# Corner Grounded C Phase Dilemma



## professor poptart (Mar 2, 2010)

I've only worked on a grounded delta once and it was a long time ago. 
This might be helpful.
http://www.kilowattclassroom.com/Archive/DELTAWYEPhasors.pdf


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## Mike Guile (Jan 14, 2010)

*wave*

Yeah. I've read that one twice today. Doesn't really help me.

Is this a 3 phase corner delta sine wave???


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## Sparkee (Sep 22, 2009)

I had a trouble call where my customer lost power to an elevator in a apartment building and when I checked power I had 240 phase to phase but no voltage from C phase to ground. It was when we had the rain storms so I assumed we lost the phase at the pole and called SoCal Edison. They checked and said the wiring was perfectly fine and they ground the C phase on purpose. It was news to me.


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## Mike Guile (Jan 14, 2010)

*spark*

Was it on a 3 pole breaker or 2 pole??


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## professor poptart (Mar 2, 2010)

Yea, I think thats right.
The diagram with the disconnect throws me a little.
I remember three phase conductors in the system I worked on, one of which read 0 v to ground.
Kind of like a neutral on a 120/240 system. Since it's grounded there is no potential to ground. Just don't get in series with it.


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## professor poptart (Mar 2, 2010)

The system I worked on definately used three pole breakers and contactors.


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## brian john (Mar 11, 2007)

Corner grounded delta.

"C" phase carries current just like a ungrounded delta or a 3 phase 4-wire wye. It does NOT carry current like a neutral, a neutral carries the unbalance of the load.

"A" or "B" phase short and the the OCP operates.

"C" phase is solidly bonded to ground.

No magic and no mysteries.


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## Sparkee (Sep 22, 2009)

Mike Guile said:


> Was it on a 3 pole breaker or 2 pole??


Three pole.


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## varmit (Apr 19, 2009)

This type service is very common on older industrial and larger commercial installations. Corner grounded systems exist as 230 volt or 480 volt and occasionally as 560 volt (rare in US). Today it is a common separately derived system for places that have a lot of 230 volt loads.

Treat your wiring methods as you would any other type of 3 phase system. Use all 3 pole devices on three phase loads. Single phase equipment will work fine also. Use only breakers for overcurrent protection. If you use fuses, by current code there is to be no fuse in the grounded phase: If the fuse ,in the grounded phase were to blow, the ground fault path to that load would be open.

The purpose of a corner ground is to provide a fault path to trip a fuse or breaker if an ungrounded phase shorts to ground. This will help to prevent equipment damage, reduce harmonics if there is a lot of noise on the system and reduce the chances of unbonded objects becoming energized. 

Your friends A/C unit: If the unit is 230 volt rated, it should work fine. If it is 208 there could be a problem with the controls.


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## Mike Guile (Jan 14, 2010)

*C Phase*

Thanks John and Varmit. 

The main thing I was having a problem with was solidly grounding the C phase at transformer.

So basically, you are getting line current on C phase even though it reads OV because it has equipotential to earth at transformer?? 

To me, initially, that = Big Explosion

The concept of grounding a phase is what I couldn't get. I still don't get it but for now I'll accept it and move on to the next mystery. 

Thanks

If anyone has any theory articles on how something can be a line and grounded at same time without exploding (short circuiting) please link if you can.


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## Mr. Sparkle (Jan 27, 2009)

brian john said:


> Corner grounded delta.
> 
> "C" phase carries current just like a ungrounded delta or a 3 phase 4-wire wye. It does NOT carry current like a neutral, a neutral carries the unbalance of the load.
> 
> ...


Ok, as a guy that needs to brush up on theory and stuff like this....

At the risk of looking like an idiot can you explain to me how C phase can carry current and be bonded to ground?

What is the purpose?

I've never seen a corner grounded Delta service.

Is there a good book on all this stuff?


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## LGLS (Nov 10, 2007)

Mike Guile said:


> Thanks John and Varmit.
> 
> The main thing I was having a problem with was solidly grounding the C phase at transformer.
> 
> ...


You can't wrap your head around it because you still have a 3 phase grounded Y system in your head for comparison. A delta has no grounded "X0"


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## raider1 (Jan 22, 2007)

LawnGuyLandSparky said:


> You can't wrap your head around it because you still have a 3 phase grounded Y system in your head for comparison. A delta has no grounded "X0"


A corner grounded delta has no grounded XO.

A center tapped 240/120 volt delta system has a grounded XO connection that is the center point of one winding.

Chris


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## bobelectric (Feb 24, 2007)

One reason for all this corner grounding, and bonded neutrals at appliances rather than 2 hots,neutral, ground was to save copper during "The War" So the "Ledgend" told me.


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## raider1 (Jan 22, 2007)

Mr. Sparkle said:


> Ok, as a guy that needs to brush up on theory and stuff like this....
> 
> At the risk of looking like an idiot can you explain to me how C phase can carry current and be bonded to ground?
> 
> ...


A "neutral" grounded conductor in a 2 wire 120 volt circuit carrys the same amout of current that the ungrounded conductor does.

The grounded C phase acts similar to a grounded conductor in a 2 wire circuit, it will carry the full current of the 3 phase corner grounded delta circuit and not the unbalanced current like in a multiwire branch circuit of a 3 phase wye system.

Chris


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## LGLS (Nov 10, 2007)

bobelectric said:


> One reason for all this corner grounding, and bonded neutrals at appliances rather than 2 hots,neutral, ground was to save copper during "The War" So the "Ledgend" told me.


In that case we should've gone 240 across the board then and there...


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## raider1 (Jan 22, 2007)

Mike Guile said:


> I'll admit right now, this is somewhat new to me, and I've been studying last few hours to try to absorb this topic.
> 
> HVAC friend calls me today to ask advice (he calls often and I normally help)
> He said he's putting in a new 3 phase roof unit and it doesn't work. He proceeds to tell me his voltages
> ...


Yes this is a corner grounded delta.



> Secondly, the thing that puzzles me still is the C phase. Is the bussbar grounded? Does it carry return current like a neutral?


The C phase is solidly grounded



> Can he screw up by hooking the lines up wrong at the motor??


No, the motor won't care what leg is the grounded leg as far as the motor is concerned it just sees 3 phases that have a voltage of 240 volts between them.



> Is this that widowmakers leg I keep hearing about? It reads 0V but it will shock the crap out of you if you touch.


No, a widowmakers leg, bastard leg or high leg comes from a center tapped delta 3 phase 240/120 system. The high leg is the phase that is not connected to the center tapped winding and usually reads 208 volts to ground.



> I've read a bunch of articles with this one attatched and diagram but I'm still not filling the last few pieces of puzzle in.
> 
> Is the C phase a Neutral or a Ground?


The C phase is a grounded conductor but not a neutral.



> How is motor getting 3 full phases?


Just because one of the phases is grounded does not mean that this is not a 3 phase system. You will still get 240 volts from phase to phase on all 3 phase, it just happens that one phase has the same potential as ground.

Chris


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## Mike Guile (Jan 14, 2010)

*Corner Grounded Delta*

Thanks Raider and LNS.

Thanks for explaining that to me at 6th grade level. Those were the missing links I needed to complete the puzzle. 

It's funny, it all seems so simple now. 

Now on to the next concept.


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## LGLS (Nov 10, 2007)

Mike Guile said:


> Thanks Raider and LNS.
> 
> Thanks for explaining that to me at 6th grade level. Those were the missing links I needed to complete the puzzle.
> 
> ...


PArt of the inability to wrap ones head around a concept is shedding the misinformation that you learned in the past, like, ground/ neutral current wants to get back to the power plant... it doesn't.

An SDS is it's own little power entity and it has no relation to anything else around it except itself. Like a battery. Once you "transform" power... it is it's own separately derived system and has no relation to the power plant, the substation, or other local separate systems.


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## Mike Guile (Jan 14, 2010)

*Crap*

Yeah. Never thought of it as an SDS and with your neutral analogy that made it click.

I'm sure my head is full of so much crap from the last 12 yrs of working around old timers that might or might not have known what they were doing. I'm on a quest to purge the misinformation. It ain't gonna be easy. 

The world is still flat right?


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## professor poptart (Mar 2, 2010)

Neutrals carry full circuit current in a 2 wire branch circuit!


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## raider1 (Jan 22, 2007)

professor poptart said:


> Neutrals carry full circuit current in a 2 wire branch circuit!


I think someone mentioned that earlier in post #16 and then again in post #18.:thumbsup:

Chris


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## brian john (Mar 11, 2007)

varmit said:


> This will help to prevent equipment damage, reduce harmonics if there is a lot of noise on the system and reduce the chances of unbonded objects becoming energized.
> 
> s.


How is a corner grounded system going to reduce harmonics?


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## brian john (Mar 11, 2007)

Mr. Sparkle said:


> Ok, as a guy that needs to brush up on theory and stuff like this....
> 
> At the risk of looking like an idiot can you explain to me how C phase can carry current and be bonded to ground?


Asking questions NEVER places anyone in a bad light, that's how I/we learn.



> What is the purpose?
> 
> Mainly in use in older industrial areas and in cases were 480 is needed. An EC will use a delta/wye transformer back-fed.
> 
> ...


I'll check on the book thing.

You can take a SDS Delta Wye and if you lift the XO bond you can ground any leg a NEC violation but ground/earth does not know the neutral from the phases, till you bond one of them.

http://www.powells.com/cgi-bin/biblio?inkey=4-0750611588-0

http://www.constructionbook.com/default2.asp?keyword=transformer


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## Mike Guile (Jan 14, 2010)

*Ending*

I talked with the HVAC guy today. He said his tech did one minor thing which is why the motor didn't work.

Hooked L1,L2, but for some odd reason skipped L3 and hooked it to L4 (which was a dummy terminal)

Evidently, he was having a brain fart day.


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## varmit (Apr 19, 2009)

brian john said:


> How is a corner grounded system going to reduce harmonics?


With one phase grounded, there is a path to help shunt noise to ground. I have had to corner ground previously ungrounded systems several times for this reason. Also, if there are several VFDs on a separately derived system, that is ungrounded, the regenerative harmonics can become a drive failure cause as the noise from each drive can become cumulative on the SDS power.


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## brian john (Mar 11, 2007)

varmit said:


> With one phase grounded, there is a path to help shunt noise to ground. I have had to corner ground previously ungrounded systems several times for this reason. Also, if there are several VFDs on a separately derived system, that is ungrounded, the regenerative harmonics can become a drive failure cause as the noise from each drive can become cumulative on the SDS power.



And this noise, generated by the load is going where?


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## varmit (Apr 19, 2009)

brian john said:


> And this noise, generated by the load is going where?


To ground.


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## brian john (Mar 11, 2007)

varmit said:


> To ground.


Is this magic you speak of, noise generated by a load from a closed source is going into the earth and you do this HOW???????????????.


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## LGLS (Nov 10, 2007)

brian john said:


> Is this magic you speak of, noise generated by a load from a closed source is going into the earth and you do this HOW???????????????.


Brian, don't you know ground is a giant sponge that absorbs all things that are bad, like harmonics, line noise, and dead people, and makes everything all nice-nice?


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## Bob Badger (Apr 19, 2009)

I do not think it has been mentioned but this could be a ungrounded delta with a ground fault.

The only way to know for sure is for the OP to look at the source to see if the C phase is intentionally grounded or not.


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## wildleg (Apr 12, 2009)

varmit said:


> With one phase grounded, there is a path to help shunt noise to ground. I have had to corner ground previously ungrounded systems several times for this reason. Also, if there are several VFDs on a separately derived system, that is ungrounded, the regenerative harmonics can become a drive failure cause as the noise from each drive can become cumulative on the SDS power.


I'm pretty sure that what brian is trying to tell you is that installing a ground does not make the harmonic voltages magically disappear. It would be helpful if you could describe what you are doing (exactly) that attenuates the noise in the systems that you have worked on. Do you have any pics of pq analyser or scope waveforms before and after that we could see, along with description of what was done ?


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## Southeast Power (Jan 18, 2009)

Mike Guile said:


> I'll admit right now, this is somewhat new to me, and I've been studying last few hours to try to absorb this topic.
> 
> HVAC friend calls me today to ask advice (he calls often and I normally help)
> He said he's putting in a new 3 phase roof unit and it doesn't work. He proceeds to tell me his voltages
> ...


I really do not understand why his three phase equipment will not work unless it derives control power form C phase to ground.
Otherwise, the equipment would not know that a phase is grounded.
Is it possible that the equipment is rejecting the utility power due to rotation? Could it be that simple?


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## Southeast Power (Jan 18, 2009)

Bob Badger said:


> I do not think it has been mentioned but this could be a ungrounded delta with a ground fault.
> 
> The only way to know for sure is for the OP to look at the source to see if the C phase is intentionally grounded or not.


Bob,
He said that he had 240 phase to phase and nothing C to ground. Ungrounded Delta can have a fault to ground (first one is free), the second one is where the trouble begins.


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## Mike Guile (Jan 14, 2010)

*dead*

you missed a few threads higher. HVAC tech put L3 on L4 which was dummy terminal. When he figured it out he hooked up L3 and everything worked fine. 
Since were still sort of on this topic (except for the mystical harmonic sponge problem above), If you measure 0V on L3 how would you know its a corner grounded delta and working correctly and how do you know L3 really is dead????? (other than touching it with your finger)


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## Mike Guile (Jan 14, 2010)

*re*

that didn't sound right let me rephrase. 

If you went on service call and got 0V on L3. How would you decipher that it is a corner grounded delta in perfect working order or L3 fuse at trans is actually out???


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## varmit (Apr 19, 2009)

Harmonics:

No, a grounded system will not make all of the harmonics and noise go away. It is an improvement over an ungrounded system. In a 208/120 system, grounding the neutral will serve a similar purpose, as the transformer windings being grounded serve as somewhat of a line reactor. With a DMM you can read the difference in imposed frequency between grounded and ungrounded.

Checking for corner ground or ground fault:

At the service or source of SDS, on load side, check phase to phase voltage, check lines to ground. Turn off main to prevent feedback from loads. Repeat voltage checks on the line side. If the readings are the same, with one phase reading 0 volt to ground, it is PROBABLY corner grounded. As a final check, trace the line back to the transformer to find the point where the phase is bonded.


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## Southeast Power (Jan 18, 2009)

Mike Guile said:


> that didn't sound right let me rephrase.
> 
> If you went on service call and got 0V on L3. How would you decipher that it is a corner grounded delta in perfect working order or L3 fuse at trans is actually out???


Thats why you should make it a practice to measure phase to phase. Phase to ground doesnt really tell the whole story.
We use to have lots of fuse boxes when I was doing service calls.
Hot to ground sometimes lies.
Phase to phase tells the truth..........


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## Mike Guile (Jan 14, 2010)

*readings*

If L3 was actually out at transformer wouldnt reading b-c still give you 240?
As well as if L3 was operating on corner grounded that would still give you 240 b-c ???? Since b -g was reading 240 then even if L3 was dead meter would still read 240V b-c wouldn't it??? 

Still confused


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## Mike Guile (Jan 14, 2010)

*difference*

what I would like to do over next 2 weeks is figure all this out

http://www.bmillerengineering.com/elecsys.htm

and be able to walk in any facility and know what system they are on by diagnostics/visual. I'll dig into transformers again today Tom Henry.

It's a tall order. Any new toys I need to buy that might help??? Any excuse to buy a new toy the better. I just bought a meggar, wiggie, T-Pro, own several basic multimeters, tics, etc... nothing major yet though (over $300).


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## Southeast Power (Jan 18, 2009)

Mike Guile said:


> If L3 was actually out at transformer wouldnt reading b-c still give you 240?
> As well as if L3 was operating on corner grounded that would still give you 240 b-c ???? Since b -g was reading 240 then even if L3 was dead meter would still read 240V b-c wouldn't it???
> 
> Still confused


True,
You you lost, call it transformer L3, you would have your full system. It would operate at 57% capacity.

We could get into the debate as to if that system is even required to have one corner grounded.


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## brian john (Mar 11, 2007)

wildleg said:


> I'm pretty sure that what brian is trying to tell you is that installing a ground does not make the harmonic voltages magically disappear. It would be helpful if you could describe what you are doing (exactly) that attenuates the noise in the systems that you have worked on. Do you have any pics of pq analyser or scope waveforms before and after that we could see, along with description of what was done ?


This is the politer response and would have been closer to my answer had the poster been inquisitive about harmonics and grounding. But I took his response as a statement one he tells customers. If that is the case then he is hurting our profession as much as electricians that violate the NEC. I work hard to make sure my responses are correct, if I am not sure I do some research on the subject. I hate the taste of crow.

While there are harmonic mitigation products that do involve grounding and will resolve harmonic issues, I see no way (and nothing I have read changes my mind on this), that simply bonding/grounding any phase will change harmonics levels.

Though if the poster has some reading or evidence of his statements I am more than open to read.


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## varmit (Apr 19, 2009)

I meant no arrogance in my statement. No, I would never tell a customer that doing anything will cure all problems. I do not claim to be an expert on physics. I do know that I have had success with improving equipment reliability by installing a ground reference on ungrounded systems. However, nothing is 100 % effective in 100% of applications.


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## brian john (Mar 11, 2007)

varmit said:


> I meant no arrogance in my statement. No, I would never tell a customer that doing anything will cure all problems. I do not claim to be an expert on physics. I do know that I have had success with improving equipment reliability by installing a ground reference on ungrounded systems. However, nothing is 100 % effective in 100% of applications.


Speaking from a UPS stand point (I seldom do anything with VFD's), the reason reliability may increase is due to how they reference the logic controllers.


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## Bob Badger (Apr 19, 2009)

Bob Badger said:


> I do not think it has been mentioned but this could be a ungrounded delta with a ground fault.
> 
> The only way to know for sure is for the OP to look at the source to see if the C phase is intentionally grounded or not.





jrannis said:


> Bob,
> He said that he had 240 phase to phase and nothing C to ground. Ungrounded Delta can have a fault to ground (first one is free), the second one is where the trouble begins.


I know what he said, do you know what I said? :laughing:

I was pointing out that using only a meter it is imposable to determine if the system is supposed to be a grounded delta system or if it is supposed to be ungrounded delta system that happens to have a ground fault at this time.

Mike would have to look for the presence or absence of a main bonding jumper to tell what kind of system this is.


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