- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The step-up transformer symbol looks the same as the step-down transformer symbol, which is identified as a grounded wye.It's not that clear to me
grounding center tap 12kv delta trans
- Thread starter Aretae
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I know that... what I'm saying is there aren't any such symbols in the OP'er's one-line.
There's indication of something being grounded, but exactly what is quite unclear (IMO).
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Aretae
Member
- Location
- at home with both wives
The utility is feeding four wire 480 into the building. The 4th wire from the utility is landed in the section containing the 480 disconnect on a lug with a connection to a ground rod.
The T1 transformer is a wye primary at 480 volts. The switchgear submittals clearly show the X1, X2, X3, X0 taps on the primary side of the step-up transformer and state that the primary side is a wye configuration.
The secondary side is clearly stated to be a delta configuration and only shows H1, H2, H3.
The "center-tap" was never there until rev 16.2 of the single line drawing. The reason for its appearance, though perhaps not material, is the municipality hired an engineer to review the single line as the AHJ has no valid experience with 12kv on a private property.
It showed up and referenced a detail used for normal ufer grounds with bonding to the cold water pipe. When I questioned it the engineer said to use an insulated 2/0 conductor, land it on a separate ground rod and do not let it touch anything else.
The transformer is not outside and is not by itself, it resides in common steel enclosure. So I asked the engineer to provide a statement that is poses no life safety issues and violates no codes requirements - they did not respond. I contended that a short to the enclosure would travel out on the equipment ground and try to get back to the "center tap" through the two feet of earth between the rods and this poses a life-safety concern. I referenced 250.84 for solidly grounded neutral systems (probably should have cited others mentioned by you all).
Thank you for the help so far....it is getting a tad contentious at this end.
aretae
The T1 transformer is a wye primary at 480 volts. The switchgear submittals clearly show the X1, X2, X3, X0 taps on the primary side of the step-up transformer and state that the primary side is a wye configuration.
The secondary side is clearly stated to be a delta configuration and only shows H1, H2, H3.
The "center-tap" was never there until rev 16.2 of the single line drawing. The reason for its appearance, though perhaps not material, is the municipality hired an engineer to review the single line as the AHJ has no valid experience with 12kv on a private property.
It showed up and referenced a detail used for normal ufer grounds with bonding to the cold water pipe. When I questioned it the engineer said to use an insulated 2/0 conductor, land it on a separate ground rod and do not let it touch anything else.
The transformer is not outside and is not by itself, it resides in common steel enclosure. So I asked the engineer to provide a statement that is poses no life safety issues and violates no codes requirements - they did not respond. I contended that a short to the enclosure would travel out on the equipment ground and try to get back to the "center tap" through the two feet of earth between the rods and this poses a life-safety concern. I referenced 250.84 for solidly grounded neutral systems (probably should have cited others mentioned by you all).
Thank you for the help so far....it is getting a tad contentious at this end.
aretae
Aretae
Member
- Location
- at home with both wives
So true on symbols
So true on symbols
Yes, so I had to start asking. I can upload the T1 specs if anyone wants to see that.
So true on symbols
I know that... what I'm saying is there aren't any such symbols in the OP'er's one-line.
There's indication of something being grounded, but exactly what is quite unclear (IMO).
Yes, so I had to start asking. I can upload the T1 specs if anyone wants to see that.
Aretae
Member
- Location
- at home with both wives
Attached Trans spec
Attached Trans spec
I have uploaded the submittal page showing the primary and secondary sides
Attached Trans spec
I have uploaded the submittal page showing the primary and secondary sides
Aretae
Member
- Location
- at home with both wives
Wye or Delta
Wye or Delta
I uploaded the cut sheet so you can see it is a delta. It is in another post a little later in here, I am working my way backward.
Wye or Delta
I uploaded the cut sheet so you can see it is a delta. It is in another post a little later in here, I am working my way backward.
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Aretae
Member
- Location
- at home with both wives
wye or delta secondary
wye or delta secondary
I tend to be confusing at times, I will try and un confuse. The primary us 480 wye, the secondary is a 12kv 3 wire.
It really is a delta
Yes, I attached the submittal page so you may confirm.
It is a dry type Nema 1
thanks
wye or delta secondary
I tend to be confusing at times, I will try and un confuse. The primary us 480 wye, the secondary is a 12kv 3 wire.
It really is a delta
Yes, I attached the submittal page so you may confirm.
It is a dry type Nema 1
thanks
Aretae
Member
- Location
- at home with both wives
OK... indoor transformer... if it is to be grounded (as so indicated on one-line diagram), it will have to be corner grounded (since it is a delta-configured secondary with no center-tapped winding). Note that it is not required that the secondary be grounded... it is simply permitted. Yet the grounding of such must utilize a grounding electrode system (GES) compliant with Article 250, Part III. It is not required to be bonded directly to enclosure steel, the primary GES, or the local ground rod for the primary GES. However, in becoming compliant with Article 250 Part III, the secondary GES will at the very least be bonded to one of the same electrodes as the primary (perhaps building structural steel).
PS: I hope the service is grounded to more than just the enclosure and one ground rod
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Aretae
Member
- Location
- at home with both wives
Article 250 Part III
Article 250 Part III
Thank you. Very informative. I believe bonding to the same electrodes as the primary would alleviate any impedance in a fault and thereby render this a safe installation. I will re read Article250 Part III.
Article 250 Part III
OK... indoor transformer... if it is to be grounded (as so indicated on one-line diagram), it will have to be corner grounded (since it is a delta-configured secondary with no center-tapped winding). Note that it is not required that the secondary be grounded... it is simply permitted. Yet the grounding of such must utilize a grounding electrode system (GES) compliant with Article 250, Part III. It is not required to be bonded directly to enclosure steel, the primary GES, or the local ground rod for the primary GES. However, in becoming compliant with Article 250 Part III, the secondary GES will at the very least be bonded to one of the same electrodes as the primary (perhaps building structural steel).
PS: I hope the service is grounded to more than just the enclosure and one ground rod
Thank you. Very informative. I believe bonding to the same electrodes as the primary would alleviate any impedance in a fault and thereby render this a safe installation. I will re read Article250 Part III.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The original one-line shows the symbol for the step-up transformer the same as for the step-down transformer.I know that... what I'm saying is there aren't any such symbols in the OP'er's one-line.
There's indication of something being grounded, but exactly what is quite unclear (IMO).
Just because the transformer that has been supplied is a 12kV delta does not mean that it is what it was supposed to be. Several months ago I was involved in a similar situation, where the contractor supplied a delta step-up transformer that needed to be operated in parallel with an existing wye system. The approving engineer never looked at what had been submitted.
Yes, you are correct in saying, the only way to ground this delta is via a corner (I have never heard of a center-tapped delta in any output voltage other then 240/120V).
And as was mentioned towards the beginning of this discussion, the X0 bushing on the 'incoming' 480V side should not be connected, it mus be left floating.
Aretae
Member
- Location
- at home with both wives
OK... indoor transformer... if it is to be grounded (as so indicated on one-line diagram), it will have to be corner grounded (since it is a delta-configured secondary with no center-tapped winding). Note that it is not required that the secondary be grounded... it is simply permitted. Yet the grounding of such must utilize a grounding electrode system (GES) compliant with Article 250, Part III. It is not required to be bonded directly to enclosure steel, the primary GES, or the local ground rod for the primary GES. However, in becoming compliant with Article 250 Part III, the secondary GES will at the very least be bonded to one of the same electrodes as the primary (perhaps building structural steel).
PS: I hope the service is grounded to more than just the enclosure and one ground rod
There are no other ground points other than what you have cited - a bond to steel with the incoming 4th wire from the utility, and out to a single ground rod, unless the utility grounds the XO on their below grade 12kv to 480 step-down transformer, separate from the structure housing the main switchgear I am referencing.
If the secondary within the switchgear we are discussing (12kv) is not bonded to enclosure steel but only to an isolated ground rod, will not a 12kv fault to steel travel out onto the GES and then try to get back to its source (the secondary tap, I guess an end-point tap as there is no center-tap, as someone explained to me) through the few feet of earth separating the secondary tap ground rod and the GES/EGC ground rod? If this is the case, does this apply (the part "the earth shall not...."):
250.4 General Requirements for Grounding and Bonding.
(A) Grounded Systems.
(5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path.
Aretae
Member
- Location
- at home with both wives
Thank you. To the best of my knowledge, the XO on the primary side of this step-up transformer is not being landed. However, since the utility is providing a 3phase 4-wire incoming off of their 12kv to 480 step-down transformer, I am assuming they have grounded their XO in order to provide the 4th wire. This 4th wire will be landed to the steel enclosure, then bonded to a 2/0 conductor that is in turn a continuous run bonded to building steel and a final ground rod. This final ground rod will be separated by a few feet of earth from the ground rod to which the tapped secondary side is landed.
The single ground rod is not an NEC-compliant GES for the 12kV secondary (a separately derived system), period.
Second, 250.53(B) requires electrodes of different grounding systems to be spaced not less than 6ft apart. So the ground rod for the 12kV secondary must be at least 6' from the ground rod for the service.
Additionally, 250.56 requires that a ground rod that does not have a resistance to ground less than 25 ohms—which is unlikely—must be augmented with another installed electrode ( the easiest of which is another ground rod not less than 6ft away)
Therein lies a question: do we use two GES's, one for service and one for the separately derived system, or one GES for both? There is some disagreement whther the latter is compliant. IMO it is. Nonetheless, for the former, that means you would likely be required to have four ground rods, instead of two for the latter, and run another GEC to buiding steel—in addition to other present electrodes named in 250.52(A)(1 thru 7).
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Cold Fusion
Senior Member
- Location
- way north
"Corner grounded 12kv Delta" ??
Jim-
When was the last time you saw one of these? I've never heard of one.
I'd say it is as you mentioned, the wrong transformer got ordered, or they are trying to use an existing step down xfm.
Aretae -
NEC design guidelines for systems over 1KV are really sketchy. I guess the code panel figures you really need to know what you are doing for premisis wiring over 1KV.
250.20.D for separately derived systems, does not apply. But 250.20.C does and that sends you to 250, section X, Gounding of Systems and Circuits of 1kv and Over.
There is no legal (code) requirement to ground the 12KV system. However, there are design considerations - none of which (that I have ever heard of) suggest corner grounding 12kV delta
So, if the engineer is telling you to, "ground the center-tap on a 12kV delta", that meets the definition of not knowing what they are doing. Its time for the owner to hire a new engineer.
cf
Aretae
Member
- Location
- at home with both wives
"Corner grounded 12kv Delta" ??
Jim-
When was the last time you saw one of these? I've never heard of one.
I'd say it is as you mentioned, the wrong transformer got ordered, or they are trying to use an existing step down xfm.
Aretae -
NEC design guidelines for systems over 1KV are really sketchy. I guess the code panel figures you really need to know what you are doing for premisis wiring over 1KV.
250.20.D for separately derived systems, does not apply. But 250.20.C does and that sends you to 250, section X, Gounding of Systems and Circuits of 1kv and Over.
There is no legal (code) requirement to ground the 12KV system. However, there are design considerations - none of which (that I have ever heard of) suggest corner grounding 12kV delta
So, if the engineer is telling you to, "ground the center-tap on a 12kV delta", that meets the definition of not knowing what they are doing. Its time for the owner to hire a new engineer.
cf
Thanks, I think it may just be a case of me not feeling comfortable while at the same time not knowing exactly what I am doing. I appreciate everyone's input, I will let you know what the final resolution is.
aretae
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Me neither, and I wouldn't want to be the first."Corner grounded 12kv Delta" ??
When was the last time you saw one of these? I've never heard of one.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
But, why?
aretae (Gary),
Check the cut sheets again. The one you posted is more understandable as being "T-2", the step-down transformer.
Unless it is established that the premises 12kV system has a neutral, Article 250 Part X does not supplement or modify any of the requirements of the previous Parts of Article 250.
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