ECN Forum Forums Technical Discussion Electrical Theory and Applications Discussion Thread For: 3Ø 3 Wire Corner Grounded Delta

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.