aelectricalman
Senior Member
- Location
- KY
We specified a capacitor bank on a 480V Delta with a floating ground system. There were supposed to be 4 conduits, each with a set of three 500 kcmil ungrounded feeders (a,b,c) and one 1/0 ground conductor per the design, for the 1600 amp bus that it was feeding. Also note that the bus fed two 800 Amp breakers each set to trip at 700 amps (so wire sizing is ok). The bank was placed immediately on the load side of the gear at the customers main disconnect.
The contractor that did this installation was called back by the owner and was told that during a routine inspection they found all the ground wires from the cap bank had melted insulation (not the conductors). Also, the breaker in the capacitor bank was tripped on one side suggesting an issue. All of the other ungrounded conductors in the bank had not heated up and no fuses were blown. All 50 capacitors and both reactors were resistance tested to ground and frame and were ok. There was no sign the bank was the problem. Upon further inspection, it appears there are two things that were wrong with the installation.
1. The contractor used EMT conduit as a raceway along with a 1/0 conductor for each ground. All knockouts were concentric so the conduits had solid connection to the enclosure. The contractor did not bond the raceways at a bushing, so there was potentially a ground loop (don't know, cannot verify without further tests).
2. The contractor (instead of sticking to the drawings) ran 4 - 4 inch conduits and used only 3 sets of conduits. Inside each set of conduits there was 4 parallel conductors for each phase (yes all four of each conductor is in their each respective conduit) along with 1 - 1/0 ground conductors. Conduit 1 - 4 parallel A phase + 1/0 ground........Conduit 2 - 4 parallel B Phase + 1/0 ground........ Conduit 3 - 4 parallel C phase + 1/0 ground......Conduit 4 is empty. This was not approved like this and I am not sure why it was done this way. I still have to look at the derating but Im not even sure if a 4 inch is acceptable ( but as I said, there is not ungrounded conductor issue).
My question is this....As much as #1 sounds like it is a viable reason for the excess currents, is there any situation where #2 could cause this issue of burned ground conductors? Thanks for your help in advance everyone..
The contractor that did this installation was called back by the owner and was told that during a routine inspection they found all the ground wires from the cap bank had melted insulation (not the conductors). Also, the breaker in the capacitor bank was tripped on one side suggesting an issue. All of the other ungrounded conductors in the bank had not heated up and no fuses were blown. All 50 capacitors and both reactors were resistance tested to ground and frame and were ok. There was no sign the bank was the problem. Upon further inspection, it appears there are two things that were wrong with the installation.
1. The contractor used EMT conduit as a raceway along with a 1/0 conductor for each ground. All knockouts were concentric so the conduits had solid connection to the enclosure. The contractor did not bond the raceways at a bushing, so there was potentially a ground loop (don't know, cannot verify without further tests).
2. The contractor (instead of sticking to the drawings) ran 4 - 4 inch conduits and used only 3 sets of conduits. Inside each set of conduits there was 4 parallel conductors for each phase (yes all four of each conductor is in their each respective conduit) along with 1 - 1/0 ground conductors. Conduit 1 - 4 parallel A phase + 1/0 ground........Conduit 2 - 4 parallel B Phase + 1/0 ground........ Conduit 3 - 4 parallel C phase + 1/0 ground......Conduit 4 is empty. This was not approved like this and I am not sure why it was done this way. I still have to look at the derating but Im not even sure if a 4 inch is acceptable ( but as I said, there is not ungrounded conductor issue).
My question is this....As much as #1 sounds like it is a viable reason for the excess currents, is there any situation where #2 could cause this issue of burned ground conductors? Thanks for your help in advance everyone..
Last edited: