Delta to Delta Connections

[iceco1]

Feeding a 480v to 240/120v delta/delta transformer from a breaker in a 480v panel. Then coming out of the transformer to a fused disconnect then feeding a 240v old MCC section. I have a few questions regarding the connections. First, I'm a little confused on whether it's corner grounded and how to deal with that. There will be no 120v loads, so do I bring the X4 tap (between X1 and X2) to ground? Will this constitute the system being grounded? I know we have to bond the frame and install a bonding jumper, just a little unsure on the rest. Any help is appreciated.
Tom

 

[roc1]

Is this three phase?it sounds like a single phase transformer to me,Why do you need the secondary one phase grounded if you have no 120 loads?The reason for grounding three phase corner delta is mainly for transient voltage.It does keep voltage from floating some plants want a floating system so if you get a ground it will not cause destruction to system.They use lights across phases to ground so they will now when something goes to ground.One ground want hurt anything two and you have problems to say the least.
roc1

 

[iceco1]

3phase 480v incoming. 240/120v secondary. My question is how to get the secondary conductors referenced to ground?
Tom

 

[iceco1]

also 3 phase secondary....

 

[sandsnow]

Your looking at this requirement:

Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts

X4 would be bonded to case and X4 would be grounded.

Treat it the same as an Xo

 

[RUWired]

Tom,X4 is only to be used if you need 120 volt circuits.(x1-x4 or x2-x4)This xfrmr is set up as a 3-wire to 3-wire.480-240.Overcurrent protection for the secondary comes from the primary o.c. device in the 480 volt gear when used as a 3-wire-3-wire set-up..Just for info,what size xfrmr is this and what size primary protection are you using.
Rick

 

[infinity]

Tom,X4 is only to be used if you need 120 volt circuits.(x1-x4 or x2-x4)This xfrmr is set up as a 3-wire to 3-wire.480-240.Overcurrent protection for the secondary comes from the primary o.c. device in the 480 volt gear when used as a 3-wire-3-wire set-up..Just for info,what size xfrmr is this and what size primary protection are you using.
Rick

But with this setup wouldn't the secondary be ungrounded? How would a ground fault open the primary OCPD?

 

[don_resqcapt19]

Trevor,

But with this setup wouldn't the secondary be ungrounded? How would a ground fault open the primary OCPD?

A single ground fault on an ungrounded system will not open the OCPD. It just changes the ungrounded system into a corner grounded system.
Don

 

[RUWired]

The secondary phase ground fault would travel through the primary egc to the source of the primary ocpd and cause an overload.This is providing the device is properly set.

 

[infinity]

Your looking at this requirement:

Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts

X4 would be bonded to case and X4 would be grounded.

Treat it the same as an Xo

Wouldn't the voltage between the ungrounded "high leg" and the ground exceed 150 volts?

A single ground fault on an ungrounded system will not open the OCPD. It just changes the ungrounded system into a corner grounded system.
Don

I agree, but wouldn't an ungrounded system need to meet a number of stringent requirements in order to be used, such as the use of ground detectors? The OP didn't say if the existing system had previously used an ungrounded transformer with a ground detection system. So I was thinking that the old MCC may need to have a grounding means brought from the new transformer.

 

[Gmack]

We must examine the beginning of the article stated.

Other systems "permitted"

We have 277 volts to ground which exceed 150 volts to ground.

II. System Grounding
250.20 Alternating-Current Systems to Be Grounded
Alternating-current systems shall be grounded as provided for in 250.20(A), (B), (C), or (D). Other systems shall be permitted to be grounded. If such systems are grounded, they shall comply with the applicable provisions of this article.

FPN: An example of a system permitted to be grounded is a corner-grounded delta transformer connection. See 250.26(4) for conductor to be grounded.

(A) Alternating-Current Systems of Less Than 50 Volts Alternating-current systems of less than 50 volts shall be grounded under any of the following conditions:

(1) Where supplied by transformers, if the transformer supply system exceeds 150 volts to ground

(2) Where supplied by transformers, if the transformer supply system is ungrounded

(3) Where installed as overhead conductors outside of buildings

Outside buildings have a 300volt to ground limit [277] IMO.

Funny how the NEC talks out of both sides of its mouth at the same time.

 

[infinity]

Gmack,

You lost me. What does this have to do with the OP?

 

[don_resqcapt19]

Rick,

The secondary phase ground fault would travel through the primary egc to the source of the primary ocpd and cause an overload.This is providing the device is properly set.

No. The secondary current cannot return to the primary...it can only return to the secondary. If the secondary is ungrouned the first ground fault doesn't really do anything.
Don

 

[don_resqcapt19]

Trevor,

Quote:
A single ground fault on an ungrounded system will not open the OCPD. It just changes the ungrounded system into a corner grounded system.
Don

I agree, but wouldn't an ungrounded system need to meet a number of stringent requirements in order to be used, such as the use of ground detectors? The OP didn't say if the existing system had previously used an ungrounded transformer with a ground detection system. So I was thinking that the old MCC may need to have a grounding means brought from the new transformer.

Under the 2005 code 250.21 would require ground detectors. These only detect a ground and do not act to open the circuit.
Don

 

[Gmack]

Don, I was just responding to Larry/Trevor issue on 150v/to ground.

Something else though is bugging me here.

Corner grounded talk in this arrangement.

 

[infinity]

Trevor,

Quote:
A single ground fault on an ungrounded system will not open the OCPD. It just changes the ungrounded system into a corner grounded system.
Don

I agree, but wouldn't an ungrounded system need to meet a number of stringent requirements in order to be used, such as the use of ground detectors? The OP didn't say if the existing system had previously used an ungrounded transformer with a ground detection system. So I was thinking that the old MCC may need to have a grounding means brought from the new transformer.

Under the 2005 code 250.21 would require ground detectors. These only detect a ground and do not act to open the circuit.
Don

So with regard to the OP, if the new transformer has an ungrounded secondary than it would need a ground detection system to comply with the 2005 NEC or it would need to be grounded.

 

[Gmack]

Sorry Trevor, I was responding to you on the 240v to ground question you asked Larry.

I think I got it right this time.

My "point was" we have other permitted systems as stated in the NEC that exceed 150v to ground.

Thats all.

 

[infinity]

Sorry Trevor, I was responding to you on the 240v to ground question you asked Larry.

I think I got it right this time.

My "point was" we have other permitted systems as stated in the NEC that exceed 150v to ground.

Thats all.

Thanks for the clarification. I was starting to think that my mind was completely fired when I couldn't decipher your reference. Well, some will say that my mind is completely fried anyway. :wink:

 

[Gmack]

Trevor, here is a tip:

There will come a day when you wont care if others think your mind/MB
is tripped.

Your an electrician, which means you will have forgotten more than they will ever know. :wink:

I know I have forgotten much.

 

[RUWired]

Don, I was using 240.4(F) as reference when i said that primary protection was ok.I see that a fault on the secondary side would have a hard time opening the secondary ocpd if the x4 was not grounded.When i follow the secondary ground fault, the path goes all the way back to the service supplied xfrmr through the coil and then back to the delta-delta primary ocpd.I would hope that if this primary ocpd was protected per 240.4(F), that it would clear the fault.
Rick