Grounding a “back fed” delta wye

[Mustang125]

I’m redoing a space where the service is 208 but the rooftops are 480. They used a “standard” primary delta to secondary wye transformer but it’s working in “reverse”. Now that the 480 side is secondary I don’t believe it’s probably grounded? If so, do you have to corner ground it? Never actually corner grounded anything and it kinda freaks me out…what do you do on the “primary” XO?

 

[hillbilly1]

You can corner ground the secondary, or install a fault detector, and leave it ungrounded. Do not ground XO on the primary. I take it that it has been working, but if the HVAC contractor upgrades the indoor blower to a frequency drive, the drives may not like the corner grounded delta.

 

[drcampbell]

The rooftop HVAC units might already use variable-frequency drives, which might be why it was never properly grounded. The technology's been deployed for thirty-plus years now.

Hesitating to change anything before investigating seems wise.
Newer VFDs will likely refuse to start and display a "phase loss" fault.
Older VFDs might be damaged by the increased line-ground voltage.

 

[texie]

I’m redoing a space where the service is 208 but the rooftops are 480. They used a “standard” primary delta to secondary wye transformer but it’s working in “reverse”. Now that the 480 side is secondary I don’t believe it’s probably grounded? If so, do you have to corner ground it? Never actually corner grounded anything and it kinda freaks me out…what do you do on the “primary” XO?

A mystery to me, but this comes up often here. Using a delta/Y in reverse comes with many issues and is something I would never do as there are numerous things to consider and some may not be solvable or safe. For starters the xformer 480 side, if grounded, all the load side gear and equipment must be approved for corner grounding-not likely. Running the 480 as ungrounded presents a whole other set of issues. The correct solution is to use a 208 Delta X 480Y xformer. They are available. But this is still a lousy idea do to the cost of all the parts, labor and the power losses with this. Keep in mind you will need all the things required for a xformer including primary and secondary OCPD, GES, etc.
If I was the owner I would find out who screwed up and make the get them right 208 volt units.

 

[hillbilly1]

Usually what happens, is they have a transformer that’s laying around, or that’s the only one the supply house stocks, or they say it would be a special order that would take weeks or months to get. I’ve found a vendor that stocks them, and have fast delivery. Ideally though it works much better to have the correct one. A lot of electricians get confused enough with open deltas, corner grounding really messes with their heads, and can end in a catastrophe! LOL!

 

[GoldDigger]

Usually what happens, is they have a transformer that’s laying around, or that’s the only one the supply house stocks, or they say it would be a special order that would take weeks or months to get. I’ve found a vendor that stocks them, and have fast delivery. Ideally though it works much better to have the correct one. A lot of electricians get confused enough with open deltas, corner grounding really messes with their heads, and can end in a catastrophe! LOL!

And regardless of the grounding or lack of it on the delta side, you generally need to leave the wye point open on the primary side. If you connect the wye point to either the supply neutral or to earth ground you risk extremely large circulating currents (more than full load) in the primary and secondary windings when the wye voltages are unbalanced by an amount which would not even be noticeable on line to neutral loads.

 

[Mustang125]

Sorry the reply function doesn't seem to be working correctly or the "enter" to start a new paragraph.... but either way, thank you all for the help. the unit has been working fine of course and no drives that I can tell, they are pretty old but now we were gutting the space for a shell and the RTU's where existing to remain, but the inspector didn't like the pipes coming into the top of the transformers and it had no GEC to building steel... so now he wants us to fix it, but even he seemed a bit stumped on the best course of action without replacing transformers, he was trying to be fair sense it was existing to remain. if I'm not supposed to bond XO, where does the GEC land? Im worried about corner grounding it and it potentially effecting the control boards or something in the units, is a fault detector my only other option, assuming it cant remain how it is? If so, can someone post a link to one? never used one...


You can corner ground the secondary, or install a fault detector, and leave it ungrounded. Do not ground XO on the primary. I take it that it has been working, but if the HVAC contractor upgrades the indoor blower to a frequency drive, the drives may not like the corner grounded delta.

 

[winnie]

The GEC always gets landed somewhere on the 'derived' system, meaning the new isolated set of currents being created at the transformer. Since this transformer is being used 'in reverse', X0 is on the primary side, already part of the grounded supply system, and not to be be connected.

Your choices for landing the GEC are:

1) Corner grounding. Simply connecting to an H terminal
This is the simplest approach. Anything that works 'line to line' will have no problem with this. The risk is if you have _any_ ground referenced components. TVS filtering on variable speed drives is the big culprit here. If you corner ground make sure the customer knows that the transformers will need to be upgraded when the RTUs get changed, since modern RTUs often use variable speed drives.

2) Adding a grounding autotransformer. This is a specialized additional transformer that 'derives' and H0 terminal from H1, H2, H3. Then you ground the H0. This is rare for 'small' transformers, but possible. Search for 'grounding transformer'. I'd expect these to be custom and thus expensive for the size, and cheaper to simply replace the transformers with proper 208V:480V delta:wye units.

3) Going ungrounded and using a ground detector. Here is an example of a ground detector. One problem is that when you have a ground fault, someone is supposed to notice the lights and get the system repaired. How likely do you expect this to be in the location??
https://beaverelectrical.com/products/GIP-4
I believe the code for an ungrounded 480V system has requirements about 'conditions of supervision' or some such.

4) Replacing the transformer.

IMHO your choices are 1 or 4, depending upon the specifics of the equipment. If it has been working well for years 'ungrounded' then it can probably tolerate corner grounded.