delta  
Radar  Offline
Member
Joined: Apr 2004
Posts: 349
Los Angeles, CA
Yoopersup,

I remember from my Navy days that shipboard AC electrical systems are commonly ungrounded delta systems. They are that way for one specific reason: Tactical reliability. ONE point in the ungrounded electrical system can become inadvertantly grounded and have no affect on the operation of the system. We would have big problems if a second point also became grounded, so we had permenantly installed ground detection circuits on all busses to detect any inadvertant ground that may develope so it could be cleared before another point became inadvertantly grounded.

So, to answer the question in the last sentence of your original post, IIF (read that IF and ONLY IF) you have a truly and completely ungrounded delta system, then you could connect any one single point to ground without affecting the operation of the circuit (no short circuit).

However, as already stated a couple of times above, static voltages to ground on even ungrounded systems are still very hazardous, and somewhat unregulated as there is no intentional ground connection to control the L-G voltages.

Hope this makes sense,
Radar


There are 10 types of people. Those who know binary, and those who don't.

#129215 - 11/05/04 12:11 AM Re: Ungrounded Delta  
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
JBD mentions the key point of the whole thread, which I should have addressed directly - yet once again I pulled a complete "Homer Simpson"...
and thus, let this be said:

"D'ohh!!!" [Linked Image]

To elaborate more on the topic of L-G Voltage testing, via Medium and High input Impedance Meters; vs. Ungrounded AC Systems (Ungrounded Deltas, to be exact!), here are readings from two different Projects containing 480 VAC 3Ø 3 Wire Ungrounded Delta Systems, derived from PoCo Transformers.

Tests from L-G using three types of Meters:

<OL TYPE=A>

[*] 1 DMM with a "Medium" input Impedance / "Sort-Of Low" Loading Effect,


[*] 1 DMM with a "Higher" input Impedance / "Much Lower" Loading Effect,


[*] 1 Voltmeter with a Low input Impedance / High Loading Effect... a "Wiggy": the typical General Purpose Solenoid type Voltage Tester.
</OL>

Project location #1:

Existing Building.
600 Amp, 480 VAC Service.
Apx. 100 Amp load total (Phase "A" around 90 Amps, "B" around 100 Amps, "C" around 95 Amps).
Apx. 200 Feet from Utility Transformers to Service Equipment (Apx. Feeder Length).

Tests taken at the Service Equipment / MDP between L-G (Grounded Metallic Enclosure).

Using Meter "A":
ØA to G: 245 VAC,
ØB to G: 260 VAC,
ØC to G: 230 VAC.

Using Meter "B":
ØA to G: 300 VAC,
ØB to G: 350 VAC,
ØC to G: 320 VAC.

Using Meter "C":
ØA to G: 0 VAC,
ØB to G: 0 VAC,
ØC to G: 0 VAC.

Project location #2:

Existing Building.
400 Amp, 480 VAC Service.
Apx. 150 Amp load total (Phase "A" around 120 Amps, "B" around 100 Amps, "C" around 150 Amps).
Apx. 80 Feet from Utility Transformers to Service Equipment (Apx. Feeder Length).

Tests taken at the Service Equipment / MDP between L-G (Grounded Metallic Enclosure).

Using Meter "A":
ØA to G: 170 VAC,
ØB to G: 160 VAC,
ØC to G: 200 VAC.

Using Meter "B":
ØA to G: 375 VAC,
ØB to G: 400 VAC,
ØC to G: 450 VAC.

Using Meter "C":
ØA to G: 0 VAC,
ØB to G: 0 VAC,
ØC to G: 0 VAC.

BTW: I also "Clamped" the DMMs with the Wiggy for another test on each system, and brought the Voltage Readings down to near zero.
"Clamped" meaning connect the leads in Parallel during tests.

Each "Wiggy" test resulted in the "Typical Capacitor Discharge" reaction (transient reading of no more than 100 ms, then no solenoid or neon lamp reaction).

Lastly, I do not remember exact values recorded at the time of the tests (this was done several years back, and those papers have since been archived somewhere!), but the listed values above are "kind of" close to what was recorded AFAIR
(As Far As I Remember!) [Linked Image]

Anyhow, that's my 2¢ contribution!

Scott35


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

#129216 - 11/08/04 09:06 AM Re: Ungrounded Delta  
Yoopersup  Offline
Member
Joined: Mar 2003
Posts: 822
Michigan
Thanks Guys
I think the best ways to check would be take a wire with a small fuse in line and go to ground on each phase(if trully ungrounded no effect if not Poof fuse gone) or if I had a small 277 volt load and put it from each phase to ground(if I get light grounded if not Trully ungrounded). Whayca think of Them apples????


#129217 - 11/08/04 10:32 AM Re: Ungrounded Delta  
JBD  Offline
Member
Joined: Jul 2001
Posts: 599
WI, USA
The fuse and wire jumper is part of the scheme used to determine a system's charging current. Just add an ammeter to measure the current.

A better method though, is to use a fused switch, that way you are not making your connections live and any possible fault is contained in a listed enclosure.


#129218 - 11/09/04 04:42 PM Re: Ungrounded Delta  
Radar  Offline
Member
Joined: Apr 2004
Posts: 349
Los Angeles, CA
Yoopersup,

Check out those static L-G voltages posted by Scott. When you make a grounding connection you might get enough of a current surge to blow your fuse. Try sticking a little impedance (resistance or inductance) in your grounding line to limit the effect.

Radar


There are 10 types of people. Those who know binary, and those who don't.

#129219 - 11/15/04 09:08 AM Re: Ungrounded Delta  
Yoopersup  Offline
Member
Joined: Mar 2003
Posts: 822
Michigan
Do they make a meter that would work in this situation. One that would tell if its Grounded or ungrounded fer sure???



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