nicks450r
Member
- Location
- Texarkana, Ar
Long story short. We had a motor giving our entire DeviceNet network fits. Showing 40-70% phase imbalances on the monitoring system. Upon investigating we found several causes including one that really concerns me. We had a grounded ( dead short) motor that failed to trip any overchrrent devices. Causing transient voltage spikes back through our entire distribution. We found that there were also a set of MOV's left in one drive that had been replaced in the mcc section. The combination of these two things caused 11 amps on one of the grounding xfmrs in the HRG and burning it up. Now that the MOV's have been removed, the motor will continue to run without throwing the transient voltage back on the line witg the HRG off due to the xfmr.
NOW FOR THE QUESTION (had to give some history)
Contractors installed this system. We have a 3200 amp swgr fed from a 2400 kVA delta-wye tranformer. Contractors did not pull the neutral to the swgr because it was ordered as a delta swgr with HRG with grounding xfmrs for a derived neutral. Therefore we have no system ground,only a case ground. If this is the case, when the HRG's are turned off, are we at risk for a catastrophic failure or accident due to the ungrounded wye feed? We do not require an external ground to grid at any motor under 75 hp in our plant. We also found that the grounding bushings were not grounded to the ground conductor in the fiberglass tray causing tgere to be no ground at the motor case other than the ground coming back with the motor leads to the isolated ground buss i. The MCC. If I am thinking correctly, we are fine as long as the HRG is operational. But, if it is off then we run a very high risk of electricution from a grounded operatinal motor. Please advise. Sorry about any typos. I'm fat thumbing from an iPhone.
NOW FOR THE QUESTION (had to give some history)
Contractors installed this system. We have a 3200 amp swgr fed from a 2400 kVA delta-wye tranformer. Contractors did not pull the neutral to the swgr because it was ordered as a delta swgr with HRG with grounding xfmrs for a derived neutral. Therefore we have no system ground,only a case ground. If this is the case, when the HRG's are turned off, are we at risk for a catastrophic failure or accident due to the ungrounded wye feed? We do not require an external ground to grid at any motor under 75 hp in our plant. We also found that the grounding bushings were not grounded to the ground conductor in the fiberglass tray causing tgere to be no ground at the motor case other than the ground coming back with the motor leads to the isolated ground buss i. The MCC. If I am thinking correctly, we are fine as long as the HRG is operational. But, if it is off then we run a very high risk of electricution from a grounded operatinal motor. Please advise. Sorry about any typos. I'm fat thumbing from an iPhone.
