francis.key19
Member
- Location
- frederick, md
Hi,
I am hopeful someone can help me understand some of the questions I have concerning Arc Flash Hazards and our power systems on site.
My plant is about 60 years old, one structure of about 120,000ft2. There are several ground electrodes installed on the outside perimeters and bonded to building steels at various place using large wires. There are four transformers installed at various time during the last 60 years by the utility company. Three of these were installed about 20 years ago, the fourth one was installed about 5 years ago. Each transformer is 2500KVA with 480VAC on secondary. The first three were installed as ungrounded delta. The newer fourth one is a Wye with neutral, but the neutral is not carried from the transformer to the switchboard inside the building (ie. only phas conductors are carried inside). All transformers are connected to ground electrodes and thus building steel. The earthing system seems to be in good shape and all equipment inside the plant have earth ground connected to them.
The plant over the years had seen many cases where one leg of the delta would be grounded, but typically the people would not know of it until a second leg is grounded or they would detected because process heaters or process parameters are out of range. Then they would go an look for it. This seems to be normal mode of operation for us here for many years. There are no ground fault detector installed at any feeders on the three transformers. Last few months, we had a company came in to monitor surges on these feeders and saw several surges (> 20 in a course of an hour of large magnitude).
I do not have any experience working with ungrounded delta system and hope that the forum can help me with basic understanding of the followings. I also read several pdfs from schneider and eaton but still not very clearly understood.
1. What is meant by ground capacitive coupling per phase? This is in the context of if the ungrounded phase are properly balance, in normal situation, one would read close to line voltage at each phase to ground (ie. 277v or thereabout for a 480V phase) eventhough there is no solidly ground, but by capcitive coupling?
2. If one leg is grounded unintentionally, the voltage at that leg to ground would be zero? This would leave the other two phases at 480VAC ?
3. If situation in number 2 to occur, I read that the current to ground would be small, would this still not pose a hazards to an operator who may have had his/her hands on the equipment that was faulted? What would the transient voltage level be at that point ?
4. What is meant by "arcing ground transients"? Is this the second leg going to ground after the firs leg gone to ground?
5. How would you find the first leg fault if the bus has several motors, starters, pumps..etc..? These systems were grandfathered in, thus no ground fault detectors were installed, any idea what the cost for the detectors would be per system? Even that, how would you find where the fault is along the bus?
6. I read the effect of leaving the first fault and the second fault come along would cause a phase to phase fault in which case the OCPD should work to clear this. Would this not cause an arcing event somewhere else along the bus? If the first fault is left alone, what is the impact on the motors life that are connected to this bus?
7. Is an arc flash hazard worse for the ungrounded system vs. a grounded system like a WYE? That is, is it more likely to have more arc flash occurences with larger incident energy per event? Would operators be more at risks with these systems?
8. Lastly, since we also have 480VAC Wye feeder and 480VAC Delta, troubleshooting techniques would be different, how would we labeled at the device to make sure people aware of the differences? Can one system cause problem for the other if conductors are inadvertently connected.?
Any suggestions to improve the safety aspects of the ungrounded systems would be most appreciated. I am not a plant electrical engineer but am very concern about the electrical safety here. As I started to read more and more, I get a lost. Thus I am hoping the forum would be kind to help me understand these issues.
I did a search on this forum for "ungrounded delta" and found 8 pages of information, while they are quite helpful (troubleshooting), I still have some questions.
Please help if you would. Any basic resources you may have (links, pdfs..etc..) would be greatly appreciated as well.
Thank you in advance.
I am hopeful someone can help me understand some of the questions I have concerning Arc Flash Hazards and our power systems on site.
My plant is about 60 years old, one structure of about 120,000ft2. There are several ground electrodes installed on the outside perimeters and bonded to building steels at various place using large wires. There are four transformers installed at various time during the last 60 years by the utility company. Three of these were installed about 20 years ago, the fourth one was installed about 5 years ago. Each transformer is 2500KVA with 480VAC on secondary. The first three were installed as ungrounded delta. The newer fourth one is a Wye with neutral, but the neutral is not carried from the transformer to the switchboard inside the building (ie. only phas conductors are carried inside). All transformers are connected to ground electrodes and thus building steel. The earthing system seems to be in good shape and all equipment inside the plant have earth ground connected to them.
The plant over the years had seen many cases where one leg of the delta would be grounded, but typically the people would not know of it until a second leg is grounded or they would detected because process heaters or process parameters are out of range. Then they would go an look for it. This seems to be normal mode of operation for us here for many years. There are no ground fault detector installed at any feeders on the three transformers. Last few months, we had a company came in to monitor surges on these feeders and saw several surges (> 20 in a course of an hour of large magnitude).
I do not have any experience working with ungrounded delta system and hope that the forum can help me with basic understanding of the followings. I also read several pdfs from schneider and eaton but still not very clearly understood.
1. What is meant by ground capacitive coupling per phase? This is in the context of if the ungrounded phase are properly balance, in normal situation, one would read close to line voltage at each phase to ground (ie. 277v or thereabout for a 480V phase) eventhough there is no solidly ground, but by capcitive coupling?
2. If one leg is grounded unintentionally, the voltage at that leg to ground would be zero? This would leave the other two phases at 480VAC ?
3. If situation in number 2 to occur, I read that the current to ground would be small, would this still not pose a hazards to an operator who may have had his/her hands on the equipment that was faulted? What would the transient voltage level be at that point ?
4. What is meant by "arcing ground transients"? Is this the second leg going to ground after the firs leg gone to ground?
5. How would you find the first leg fault if the bus has several motors, starters, pumps..etc..? These systems were grandfathered in, thus no ground fault detectors were installed, any idea what the cost for the detectors would be per system? Even that, how would you find where the fault is along the bus?
6. I read the effect of leaving the first fault and the second fault come along would cause a phase to phase fault in which case the OCPD should work to clear this. Would this not cause an arcing event somewhere else along the bus? If the first fault is left alone, what is the impact on the motors life that are connected to this bus?
7. Is an arc flash hazard worse for the ungrounded system vs. a grounded system like a WYE? That is, is it more likely to have more arc flash occurences with larger incident energy per event? Would operators be more at risks with these systems?
8. Lastly, since we also have 480VAC Wye feeder and 480VAC Delta, troubleshooting techniques would be different, how would we labeled at the device to make sure people aware of the differences? Can one system cause problem for the other if conductors are inadvertently connected.?
Any suggestions to improve the safety aspects of the ungrounded systems would be most appreciated. I am not a plant electrical engineer but am very concern about the electrical safety here. As I started to read more and more, I get a lost. Thus I am hoping the forum would be kind to help me understand these issues.
I did a search on this forum for "ungrounded delta" and found 8 pages of information, while they are quite helpful (troubleshooting), I still have some questions.
Please help if you would. Any basic resources you may have (links, pdfs..etc..) would be greatly appreciated as well.
Thank you in advance.