Hello all,

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Corner Grounded 3Ø 3 Wire Delta
https://www.electrical-contractor.net/ubb/Forum15/HTML/000078.html

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Scott35 S.E.T.

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Scott35 S.E.T.

I'll throw in a question.

Just how common were corner-grounded delta services, compared to ungrounded delta or the 4-wire delta with high-leg?

I have only ran across two Corner Grounded 3Ø 3 Wire Delta Systems in 20+ years (both 240VAC), but then again I don't work on very many Industrial Projects.

Did notice recently, in a few local Industrial zoned areas of Anaheim, a couple pole mounted setups which grounded the "Long Jumper" terminations.
These were three individual 1Ø pots connected Delta/Delta.
The "Long Jumper" is in reference to the longest intertie connecting wire, which runs from the far-left "X" terminal on the first pot, to the far-right "X" terminal on the third pot. The one that comprises "The Top Of The Triangle".

Scott35 s.e.t.

I have never seen this set up can anyone fill me in as to the advantages of this?

Very good diagrams to go along with this though.

Iwire,

It's a Grounded AC System. Advantage is the system has a Grounded Conductor, so Voltage to Ground is stabilized to somewhere around 240 VAC maximum, whereas an Ungrounded version of this same Delta system would have an unstable Voltage to Ground (plus no Grounded Conductor).

This system does not need the Ground Fault Sensing equipment of it's Ungrounded Counterpart.

It is mostly used on 240 VAC systems, whereas the Ungrounded version was commonly used for 440-480 VAC systems.

This system (the grounded Delta) was posted here for discussion, in order to point out it's main characteristics, which differ it from an Ungrounded Delta (or a 4 wire Delta).
Another reason for posting it was to try and cover it's Grounded Conductor.

Using a 4 Delta for example, the Grounded Conductor of that system would be the Center Tapped "Noodle", derived from the Midpoint of the "Lower" coil (between terminals X1 and X2 on the lower part of the Triangle).
That Center Tap works in the same fashion as the Center Tapped Noodle of a 1Ø 3W Secondary - because it is the same thing!

In this scenario, the Center Tap (Noodle) will be at 1/2 Potential of the coil (magnetic flux drives across the entire coil, but drives currents at 1/2 the full coil's potential between the centertap and one end of the coil).
Let me know if you need further information regarding this function.

Along with the center tap's 50% potential, the currents will flow in a "Balanced" fashion across the entire coil's length, and any imbalanced currents will be driven through the center tap, still at 1/2 potential.

On a 4 wire Wye, the Common Grounded Conductor is the point where all three coils are "Bound Together" at the star point. This Circuit Conductor does not act like the Center Tapped "Noodle", since it is more like an extension of each 1Ø 2 Wire coil, so the term Common is the more logical choice here.

This Common point is Grounded, and the Circuit Conductor becomes the Grounded Conductor for a 3Ø 4W Wye system.

BTW: If these systems were not grounded, the Circuit Conductors described above would function just as they would if they were Grounded. The difference being an unstable Voltage to Ground systemwide.

Now that we have briefly covered commonly found Grounded Systems, apply this to the 3Ø 3W Corner Grounded Delta.

In this system, we take one (and only one) Secondary Line output, and drive it to Ground Potential by Physically and intentionally bonding it to a Ground reference (in this case, earth ground is reference). This results in a grounded AC system, such as the ones mentioned previously.

In the example schematics, the Line output which would be identified as ØC becomes the Grounded Conductor. It's still ØC and currents flow just like usuall!

Voltage to ground is stabilized at 240 VAC (ØA and ØB will read 240 VAC to ground, and 240 VAC will be read between any L-L points)

Line C is a Grounded Conductor, so it must be treated as would a typical Grounded Conductor - hence the illustrations depicting the Grounded Conductor's placement and location throughout the complete system.

One scenario that should be pointed out for discussion is how the HID Ballasts are connected in a Multiwire-Like circuit.

Notice the useage of the common Grounded Conductor as to where the screwshells terminate?

Hope this brief text is helpful to everyone.

Scott35

I’ve found one utility where it seems to have been installed for new service about 40 years ago—Los Angeles Department of Water and Power. {The neighborhood also had the lovely ungrounded-delta 4800V primary distribution.} A pretzel plant was originally built with a 240V 800-amp 3ø 3-wire corner-grounded switchboard, with a hodgepodge of wiring over decades of existence. The plant-maintenance guys were puzzled by it, for some circuits had 2-pole 250V fused switches, served with two black conductors and a white conductor, but with ~240V (!) between all three conductors. In some areas, even though it was a grounded circuit conductor that should have been white, the installer put in non-white cable, apparently because it was three phase, a he’d be dammed if a motor would have a white wire to a three phase motor! To make matters worse, they had assorted 2-pole and 3-pole fusible switches, and some circuit-breaker panelboards that had 2-pole and 3-pole breakers, evidently depending on the free will of the installing contractor/electrician. For further confusion, even for corner-grounded service, motor-overload relays have to be installed in each phase, and connecting white wires to overload relays had the whole maintenance crew {about 8 guys; none over age 30} perplexed. It was hard getting them to understand that even “the white wire” to a motor needed to correctly read close to the same current as the “hot” leads.

Then to make it even more worse, because of load growth, a contactor installed a new 120/240V 1ø 3-wire 400-amp switchboard, but used no unique color code to differentiate between the single-phase system and the three-phase system, apparently because it was all “240 volts.” None of the maintenance crew had any significant electrical experience, so until we sat down with a big easel pad and markers and voltmeters testing energized equipment on the plant, it was a complete mystery. When I left, I think two guys had it figured out, and they promised to get it into others’ heads what readings to expect in troubleshooting.

To be honest I would have been puzzled too.

All I run into is 480/277 4W , 208/120 4W and 120/240 3W even when we have done places with many motor loads. The only time I run into 230 is when I need to boost it from 208 for loads that will not run at 208 Are these other systems still being installed?

A typical utility spec, from http://www.pge.com/customer_services/business/tariffs/doc/ER2.doc for 240/120V service, lists minimum 3-phase connected load of 5 HP—up to 500 kVA maximum demand.

Realistically, from copper-savings and starter-size economy, it would be common for a maximum connected/combination 230V load to be roughly 25-50HP. IMO, an approximate 240-versus-480V breakpoint seems to be about 30HP for single motors. Where 480V is served to a facility, it’s routine for 1HP and up motors to be powered at this voltage. Motor catalogs list stock dual-voltage [230/460V] 3ø motors starting at 1/8 HP.

From a safety aspect, note that 240/120V∆ has one phase 208V-to-ground. 240V corner-grounded delta is 240V-to-ground on two phases. 480Y/277V has, of course, 277V-to-ground on three phases. The relative difference in potential shock hazard for these typical service voltages is not markedly different.

When I first got started in the trade I worked for a company that did repair work for a coffee factory.

It had sensor lights on the wall in a "Y" formation and I believe it was 240 3 phase ungrounded, and if I remember my boss told me as long as the lights where dim all was OK and if one section of bulbs went bright we had a ground that had to be fixed.

Am I right so far?

I can see an advantage here for safety (no potential to ground) but what is the advantage if grounded.

Like I said I am used to 480 and if you follow code it seems like a safe and efficient way to power motors.

I hope my questions are not to basic but I would like to understand this.

I started to write a response explaining but then I figured Mike Holt does a much better job. You need real player to view.

Thanks for the link, I grabed that file but I will have to download realplayer later (I am on dial up and realplayer will be large)
and I guess I will need to put some speakers on my machine.

It's said that America is a land of great contrast, and I guess that extends to the variety of electrical systems that have been installed over the years as well.

I notice that Mike Holt says he likes the word "earthing," which of course is the standard British term for "grounding."

Great bunch of replies here, from all you members! Thanks for the stories and info!

Bjarney, LOL about explaining the Grounded Conductor to the Maintenance Crews! Great story!

Iwire (may I call you Bob? - makes more friendly this way),
Hope this thread covered the topic enough and helped explain the system.
I am not the best for describing things in a simple or concise way, so if you have more questions now than before, feel free to let me know.

Paul,
I like the term "Earthing" a little more than a general term like "Grounding", but I am biased by thoughts of other systems and what the term "Grounding" could include!

Could be very relavent to a common envelope / frame, which is by no means even near Earth Ground! (Automotive Electrical Systems, Aircraft Electrical Systems, Electronic Equipment, Space Exploration Equipment, etc.).
I have to keep in the context of Terrestrial based AC Power Systems, in order to keep from causing unnecessary confusion to people, in regards to Grounding methods.
If this was more accepted terminology, I would go all-out in descriptions!
(humor, but you get the point!)

Thanks again, everyone! Glad the material was enjoyed and presentable!

Scott35

From my point of veiw, a grounded Y is much safer. Working many years in very old industrial plants, if any of the ungrounded phases faults they are more likey to trip the breaker. With all the bad grounding in old machinery, it can be fun trying to find a fault. The old time electricans would always tap the machine with the back of the hand before starting to work around them. Sorry to say I'm one of them! On a 480 Y ungrounded system, I read 300 volts between two presses with shorted motor winding.

By Scott,


Scott

Of course you can call me Bob I agree it is more friendly.

Yes I learned a lot from this thread, and as far as being concise I suffer from that too, many times I have to cut my replies by 50% before I post them.

I have been looking at your train pictures a little at a time (dial-up) do you have any shots of the controls and machinery.

Talk to you later, Bob

Scott, I have one for you.

I noticed in the schematics for the grounded conductor, there is no fuse. I assume it is the same theory on fusing the nuetral in a 4 wire system, it is the return back to the source and you never fuse the grounded conductor. Hope I am on the right track here.

No comes the Question.

Why then do I see a corner grounded 3 wire system coming into a MDP which is OCPD is breakered. 1600A. Then it leaves as a branch circuit breakered and feeds a 400A bus duct. The bus ducts themselves have fused disconnects feeding machines that have fuses in them? All 3 phases fused.

Isn't this fusing of the GC a hazzard?

Note: system is 240V Bphase grounded.

Thanks

One more while I'm thinking about this.
I have a plant that maintains its own substation. 34000-4160VAC.
The 4160 feeds a 3w 480V CG delta in the rear of the plant. It has a seperate feed of 4160 feeding "pod" SP transformers 208V on the side of the building. 208 3ph 4 w.

Now then, in theory the grounded conductor of the 480 is electrically the same point as the "nuetral" on the 208 side.

Any shematics of this around? I still have a hard time figuring out why the 480 doesn't "bleed" into the nuetral of the 208 side.


Thanks again.

 
Dnkldorf—some interesting questions… If I follow your explanation, aside from medium-voltage distribution, there is a 480V corner-grounded system, a 240V corner-grounded system, and a 208Y/120V system essentially “under one roof.”

All have grounded-circuit conductors, but the only true neutral is in the 208Y/120 system—for it carries unbalanced current in multiwire feeders and branch circuits, where the “white wires” in the 480 and 240 systems do not.

About overcurrent protection in grounded-circuit conductors—in the case of running-overload protection in three-phase motors—there must be overcurrent-protective devices in each of three phase conductors—ungrounded or grounded. For this reason, if time-delay fuses are used for motor running-overload protection, then a fuse is required in the grounded-circuit conductor of each motor branch circuit—which is indeed a “white wire.’

On the 208-to-480V “bleed” problem, the only way that could happen would be to use a remote 480V grounded-circuit conductor as the ground electrode for which of the 208Y/120V-transformer XO terminals—which would be unlikely to occur intentionally or accidentally.

Bjarney,
first off thanks for your answers.

They 240 CG system, with the bus ducts is is a totally different plant. Thanks for the explanation about the grounded conductor being fused for the motor loads.

The second post was a Machine shop with the 480V CG in the rear of the plant and the 208 3p 4 w under the same roof.

If you could, Could you elaborate on the "remote" 480v Ground circuit conductor. I haven't look into this yet, but I believe the ground electrodes for the whole system should come from the sub-station, should they not? And then the ground should "tie" into the building steel, the tranformer grounds should bond to the building and so on. Is there a schematic "drawing" that would show how to properly ground such a facility like this and where the ground electrodes should be found?

Thanks again.

I happen to run into this type of 3 phase 240volt service today. I still don't understand the theory behind it but will chew on it till I do.
While I am learning, is any 3 phase 3 wire motor load OK to run on this system? Should this panel be used only for 3 phase loads? The panel I ran into today has a 2 pole 20 in it feeding a forced hot air furnace.

[This message has been edited by Joey D (edited 01-12-2005).]

 
Joey, it should not be a misapplication to serve a 240V 1ø 2-wire load from a 240V 3ø 3-wire system. Simply be aware that you may not find 120V-to-ground on the 2-wire circuit.

Hey, hey, hey... I remember this discussion thread! Just got the Mod. Notification today (wonder what happened to the notifications for the December posts, plus Iwire's last post?)

Anyhow, I'll address the replies in order (not too much to address, as Bjarney has done an excellent job already!).

*To Bob ("Iwire"):

There are a few pictures of the Locomotive's Operation Control Stand, but not any of the Relay Logic based Control Equipment.
There's an "On The List To Be Scrapped" SD45 T-2 which would make a great "Model" to get Control Equipment and Prime Mover pictures from. It's near me at a Scrap Yard.

Will let you know whazzup with these pics!

Dnkldorf:


Bjarney covered this one well. Only thing I could add would be the "Disconnection Of All Ungrouded Conductors + The Grounded Conductor Simultainiously" thingee - as regarding a Circuit Breaker + EXO Disconnect Switch.


The 480 VAC 3Ø 3 Wire Corner Grounded Delta is one SDS (Separately Derived System), and the 208Y/120 VAC 3Ø 4 Wire is another SDS... is this correct?

They are both Isolated from each other (so to say...) and there is only one connection to each system - and both are driven to "Earth-Ground" Potential.
There will still be a measurable Voltage between the two systems, and with this, a measurable level of Current will flow between them also.
The Voltage level between the two systems will fall well below 208 Volts - more like no higher than 25 Volts at the farthest points.

There should be some Schematics around, which cover these setups. If not, let me know so one can be generated.
*BTW: If the Transformer's Primary is a Grounded Delta with a Wye Secondary, that one would be an odd-ball setup and definitely not in the list of Transformer Drawings posted on-line here!
Let me know if you have this setup, and I will discuss the oddity with the "Elders" for reasons of "Whaddaheck!!!"


Joey D:


As Bjarney has mentioned, 240 VAC 1Ø 2 Wire loads and 240 VAC 3Ø 3 Wire Loads - Motor Loads, are applicable on this System.
Just don't expect to use any 120 VAC Loads on this System directly, without letting out smoke!


Scott35

Thanks guys. I have never seen this system before and other electricans I know had no idea. Just when you think you seen it all.

Check this out! I work as a hobby on private rail passenger cars and in some of them they use 120v 3ph corner grounded delta. All of the 120 volt circuit breakers are 2pole when you check pole to pole the meter reads 120v 2 of the poles will read 120 volts to ground (in this case car body) and the third phase will read 0v to ground. After doing commerical and industrial work for 30 years railroad electric can be strange.

Scott,
Thanks for your reply, I can't find a schematic on this setup anywhere.

This is my concern, if both systems are SDS should there be a common grounding point.
Or should both systems be electricly connected somewhere. I guess the point I am trying to make is if the "system"(4160) is grounded at one point and that being the reference, if you install ground rods at each other system, wouldn't there be a ground loop problem?
I have a hard time explaining things so bear with me here.

What I am looking for is a schematic showing how to effectively bond all systems, from the 4160 out of the substation, to the 4160X480 transformer, to the 4160x208 "pods", to the grounding electrodes, and finally to the building steel itself.(and the fences around them)
The reason I am looking for this is the building has what I suspect is a grounding problem, but with all these different "systems" involved, and my inexperience with multiple setups, I don't know how it is done or where to look.

I hope I don't sound like a bonehead here.

Can you help?