# 480 delta



## french connection!! (Dec 13, 2007)

install a 120/208 to 480 delta 15 kva transformer this week for a test facility bonded to XO and I'd like to know if there's any good equipment out there for ground fault detection .


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## wildleg (Apr 12, 2009)

yes .


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## Chris Kennedy (Nov 19, 2007)

You bonded the delta secondary to XO?


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## Big John (May 23, 2010)

If you bonded the secondary, you have a solidly grounded system and any breaker with GFP or GFCI capability will operate. 

Or am I misunderstanding?


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## Tsmil (Jul 17, 2011)

Just wanted to clarify.. Is the primary the 120/208 or is the 480 delta?


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## french connection!! (Dec 13, 2007)

120/208 v 3 phase step up transformer to 480 volt delta


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## Tsmil (Jul 17, 2011)

Check this link out. This is the stuff we have been using for delta systems.

http://www.i-gard.com/index.php?q=Default/home


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## Big John (May 23, 2010)

Aha. If you bonded your primary XO, take it back off; it should already be bonded at the feeder.

Unless you have a spec for ungrounded secondary, I wouldn't even mess around with that, I'd do a corner ground and call it good.

Otherwise, there are several options. They can be as simple as installing three pilot lights for fault indication (which nobody ever looks at), or as complicated as an off-the-shelf detection system with alarms and pulser fault locator. How much money do you want to spend?


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## french connection!! (Dec 13, 2007)

it's driving me nut the past few days ,I did it that way cause that what they want but I wanted to do a 480 v delta corner ground and then been told by others not to and bound X0 , when I ask all week long by calling different contractor and engineer , the answers are never the same .


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## Big John (May 23, 2010)

Think of the a sub panel termination. Would you put the bond screw in a sub panel?

The primary of the step up transformer is the same thing. Just because the bonding point is there doesn't mean you have to use it. The primary feeder is already bonded elsewhere.


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## varmit (Apr 19, 2009)

As stated above, DO NOT BOND THE LINE SIDE XO. If you connected a "neutral" to this terminal, it is already bonded at the building service (hopefully).

If this is only a 15 kva transformer, I would do a corner ground unless there are specs calling for something more elaborate (expensive). If you do not install a corner ground or a ground fault relay that will drop out the power on a ground fault, there is the risk of burning up equipment if a ground fault occurs, since an ungrounded system has no way to trip fuses or breakers except for an overload. 

With a corner ground, one phase in bonded to ground intentionally. If another phase shorts to ground, the the circuit protection (fuses or breaker) can trip.


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## Tsmil (Jul 17, 2011)

varmit said:


> As stated above, DO NOT BOND THE LINE SIDE XO. If you connected a "neutral" to this terminal, it is already bonded at the building service (hopefully).
> 
> If this is only a 15 kva transformer, I would do a corner ground unless there are specs calling for something more elaborate (expensive). If you do not install a corner ground or a ground fault relay that will drop out the power on a ground fault, there is the risk of burning up equipment if a ground fault occurs, since an ungrounded system has no way to trip fuses or breakers except for an overload.
> 
> With a corner ground, one phase in bonded to ground intentionally. If another phase shorts to ground, the the circuit protection (fuses or breaker) can trip.


I work with ungrounded delta systems on a regular basis. Could you please explain to me how a ground fault gives you the risk of burning up equipment?


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## varmit (Apr 19, 2009)

Tsmil said:


> I work with ungrounded delta systems on a regular basis. Could you please explain to me how a ground fault gives you the risk of burning up equipment?


If there is a ground fault, there is no direct fault path to clear the over current device, therefore the the wiring can essentially become a fuse that can overheat, melt, or damage the equipment until an overload or phase to phase short clears the circuit over current device.

I also work on a lot of ungrounded systems. I have seen this problem first hand a few times. It can be a challenge to repair with the wire melted to everything.


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## Tsmil (Jul 17, 2011)

varmit said:


> If there is a ground fault, there is no direct fault path to clear the over current device, therefore the the wiring can essentially become a fuse that can overheat, melt, or damage the equipment until an overload or phase to phase short clears the circuit over current device.
> 
> I also work on a lot of ungrounded systems. I have seen this problem first hand a few times. It can be a challenge to repair with the wire melted to everything.


I have only seen this upon the occurrence of a second ground fault. The first ground fault basically does the same thing as corner grounding the delta. Problem only occurs if you leave the fault and wait for the second fault.


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## varmit (Apr 19, 2009)

Tsmil said:


> I have only seen this upon the occurrence of a second ground fault. The first ground fault basically does the same thing as corner grounding the delta. Problem only occurs if you leave the fault and wait for the second fault.


I was speaking of utilization equipment, not electrical distribution equipment.

As an example: On some process heating equipment some of the smaller wires to an element, cause a high resistance short to ground due to insulation failure. Due to capacitive coupling there is enough resistance to set up a heating situation at the location of the high resistance short.
If the short is a low resistance short, as would be a corner ground, there would probably be no operational problem, but if the equipment was not bonded properly, there could be a voltage potential between the production equipment and the other structure or equipment.

This is why some locations that use ungrounded systems use additional equipment bonding methods. I do a lot of work in old poorly maintained plants where the above scenario can be a real problem.

At the distribution level, that is the purpose of an ungrounded system, to allow the system to continue to operate with a ground fault on one phase until it can be repaired.


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## Big John (May 23, 2010)

varmit said:


> ...On some process heating equipment some of the smaller wires to an element, cause a high resistance short to ground due to insulation failure....


 While I've seen this occur with old ungrounded systems, I wouldn't say it's a problem that exists because they're ungrounded, it's a problem that exists because they're old.

If you had a grounded system with a high resistance fault, the heating would likely be even worse. And actually, that's how I've found those in the past: A second hard ground fault develops and suddenly the high resistance faults become an issue.

My major concerns with ungrounded systems are
- The lack of maintenance to keep them operational. 
- The illusion of safety from people who think "ungrounded" means "shock proof"
- And that an arcing ground fault can cause multiple insulation failures throughout. I saw that happen where it was ultimately much more destructive than a single high current fault would've been in a grounded system.


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## varmit (Apr 19, 2009)

Big John said:


> While I've seen this occur with old ungrounded systems, I wouldn't say it's a problem that exists because they're ungrounded, it's a problem that exists because they're old.
> 
> If you had a grounded system with a high resistance fault, the heating would likely be even worse. And actually, that's how I've found those in the past: A second hard ground fault develops and suddenly the high resistance faults become an issue.
> 
> ...


I agree.


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