Corner grounded delta for HVAC unit

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acolella

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Years back, I had a call on a rooftop HVAC unit with an electrical issue that was the only piece of equipment in the building that was supplied from a corner grounded delta. This was the first time I had seen such a system. I understand the system but I'm not quite sure why it was applied in this situation.

The building had a 208Y/120 3Ø system. The rooftop unit was a 460V 3Ø unit. There was a transformer in the electrical room to step up the 208V to 480V for the rooftop unit. This transformer was dedicated for the rooftop unit. When I was troubleshooting I found that the B phase also had a ground wire ran to a ground rod in the floor. I was puzzled at first, as I'm sure many are when they first see this type of system without having any prior knowledge of one.

If I remember correctly, the transformer was a standard 480Δ-208Y/120 unit that was back-fed with 208V from the building ran to the secondary and 480V ran from the primary to the rooftop unit.

So, I have a few questions.
-Why use a corner grounded system here?
-Could an ungrounded delta been utilized?
-Why not use a 208Δ-480Y/277 unit and ground X0? Most likely not sitting on shelves anywhere?
-Is back-feeding a standard delta/wye transformer in this manner an acceptable practice?

Thanks!
 
ActionDave

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acolella said:
.......So, I have a few questions.
-Why use a corner grounded system here?
-Could an ungrounded delta been utilized?
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You either need to corner ground or install a ground detection system. Corner grounding is much simpler.
-Why not use a 208Δ-480Y/277 unit and ground X0? Most likely not sitting on shelves anywhere?
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Usually not sitting on the shelf, but not hard to get.
-Is back-feeding a standard delta/wye transformer in this manner an acceptable practice?
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Yes it is, although I think it's better to order a step up transformer with a wye secondary.
 
electrofelon

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It was probably done that way because someone either had the incorrect transformer lying around and didn't want to but the correct one, or they ordered the wrong one. So once you have a 480 Delta, as the other kitty said, you either gotta corner ground or not ground.

I've been there. Upgraded an old 600V service but the elevator still needed 600. There was a 480 Delta to 208/120 wye lying around so I used it and corner grounded it.

A few code cycles ago, wording was added stating that a reverse fed transformer must be marked as being suitable for such.
 
tom baker

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-Why use a corner grounded system here?
Maybe the HVAC was 480 and the service changed to 120/208
-Could an ungrounded delta been utilized?
Would need ground detection
-Why not use a 208Δ-480Y/277 unit and ground X0? Most likely not sitting on shelves anywhere?
Yes what was used maybe what was available
-Is back-feeding a standard delta/wye transformer in this manner an acceptable practice?

Its much easier to use a delta wye for a step up. They have always been available but like pointed out, someone may have step down laying aroung
 
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acolella

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Appreciate the replies! I didn’t even think about ground detection with ungrounded.
 
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acolella

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electrofelon said:
It was probably done that way because someone either had the incorrect transformer lying around and didn't want to but the correct one, or they ordered the wrong one. So once you have a 480 Delta, as the other kitty said, you either gotta corner ground or not ground.

I've been there. Upgraded an old 600V service but the elevator still needed 600. There was a 480 Delta to 208/120 wye lying around so I used it and corner grounded it.

A few code cycles ago, wording was added stating that a reverse fed transformer must be marked as being suitable for such.
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Okay, so I’m a little confused by this. So, you needed to derive a 600V system for the elevator? What was the 600V service upgraded to? How did you apply the 480 to 208/120 unit?
 
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priyankasethi

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Corner-grounded delta frameworks are likewise alluded to as grounded B stage frameworks, grounded stage administrations, and end-grounded delta frameworks. Grounded System A framework that has in any event one conductor or point purposefully associated with ground, either decidedly or through an impedance. Appropriate use of shaped case circuit breakers on three-stage frameworks, other than unequivocally grounded wye, especially on corner grounded delta frameworks, considers the electrical switch's individual shaft interfering with ability. Grounded Conductor No overcurrent gadget will be associated in arrangement with any conductor that is purposefully grounded, except if one of the accompanying two conditions are met: The overcurrent gadget opens all conductors of the circuit, including the grounded conductor, and is planned so that no post can work freely. In Grounded Conductors One shaft of the regulator will be allowed to be set in a forever grounded conductor, given the regulator is planned so the post in the grounded conductor can't be opened without at the same time opening all conductors of the circuit. 3 Corner-Grounded Delta Systems Data Bulletin Theory 2700DB0202R2/09 02/2009 A corner-grounded delta framework has the accompanying qualities: Grounding one stage settles the voltage of the other two stages to ground. I-Line Circuit Breaker Panelboards, 480 HVAC I-Line panelboards are UL Listed for use on 480 Vac grounded B stage frameworks with the properly appraised circuit breakers introduced when utilizing the B stage as grounded.
 
electrofelon

electrofelon

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Location
Cherry Valley NY, Seattle, WA
priyankasethi said:
Corner-grounded delta frameworks are likewise alluded to as grounded B stage frameworks, grounded stage administrations, and end-grounded delta frameworks. Grounded System A framework that has in any event one conductor or point purposefully associated with ground, either decidedly or through an impedance. Appropriate use of shaped case circuit breakers on three-stage frameworks, other than unequivocally grounded wye, especially on corner grounded delta frameworks, considers the electrical switch's individual shaft interfering with ability. Grounded Conductor No overcurrent gadget will be associated in arrangement with any conductor that is purposefully grounded, except if one of the accompanying two conditions are met: The overcurrent gadget opens all conductors of the circuit, including the grounded conductor, and is planned so that no post can work freely. In Grounded Conductors One shaft of the regulator will be allowed to be set in a forever grounded conductor, given the regulator is planned so the post in the grounded conductor can't be opened without at the same time opening all conductors of the circuit. 3 Corner-Grounded Delta Systems Data Bulletin Theory 2700DB0202R2/09 02/2009 A corner-grounded delta framework has the accompanying qualities: Grounding one stage settles the voltage of the other two stages to ground. I-Line Circuit Breaker Panelboards, 480 HVAC I-Line panelboards are UL Listed for use on 480 Vac grounded B stage frameworks with the properly appraised circuit breakers introduced when utilizing the B stage as grounded.
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That is hilarious. Did you have Google translate that?
 
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