3 phase 277/480 problem

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danstewart75

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Location
Ga
Occupation
Electrical
Power company recently replaced transformers and now that businesses are opening back up.
Customer had lost some lights. This is a 277/480 volt system.
Measured voltage on tenant panel 480 A-B, A-C, B-C.
But read 480 A to N, 0 B to N, 480 C to N. (Read the same voltage measured to Ground)

Went to electrical room, couldn’t really get into main ( no AF gear) but did remove one dead front and 4 - 200 amp breakers all read the same as the aforementioned tenant panel ( but I read to Ground as Neutral wasn’t safely accessible, but neutral SHOULD be bonded at main... ).

120 volt lighting and receptacles seem to work, the mall is 95% closed, so it’s not possible to enter all tenants, and no complaints been made yet but the one. 120 seem to all be working in tenant I visited ( couldn’t check everything, overly inventoried dress store, just checked panels). And honestly with 480 not reading proper I did not look too much down stream.

Supposedly Ga Power changed the transformers ( underground vault , blew up) two days before the corona shut down order. Word is the tenants had no issues but the one I initially investigated, yet I find this voltage anomaly on 6 or 7 disconnects in the electrical room. The main switchboard feeds straight underground maybe 50’-75’ to the utility transformer vault.

Thoughts? Reminds me of the 240 open Deltas we have in industrial parks around here. Is this a faulted/open situation effectively creating an open Wye? I’ve lost phases before ( fuses, breakers, burnt wires) but what buffalos me here is where’d the neutral go?
 
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oldsparky52

Senior Member
Location
Wilmington, NC USA
Sounds like B phase is grounded and the neutral is floating. You said the neutral wasn't safely accessible, did you see the neutral? It sounds like a grounded Delta from the voltage readings. I'd really chase down that neutral.
 
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danstewart75

Member
Location
Ga
Occupation
Electrical
Something wonky is going on for sure. Tenant had Ga Power out and they said blown fuse on B phase. I honestly don’t know how the lineman/meter man could not see and understand the problem with these voltage readings.
Also, 4 parallel feeds from vault to Main.
If I could see how they hooked it up the voltage would make sense, but I haven’t been able to wrap my head around this.
 
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danstewart75

Member
Location
Ga
Occupation
Electrical
And the mall manager wants an explanation.....Ga Power doesn’t necessarily have the best engineers.....and some are combative and condescending to us lowly contractors ....the horror stories I could tell, but I’m wandering.....

Hopefully tomorrow I can go back out,with AF suit, kill the main and read line side of main.
 
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hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
Definitely need the poco back out there, both scenarios are possible, corner grounded delta more likely, unless they left the neutrals disconnected, Since transformers being plural, they may have pole mounts in an enclosure, with buss bars, and that may be where it got screwed up.
 
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GoldDigger

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Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
ActionDave said:
Sounds like the POCO hooked the service back up as a corner ground delta instead of a wye. That would mean all the 120V would be fine, but any 277V lighting would be toast.
Click to expand...
Indistinguishable, from readings just on the phase lines and ground without access to a hypothetical neutral, would be 480Y/277 with one "corner" grounded and the neutral not connected at all. One possible way of getting there would be an accidental hard ground fault on the B phase which opened the neutral in some way.
 
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Jamesco

Senior Member
Location
Iowa
Occupation
Master Electrician
danstewart75 said:
This is a 277/480 volt system.
Measured voltage on tenant panel 480 A-B, A-C, B-C.
But read 480 A to N, 0 B to N, 480 C to N.
Click to expand...

3 transformers wired in a WYE configuration.
"Measured voltage on tenant panel 480 A-B, A-C, B-C.' OK.

"But read 480 A to N, 0 B to N, 480 C to N."

The Lineman connected the neutral conductor on the B ph instead of on the common WYE connection.

Both the hot B ph conductor and neutral conductor are connected to the hot B phase bushing.

The OP didn't measure any 277V. The neutral conductor was not connected to the common WYE connection of the 3 transformers.
 
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