grounding center tap 12kv delta trans

[Aretae]

The shield it is not an EGC, IMO. But then again, it is a grounding conductor and 99% of the MV/HV installations I've been on did not run a separate EGC (i.e. in addition to the conduit or cable tray).

The engineer has us running EGC's out of the generator though we have well bonded cable tray from it to the ATS.

Do you think this will change the lack of EGC's as you have described?:

250.184 Solidly Grounded Neutral Systems.
Solidly grounded neutral systems shall be permitted to be either single point grounded or multigrounded neutral.
For systems over 1000 volts, the Code permits solidly grounded neutral systems that are either single-point grounded or multigrounded systems. For the 2005 Code, 250.184 was reorganized, and new requirements for the installation of single-point grounded systems were added. Circuits supplied from a single-point grounded system are required to have an equipment grounding conductor run with the circuit conductors, and this conductor is not to be used as a conductor for continuous line-to-neutral load.

 

[Aretae]

Detailed installation notes

Detailed installation notes

That's a major ouch. Every response that comes to mind isn't acceptable in polite company. "Excuse me, they are the engineers, we are the craft. The engineer of record comes up with the 'interpretation'; we put in the bolts. Even so, we are team players, if we knew of an industry standard installation method that was even close, we would suggest it in a second. There isn't one that fits what they are saying"

We have stated this explicitly more than once, we thinj the general should say soomething.

You are going to have a hard time making chicken soup out of this one. Okay, moving on:

The system is:
480V feeding a 500kva, 480Y/12460D xfm.
Three shielded conductors from xfm1 to xfm2
Xfm2 is a 500kva, 12460D/480Y
xfm secondary FLA (12460V side) is ~23A
Secondary fuses are 65E

"They" (AHJ?) want:
1. An EGC between the two transformers.
2. The 12460v system is to be grounded.

Engineer says:
1. Ground xfm1 12460V "centertap" (The "centertap" issue is a problem, but aside from that, separate ground rod - that is not connected to xfm case? I'm assuming there is an EGC from the 480V feeder source to xfm1 case)

Yes, but then they also reference a ground rod detail for both the 480 and the 12kv with a common bond to a cold water pipe, no more isolated ground rod per verbal direction.

2. Use the 12460V shields for the EGC. (Smart $ had a good comment here ? Is there a metallic conduit, cable tray, or cable sheath between the two xfms?)

1700' PVC

Some guesses and observations:
Conductors are #2 (probably the smallest you can easily buy)
Shield grounds are #12
At 5% impedance that gives <500A SSC
At 400A, 65E fuses will blow in 1 - 2sec. Makes one wonder how long a cable shield, by itself, would last at 400A.

This, sadly, is where I lack knowledge, the calcs for blow time and sustained fault current.

Smart $ had another good comment: ?? ask the AHJ for the method they will approve?

Gary - jump in here, verify, correct, or add. I?m pa3b now. I?ll look at it again after you respond.

cf

Thanks! I look back at the thread and wonder at the time it took you to piece it together, I should have done this for everyone.

 

[Aretae]

PA2b, PA3b

PA2b, PA3b

Hummm.... That's a pretty tough question from my side of the keyboard. And I'm pa2b - which doesn't always make for good answers.
jump in here, verify, correct, or add. I?m pa3b now. I?ll look at it again after you respond.[/FONT][/COLOR]

cf

Sorry, what is a PA2b, PA3b?

 

[jim dungar]

250.184 Solidly Grounded Neutral Systems.
Solidly grounded neutral systems shall be permitted to be either single point grounded or multigrounded neutral.
For systems over 1000 volts, the Code permits solidly grounded neutral systems that are either single-point grounded or multigrounded systems. For the 2005 Code, 250.184 was reorganized, and new requirements for the installation of single-point grounded systems were added. Circuits supplied from a single-point grounded system are required to have an equipment grounding conductor run with the circuit conductors, and this conductor is not to be used as a conductor for continuous line-to-neutral load.

250.184 is not and can not be made applicable to your 'existing' step-up transformer. Your 12kV system does not have a neutral point.

If the AHJ wants the 12kV system (not just the 'metal' parts) grounded, your only choice is to purchase and install an artificial neutral under 250.182. Only then, can you begin to apply any of the requirements of 250.180. You will also need to pull new cables per 250.184(A)(2) and (B)(8)

 

[Smart $]

The AHJ hired an engineer to review this. It was s/he who mandated the ground.

My, my... what a dilemma

Are you going through or cc'ing the AHJ with your RFI's. If not, I suggest you get them up to date on this dilemma. There is no easy fix for the discrepancies (except to run the 12kV system ungrounded).

 

[Aretae]

No neutral

No neutral

250.184 is not and can not be made applicable to your 'existing' step-up transformer. Your 12kV system does not have a neutral point.

So the corner tap does not constitute a neutral, though not run with the feeders?

If the AHJ wants the 12kV system (not just the 'metal' parts) grounded, your only choice is to purchase and install an artificial neutral under 250.182. Only then, can you begin to apply any of the requirements of 250.180. You will also need to pull new cables per 250.184(A)(2) and (B)(8)

A wealth of new information. I am sure the intent is not to create a neutral but then again, I am not exactly sure what it is for then. I thought a tap off of a transformer, whether a center tap wye or a corner tap delta creates a neutral, even if one makes no use of it. This is good for me to learn. I had better cease calling a neutral and stick with corner tap.

 

[Aretae]

cc the AHJ

cc the AHJ

My, my... what a dilemma

Are you going through or cc'ing the AHJ with your RFI's. If not, I suggest you get them up to date on this dilemma. There is no easy fix for the discrepancies (except to run the 12kV system ungrounded).

WOW. No I have not. If it is not ultimately resolved and all of us electricians think it is safe, think is an interesting word right now, then I will talk to the AHJ directly at that time.

 

[Cold Fusion]

250.184 is not and can not be made applicable to your 'existing' step-up transformer. Your 12kV system does not have a neutral point.

If the AHJ wants the 12kV system (not just the 'metal' parts) grounded, your only choice is to purchase and install an artificial neutral under 250.182. Only then, can you begin to apply any of the requirements of 250.180. You will also need to pull new cables per 250.184(A)(2) and (B)(8)

I don't have much to add. Most all has been said by others. The wrong transformer was ordered. Xfm1 should have a 480D/12460Y. It might be worth checking if the right one was ordered and the wrong one shipped. Be nice if someone would pay for a new xfm, provided the customer didn't mind waiting. I also don't have a clue why there wasn't an EGC speced for the 12460V raceway.

Jim pretty well has the reasonable responses nailed. Grounding transformer and pull an EGC. I had delusions of digging up the PVC and laying a DB conductor along side of the MV conduit. Then reality struck: The cost of digging up the conduit, without breaking it, is likely a lot more than pulling the conductors out and pulling back in with an EGC. If you are careful (and lucky), you could re-use the existing conductors

As Jim pointed out, the minimum size (ampacity) of the EGC is specified by code. I don't see any way the shields could provide the required size.

--- There is no easy fix for the discrepancies (except to run the 12kV system ungrounded).

For some reason ungrounded really bugs me. It's not code issues, it's design issues. I really would not want to put in an ungrounded system without ground detectors, and ground detectors require someone to look at them occasionally and know what to do, and from your posts, this would not be considered a supervised installation.

Jim -
Have you ever worked with un-grounded MV systems? I don't recall any. Lots of impedance grounded - mostly high impedance, some solidly grounded, but no un-grounded. Where/why would one use one?

What to do?
It appears the suituation has gone adversarial - which generally means everybody is screwed, even if the job finishes okay. Bummer

1. Start with the GC:
You are not going to "interpret" the engineers rfi responses. They don't provide a detail and there is no industry standard method - you are not putting it in. There is no upside for you to follow the GC saying you have enough information. If any thing goes wrong the engineer says, "We didn't say to do that."

A. There is no way to ground the 12460V as proposed by engineering. 12460D does not have a "centertap". Suggest engineering provide a spec and detail for a grounding transformer. This falls under the, "We're trying to help out" part - not the, "We're trying to tell you your job" part.

If they don't like that, then engineering provide a detail on how to ground the 12460D - using the three available terminals, H1, H2, H3 There is no industry standard method to do this.

B. If engineering/GC expect you to size the EGC, as Jim noted, point out 250.184.B.c. You consider this an engineering task to provide EGC sizing.

However, if they insist, you will size per 250.66, #8CU. And the three shields don't look like they meet that. And that means a re-pull - on their dime.

2. Enlist the support of the AHJ:
As smart $ suggested, get the AHJ to buy-in on the solutions.

A. Use the shields as the EGC conductors. Or re-pull with an EGC.

B. Any engineering specified screwy grounding method ("Screwy" defined as other than a grounding transformer or a new 480D/12460Y xfm1)

3. Put in what ever they tell you to, exactly the way they tell you to do it. The job has gone south. Document it all you can, get insulated from it.

4. Make sure you keep your short sword close at hand. It may be the only way to get completely free from this job

Gary -
I normally don't provide free advice. I don't mind helping out, it's just that I figure advise is worth what you pay for it. Keep that in mind when you are reading this.

So what do you suggest Gary do? :wink:

smart $ - I hope this meets your expectation. I don't expect to go any farther with the information at hand.

cf

 

[Cold Fusion]

So the corner tap does not constitute a neutral, though not run with the feeders?

--- I had better cease calling a neutral and stick with corner tap.

How about, "grounded conductor"?

cf

 

[Aretae]

Getting Paid

Getting Paid

Gary -
I normally don't provide free advice. I don't mind helping out, it's just that I figure advise is worth what you pay for it. Keep that in mind when you are reading this.

smart $ - I hope this meets your expectation. I don't expect to go any farther with the information at hand.

cf

I completely concur. The input and advice on this matter is invaluable to me and I believe everyone deserves to be paid to provide their expertise on something. All I can say is thank you for what everyone has provided.

 

[Aretae]

I don't have much to add. Most all has been said by others. The wrong transformer was ordered. Xfm1 should have a 480D/12460Y. It might be worth checking if the right one was ordered and the wrong one shipped. Be nice if someone would pay for a new xfm, provided the customer didn't mind waiting. I also don't have a clue why there wasn't an EGC speced for the 12460V raceway.

We would like to see an EGC and a grounded secondary. The DELTA has not been ordered, it is in the submittals under review by the engineers now.

Jim pretty well has the reasonable responses nailed. Grounding transformer and pull an EGC. I had delusions of digging up the PVC and laying a DB conductor along side of the MV conduit. Then reality struck: The cost of digging up the conduit, without breaking it, is likely a lot more than pulling the conductors out and pulling back in with an EGC. If you are careful (and lucky), you could re-use the existing conductors.

Only about 500' of PVC has been run, almost all of it now encased. I am putting together an electrician's "suggestion". We just do not want to see anyone get hurt and do not want to spend 3k for another engineer to review the engineered drawings ALREADY reviewed by an engineerr hired by the AHJ. We feel that the issues we have raised warrant a redesign by the engineers, not by us (not meaning all of you) guessing what is right.

As Jim pointed out, the minimum size (ampacity) of the EGC is specified by code. I don't see any way the shields could provide the required size.

We have asked the supplier if shields are ever sized for ground faults

For some reason ungrounded really bugs me. It's not code issues, it's design issues. I really would not want to put in an ungrounded system without ground detectors, and ground detectors require someone to look at them occasionally and know what to do, and from your posts, this would not be considered a supervised installation.

It bugs us too. We would like to see current flow right back to the transformer on a short to steel and cause the main to trip. Definetly not a supervised installation where anyone is going to be monitoring any high impedance grounded system.

What to do?
It appears the suituation has gone adversarial - which generally means everybody is screwed, even if the job finishes okay. Bummer

Yes, very sad. We started off with very subtle RFIS like " Please confirm no EGC with feeders", this gravitated to requesting a statement that the installation poses no life safety issues and violates no code citations made.

1. Start with the GC:
You are not going to "interpret" the engineers rfi responses. They don't provide a detail and there is no industry standard method - you are not putting it in. There is no upside for you to follow the GC saying you have enough information. If any thing goes wrong the engineer says, "We didn't say to do that."

A. There is no way to ground the 12460V as proposed by engineering. 12460D does not have a "centertap". Suggest engineering provide a spec and detail for a grounding transformer. This falls under the, "We're trying to help out" part - not the, "We're trying to tell you your job" part.

If they don't like that, then engineering provide a detail on how to ground the 12460D - using the three available terminals, H1, H2, H3 There is no industry standard method to do this.

B. If engineering/GC expect you to size the EGC, as Jim noted, point out 250.184.B.c. You consider this an engineering task to provide EGC sizing.

However, if they insist, you will size per 250.66, #8CU. And the three shields don't look like they meet that. And that means a re-pull - on their dime.

2. Enlist the support of the AHJ:
As smart $ suggested, get the AHJ to buy-in on the solutions.

A. Use the shields as the EGC conductors. Or re-pull with an EGC.

B. Any engineering specified screwy grounding method ("Screwy" defined as other than a grounding transformer or a new 480D/12460Y xfm1)

3. Put in what ever they tell you to, exactly the way they tell you to do it. The job has gone south. Document it all you can, get insulated from it.

4. Make sure you keep your short sword close at hand. It may be the only way to get completely free from this job


cf

I am and have been documenting everything, even following up verbal conversations with emails stating my understanding of them. I also asked the GC to have me removed from the job, no go (though we are really butting heads on this). I felt if they asked to have me removed then the company I am working for would not be cast is incapable of fielding a proper crew.

You have all been of tremendous aid, I wish I could repay. I will post my final information going to the GC and will let you all know how it works out!

 

[Smart $]

...smart $ - I hope this meets your expectation.

Beyond

 

[Aretae]

Grounded Conductor not neutral

Grounded Conductor not neutral

How about, "grounded conductor"?

cf

My ignorance again, I had always thought the grounded conductor was the neutral.

 

[Cold Fusion]

--- The input and advice on this matter is invaluable to me ---.

Glad we could help. Really hope we helped.

--- I believe everyone deserves to be paid to provide their expertise ---

Well, I got paid.:smile: Had to think through how I would deal with this. Would I tell them you're buying a new xfm? Would I tell them grounding xfm? How strong would I get about the re-pull with an EGC - or would I except the shields as an EGC? So when this come up, now I already know.

Mostly I just wanted to warn you that the advice may be worth what you paid for it. Nothing says I'm not clear screwed up - truly, that happens regularly

--- All I can say is thank you for what everyone has provided.

First, like I said, I really hope we helped and didn't just screw you up.

Second, you're welcome.

The way I read your post 71, you are doing as good as you can with what you have to work with. I'll be interested to see how it comes out

cf

 

[Aretae]

Final submittal to GC

Final submittal to GC

Glad we could help. Really hope we helped.

Well, I got paid.:smile: Had to think through how I would deal with this. Would I tell them you're buying a new xfm? Would I tell them grounding xfm? How strong would I get about the re-pull with an EGC - or would I except the shields as an EGC? So when this come up, now I already know.

Let's hope a complete design negates the need.

Mostly I just wanted to warn you that the advice may be worth what you paid for it. Nothing says I'm not clear screwed up - truly, that happens regularly

I paid for it in donated time to the company I work for. I have put about 16 hours into the questions and asking/reviewing all I can. I did not include any of this time in my time sheet, so I actually had my first sub-40 hour week. I do this because I really want to know the safest way to do this work, where else is there an online "class" like this? Therefore, again, the advice is invaluable.

First, like I said, I really hope we helped and didn't just screw you up.

Second, you're welcome.

The way I read your post 71, you are doing as good as you can with what you have to work with. I'll be interested to see how it comes out

cf

I have put together one single line with all of the details taken from separate drawings that are referenced on the original single line, then inserted questions/thoughts based on everyone's input from here in RED and highlighted apparent contradictions in RED.

I have again asked them to remove me from the project if they think I am not performing to the standards of an installer. The neat thing about being at the stage of
"willing to walk away" makes the threat of removal because of speaking out a non issue.

 

[Aretae]

Changing T1

Changing T1

The first email came out, they are changing T1 to a delta/wye. We will see what they do with the EGC. Thanks Everyone!
gary