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!
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!