- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
Consider the following (highly simplified) one line. There is a transformer inside the UPS, it takes in 480 and outputs 208/120V. There is a second feeder, by way of a second transformer outside the room, that serves the bypass inputs of the UPS.
View attachment UPS Setup.pdf
I will call the two feeders 1 and 2. Feeder 1 is created inside the UPS, and Feeder 2 is from the outside transformer to the bypass of the UPS. All feeders are from solidly grounded wye systems.
Would these voltages be normal, or would you be concerned?
Feeder 1:
A - N = 120V
B - N = 120V
C - N = 120V
A - B = 208V
B - C = 208V
C - A = 208V
Feeder 2:
A - N = 120V
B - N = 120V
C - N = 120V
A - B = 208V
B - C = 208V
C - A = 208V
Then we measured between sources: "f1A" below means "Feeder 1, Phase A".
f1A - f2A = 120V
f1A - f2B = 120V
f1A - f2C = 240V
f1B - f1A = 120V
f1B - f2B = 240V
f1B - f2C = 120V
f1C - f2A = 240V
f1C - f2B = 120V
f1C - f2C = 120V
Now, these readings were unexpected to us, and we all had varying theories on why we were seeing what we saw.
We expected to see a zero voltage between phases, indicating that the two phases were the same phase, even if f1A were zero volts to f2C - we expected to find three zeroes in there somewhere.
My feeling is that, despite the two transformers being derived from the same 480V source, and having interconnections via grounding conductors, that they were two independent sources and that readings between the sources were irrelevant and meaningless.
Another electrician felt that there had to be a winding open somewhere, a motor load connected inside the UPS somehow, causing an oddly high voltage between phases of the two systems.
In theory, only one source will serve the load panel at a time, so we decided to roll with what we had.
Thoughts?
View attachment UPS Setup.pdf
I will call the two feeders 1 and 2. Feeder 1 is created inside the UPS, and Feeder 2 is from the outside transformer to the bypass of the UPS. All feeders are from solidly grounded wye systems.
Would these voltages be normal, or would you be concerned?
Feeder 1:
A - N = 120V
B - N = 120V
C - N = 120V
A - B = 208V
B - C = 208V
C - A = 208V
Feeder 2:
A - N = 120V
B - N = 120V
C - N = 120V
A - B = 208V
B - C = 208V
C - A = 208V
Then we measured between sources: "f1A" below means "Feeder 1, Phase A".
f1A - f2A = 120V
f1A - f2B = 120V
f1A - f2C = 240V
f1B - f1A = 120V
f1B - f2B = 240V
f1B - f2C = 120V
f1C - f2A = 240V
f1C - f2B = 120V
f1C - f2C = 120V
Now, these readings were unexpected to us, and we all had varying theories on why we were seeing what we saw.
We expected to see a zero voltage between phases, indicating that the two phases were the same phase, even if f1A were zero volts to f2C - we expected to find three zeroes in there somewhere.
My feeling is that, despite the two transformers being derived from the same 480V source, and having interconnections via grounding conductors, that they were two independent sources and that readings between the sources were irrelevant and meaningless.
Another electrician felt that there had to be a winding open somewhere, a motor load connected inside the UPS somehow, causing an oddly high voltage between phases of the two systems.
In theory, only one source will serve the load panel at a time, so we decided to roll with what we had.
Thoughts?