neonbkw
Member
- Location
- central PA
Hello. Going to be doing some troubleshooting on a brand new service this week and I'm looking for some insight. Here are the specs.
We installed an 800 amp 480 volt 3 wire ungrounded service, due to POCO only having 2 primary lines on the pole and using an open delta config. Service disconnect is a 800 amp main in an 84" I-Line panel with no neutral bar. Grounding system consists of Ufer, 2 rods, building steel, and 50 feet of bare 2/0 in a trench. Ground system resistance tested at 1.9 ohms.
There are currently only 3 breakers being utilized in the 480 volt I-Line panel.
The first is a 400 amp feeder to a 225 kva 480 delta to 120/208 wye transformer to feed a second I-Line panel which serves as a MDP for most of the existing loads.
The second is a 40 amp feeder to a 30kva 480 delta to 120/240 delta transformer to feed a QO panel for a few pieces of equipment that wouldn't tolerate 208.
The third is a 20 amp branch circuit to the required ground fault detection unit which is a UL listed manufactured box with 3 light bulbs.
Ok here is the confusing part. On the day of POCO transfer from the old service to the new, we hooked up a rented 65kva generator to the 480 volt service panel thru a spare 100 amp breaker, while having the main LOTO'ed for POCO safety. After making a few last minute connections we had the entire facitily on generator power. Of course all production was halted for the day but the front office was able to remain open. Mission accomplished, or so we thought. Upon POCO's completion of work we successfully transferred load back off generator and onto utility and started to wrap up for the day. I switched on the ground fault monitor and it showed a fault on phase A! I verified with a meter and had 480v B-g, 480v C-g, and 20v A-g. Very strange.
It was quitting time at the plant and they don't mess around. We are essentially booted out the door at 5 o clock. The only thing I got to check was for a ground fault in the monitor itself due to a blown and shorted bulb or something along those lines, but the voltages remained the same with that breaker turned off.
What I don't understand is if there is a ground fault in my equipment why didn't the generator breaker trip? We were operating with a solidly connected EGC from gen to panel. Amps were around 30 on all 3 legs on gen power.
Doesn't add up, unless the fault is on POCO side. I really could not see from the ground that they had intentionally corner grounded the delta, but they did have a 4th wire run from their MGN to the CT cabinet. There is also a small device with 4 wires on the pole under the transformer bank. Reminds me of a small round SPD like you would see hanging off of a panel. 3 black wires connected to the 3 phases and a green to the MGN.
I do plan to megger both of my transformers and their feeders before I call the POCO engineer and try to fight it out with him.
Sent from my LG-V410 using Tapatalk
We installed an 800 amp 480 volt 3 wire ungrounded service, due to POCO only having 2 primary lines on the pole and using an open delta config. Service disconnect is a 800 amp main in an 84" I-Line panel with no neutral bar. Grounding system consists of Ufer, 2 rods, building steel, and 50 feet of bare 2/0 in a trench. Ground system resistance tested at 1.9 ohms.
There are currently only 3 breakers being utilized in the 480 volt I-Line panel.
The first is a 400 amp feeder to a 225 kva 480 delta to 120/208 wye transformer to feed a second I-Line panel which serves as a MDP for most of the existing loads.
The second is a 40 amp feeder to a 30kva 480 delta to 120/240 delta transformer to feed a QO panel for a few pieces of equipment that wouldn't tolerate 208.
The third is a 20 amp branch circuit to the required ground fault detection unit which is a UL listed manufactured box with 3 light bulbs.
Ok here is the confusing part. On the day of POCO transfer from the old service to the new, we hooked up a rented 65kva generator to the 480 volt service panel thru a spare 100 amp breaker, while having the main LOTO'ed for POCO safety. After making a few last minute connections we had the entire facitily on generator power. Of course all production was halted for the day but the front office was able to remain open. Mission accomplished, or so we thought. Upon POCO's completion of work we successfully transferred load back off generator and onto utility and started to wrap up for the day. I switched on the ground fault monitor and it showed a fault on phase A! I verified with a meter and had 480v B-g, 480v C-g, and 20v A-g. Very strange.
It was quitting time at the plant and they don't mess around. We are essentially booted out the door at 5 o clock. The only thing I got to check was for a ground fault in the monitor itself due to a blown and shorted bulb or something along those lines, but the voltages remained the same with that breaker turned off.
What I don't understand is if there is a ground fault in my equipment why didn't the generator breaker trip? We were operating with a solidly connected EGC from gen to panel. Amps were around 30 on all 3 legs on gen power.
Doesn't add up, unless the fault is on POCO side. I really could not see from the ground that they had intentionally corner grounded the delta, but they did have a 4th wire run from their MGN to the CT cabinet. There is also a small device with 4 wires on the pole under the transformer bank. Reminds me of a small round SPD like you would see hanging off of a panel. 3 black wires connected to the 3 phases and a green to the MGN.
I do plan to megger both of my transformers and their feeders before I call the POCO engineer and try to fight it out with him.
Sent from my LG-V410 using Tapatalk
