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#128413 - 03/08/03 03:56 PM Discussion Thread For: 3Ø 3 Wire Corner Grounded Delta
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Hello all,

This thread may be used to discuss the items posted in the Technical Reference Area under the topic:

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

To discuss something, create a "Reply To Topic" for this thread.

Feel free to ask some questions or make comments as needed.

Have fun everyone!

Scott35 S.E.T.
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#128414 - 03/14/03 08:18 PM Re: Discussion Thread For: 3Ø 3 Wire Corner Grounded Delta
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
No takers?

Did anyone like or dislike the stuff in this post?
Any suggestions?
Any suggested items to post?
Anyone want to contribute?

Scott35 S.E.T.
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#128415 - 03/15/03 03:00 AM Re: Discussion Thread For: 3Ø 3 Wire Corner Grounded Delta
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
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?

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#128416 - 03/15/03 05:59 AM Re: Discussion Thread For: 3Ø 3 Wire Corner Grounded Delta
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
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.
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#128417 - 03/15/03 12:29 PM Re: Discussion Thread For: 3Ø 3 Wire Corner Grounded Delta
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
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.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#128418 - 03/15/03 02:24 PM Re: Discussion Thread For: 3Ø 3 Wire Corner Grounded Delta
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
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
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#128419 - 03/15/03 06:05 PM Re: Discussion Thread For: 3Ø 3 Wire Corner Grounded Delta
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
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.

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#128420 - 03/15/03 06:28 PM Re: Discussion Thread For: 3Ø 3 Wire Corner Grounded Delta
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
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?
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#128421 - 03/15/03 08:25 PM Re: Discussion Thread For: 3Ø 3 Wire Corner Grounded Delta
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
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.

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#128422 - 03/16/03 04:53 AM Re: Discussion Thread For: 3Ø 3 Wire Corner Grounded Delta
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
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.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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