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

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?