grounding center tap 12kv delta trans

[Aretae]

I am an electrician being asked to install a ground that appeared on the latest reved single line. The transformer is a 480V wye step-up to 12kv delta. The ground wire is to be insulated and not be bonded to the equipment ground. Since the transformer resides in a common switchgear enclosure and is not insulated from it, I am concerned that a fault to the steel switchgear enclosue would not be a direct ground fault.

The ground rod for the center tap will reside a couple of feet away from the ground rod for the equipment ground. It appears to me that a fault to the metal enclosure would go out on the equipment ground and then through the earth to the center tap. I think there will be a voltage potential. Am I wrong to think that safety is at risk here? I would rather be wrong and be embarrassed then right and not say anything.

I can upload the relevant portion of the single line if necessary, I am not sure you can see the one I posted at:
http://files.engineering.com/getfil...-b466-d5a7c63700ac&file=a_12kv_iso_Ground.pdf

Thank you,
aretae

 

[LarryFine]

The ground wire is to be insulated and not be bonded to the equipment ground.

If, by "ground" you mean the primary neutral, that's correct. Connect absolutely nothing to it.

Since the transformer resides in a common switchgear enclosure and is not insulated from it, I am concerned that a fault to the steel switchgear enclosue would not be a direct ground fault.

The steel is still bonded to the supply neutral, so that's not an issue.

The ground rod for the center tap will reside a couple of feet away from the ground rod for the equipment ground.

No! First of all, no connection to the neutral. Second, you should not have separate, unbonded rods like that.

Bottom line: wire the primary of that transformer as if it were a Delta primary. No separate rod, no connection to the neutral. Just the lines. Supply neutral bonded as usual. Nothing else.

 

[Aretae]

If, by "ground" you mean the primary neutral, that's correct. Connect absolutely nothing to it.

I mean the secondary neutral. The primary feed is a 480VAC 3phase 4 waire from the utility. The secindary is the bumped up 12kv that started out as a three wire feed to another transformer (step-down), then suddenly showed up with the center tap ground, but no neutral or ground being run with the 12kv conductors through the conduit to its ultimate destination about 1700 feet away.

The steel is still bonded to the supply neutral, so that's not an issue.

OK

No! First of all, no connection to the neutral. Second, you should not have separate, unbonded rods like that.

I agree with this, they should have no potential between them.

Bottom line: wire the primary of that transformer as if it were a Delta primary. No separate rod, no connection to the neutral. Just the lines. Supply neutral bonded as usual. Nothing else.

The 480 incoming is grounded in the below grade vault outside the structure. This vault houses a utility 12kv step-down to 480 transformer, wye secondary. The utility is providing a fourth wire, but there is no "ground" being brought over. The feeders from the first transformer enter the client's main switchgear room, are metered, then through a 480 disconnect into the step-up to 12kv transofrmer. I wish I could upload the single line here.

 

[K2500]

I mean the secondary neutral. The primary feed is a 480VAC 3phase 4 waire from the utility. The secindary is the bumped up 12kv that started out as a three wire feed to another transformer (step-down), then suddenly showed up with the center tap ground, but no neutral or ground being run with the 12kv conductors through the conduit to its ultimate destination about 1700 feet away.

That sounds like a problem, the secondary should be grounded and bonded. Is this instalation governed by the nec or nesc?

 

[KevinVost]

If you are sending it 1700' and stepping down at the other end, you should not need a grounded conductor. I am confused as to why you are being asked to center tap the transformer. I agree with the others, gorunding and bonding requied on the secondary and you should be pulling a ground in with the conductors, unless you are using the 2005 code and are taking advantage of 250.32???(I don't know if that is the right section for seperate buildings, don't have my code book with me ) If you are under the 2008 code you must pull a EGC with the phases.


If this work is being done for a utility co, then the NEC has no jurisdiction.

 

[LarryFine]

I mean the secondary neutral.

But, you said:

The transformer is a 480V wye step-up to 12kv delta.

You would still bond the secondary neutral. Is there OCP?

 

[LarryFine]

The utility is providing a fourth wire, but there is no "ground" being brought over.

No, the utility rarely provides separate neutral and EGC. That 4th wire should be brought to the primary disco, and to local electrodes.

 

[Aretae]

If you are sending it 1700' and stepping down at the other end, you should not need a grounded conductor. I am confused as to why you are being asked to center tap the transformer. I agree with the others, gorunding and bonding requied on the secondary and you should be pulling a ground in with the conductors, unless you are using the 2005 code and are taking advantage of 250.32???(I don't know if that is the right section for seperate buildings, don't have my code book with me ) If you are under the 2008 code you must pull a EGC with the phases.


If this work is being done for a utility co, then the NEC has no jurisdiction.

This is a private residence with multiple support structures. The engineer is always referencing the CEC (California Electric Code) so I thought it must be based on the 2005 NEC, however, as there is a permit for each structure, I have asked for the code applicable to each structure. I am completely confused myself, that is why I finally asked in here. They told me the center tap is to "provide a reference".

 

[Aretae]

But, you said: QUOTE ARETAE - The transformer is a 480VAC WYE step-up to 12 kv.

This is why I am calling the 12kv the secondaary, should the 12kv be deemed a primary in a step-up configuration?

You would still bond the secondary neutral. Is there OCP?

There is overcurrent protection at both ends, again, I wish I could upload the single line.
Utility 12 kv to below-grade stand-alone step-down trans.
480 3ph 4w out into Main Switchboard drip loop section (MSB in separate structure)
Metering
480 disconnect (CB) 600 amp 3-pole
Air gap
Step-up trans
Medium Voltage fused disconnect
(All of the above, from drip loop on = MSB)
3-wire Out into the property in PVC
1700 feet to stand-alone step-down trans with primary fuses (grounded secondary - 480)
480 secondary out to a 600/3 CB
On to a transfer switch (Utility or 400kva generator).

 

[Aretae]

No, the utility rarely provides separate neutral and EGC. That 4th wire should be brought to the primary disco, and to local electrodes.

I need to look at his onsite today. The Main disconnect is 3 pole, I do not know the landing scheme for the 4th wire, unless it is landed to the grounding conductor shown for that section (not clear on the single line). With all sections bolted together, the entire cabinet would then be bonded to this grounded conductor, which in turn would be bonded to the equipment ground. Neither of which is bonded to the center tap of the 12kv side of the step up transformer. However, again, the equipment ground rod will reside about two feet away from the ground rod for the center tap ground of the 12kv. Back to the scenario of a 12kv short to steel frame.....

 

[Aretae]

upload portion of single line

upload portion of single line

I see this option to upload, I hope it works.

 

[jim dungar]

It seems you are using some confusing terminology.

You said the transformer was wired as a step-up delta. I believe you really meant it is wired as a step-up to 12kV 3-wire.

Then, you are talking about a center tap on the 12kV side. I believe you are really describing the center (neutral) point of a wye connected transformer.

Transformers windings are identified, per ANSI, as "H" for the high voltage windings and "X" for the low voltage regardless of primary and secondary.

On most liquid-filled wye-wye transformers the center (neutral) point for the high and low voltage sides are internally bonded together and then brought out to a bushing only as X0.

 

[Smart $]

...On most liquid-filled wye-wye transformers the center (neutral) point for the high and low voltage sides are internally bonded together and then brought out to a bushing only as X0.

One-line indicates the following:

TRANSFORMER ?T-1?
500KVA DRY TYPE
480V 3? 3W PRIMARY
12KV 3? 3W SECONDARY
NEMA 1​

 

[jim dungar]

One-line indicates the following:

TRANSFORMER ?T-1?
500KVA DRY TYPE
480V 3? 3W PRIMARY
12KV 3? 3W SECONDARY
NEMA 1
​

You got me on the cooling medium for the step-up unit.

But,the wiring diagram clearly shows, and describes a grounded-wye 12kV output with only 3-wires going to the load. This is not the same as a 3-wire delta output.

 

[LarryFine]

This is why I am calling the 12kv the secondaary, should the 12kv be deemed a primary in a step-up configuration?

No, you said the correctly. Originally, you said the 480v side was a Wye, which has three lines and a neutral. In your case, that is the primary, because that's the side you're powering.

I was saying that, when you have a Wye, which means you have a neutral, it should not be connected to anything; not the supply neutral, any electrode, or any enclosure. None, nada.

 

[LarryFine]

The Main disconnect is 3 pole, I do not know the landing scheme for the 4th wire, unless it is landed to the grounding conductor shown for that section (not clear on the single line). With all sections bolted together, the entire cabinet would then be bonded to this grounded conductor, which in turn would be bonded to the equipment ground.

That makes sense.

Neither of which is bonded to the center tap of the 12kv side of the step up transformer.

Why not?

However, again, the equipment ground rod will reside about two feet away from the ground rod for the center tap ground of the 12kv. Back to the scenario of a 12kv short to steel frame.....

There should not be a separate rod for the 12Kv neutral. It should be solidly bonded in the transformer or in the OCP enclosure (but not both) and tied into the electrode system.

A 12Kv short should be a short.

 

[KevinVost]

The one line does not say WYE or DELTA anywhere, it simply says 3 phase 3 wire for the 12kv. I would think that on the 12kv side of the transformer you have a neutral (grounded conductor) available (WYE configuration) in the equipment. If so, what he is showing you in the transformer is the main bonding jumper from neutral to ground. Even if you are not using the neutral in the feed to the other building you still need to make the connection happen at the transformer since you are making a seperately derived system (you do not have to make this connection at the OCD since this is all one piece of gear). This would explain the "provides a reference" statement made to the OP.

I would also agree with the others, that the ground rods should all be tied together since art 250 tells us that all electrodes shall be bonded together.

Please be careful of the use of the word "centertap" as it usually indicates the creation of a "high leg" in a 3 phase delta service.

 

[K2500]

Should the high side of the step down not have a neutral brought to it some how? It apears to be wye-wye, so would'nt an unbalance cause unstable voltage without it?

 

[Smart $]

...But,the wiring diagram clearly shows, and describes a grounded-wye 12kV output with only 3-wires going to the load. This is not the same as a 3-wire delta output.

It's not that clear to me

Winding configuration is neither stated explicitly nor schematically. I generally see it indicated similar to...

But then there is the annotation...

Who put it there? ...and what bearing does it have on the drawing???

 

[Smart $]

There should not be a separate rod for the 12Kv neutral.

Hmmm... if this is an outdoor xfmr, there can be. OP'er referred to support structures... but has yet to enlighten us as to indoor/outdoor conditions.