Ungrounded System
- Thread starter danilo
- Start date
- Status
- Location
- Massachusetts
Re: Ungrounded System
You do all your work as you normally do.
Grounding electrodes, grounding electrode conductors, equipment grounding conductors etc.
The only change will be the elimination of the main bonding jumper.
You will not really have a ground fault path as that is the point of using an ungrounded system.
Under normal operating conditions there is no grounded conductor.
If at some point one of these ungrounded conductors contacts grounded parts nothing happens other than you now have a grounded delta system. It should also activate an indicator to let you know a ground fault is in the system.
At this point the ground fault should be tracked down and repaired.
If you do not fix the first ground fault and another phase ground faults you will have a line to line fault through the bonded metal enclosures and / or equipment grounding conductors. If this second ground fault happens all your bonding / grounding becomes a very important factor in opening the overcurrent protection.
You do all your work as you normally do.
Grounding electrodes, grounding electrode conductors, equipment grounding conductors etc.
The only change will be the elimination of the main bonding jumper.
You will not really have a ground fault path as that is the point of using an ungrounded system.
Under normal operating conditions there is no grounded conductor.
If at some point one of these ungrounded conductors contacts grounded parts nothing happens other than you now have a grounded delta system. It should also activate an indicator to let you know a ground fault is in the system.
At this point the ground fault should be tracked down and repaired.
If you do not fix the first ground fault and another phase ground faults you will have a line to line fault through the bonded metal enclosures and / or equipment grounding conductors. If this second ground fault happens all your bonding / grounding becomes a very important factor in opening the overcurrent protection.
Re: Ungrounded System
Thanks Bob..
yeah thats what im worried about if there would be an unsymmetrical fault in the system. assuming the 2nd fault occured, would that mean the fault current would go through the earth?or could it create unbalanced system to trigger high current on other legs thus trips the disconnecting means?
regards,
Danilo
Thanks Bob..
yeah thats what im worried about if there would be an unsymmetrical fault in the system. assuming the 2nd fault occured, would that mean the fault current would go through the earth?or could it create unbalanced system to trigger high current on other legs thus trips the disconnecting means?
regards,
Danilo
- Location
- Massachusetts
Re: Ungrounded System
Danilo The earth should not play any role at all in operating overcurrent devices on either ungrounded or grounded systems.
In the case of a second fault on an opposing phase in an ungrounded system very little current if any will flow through the earth. If any current flows through the earth it will only be because the earth ended up in parallel with your EGC via incidental contact.
The majority of current will flow through the equipment grounding conductor and / or metal enclosures.
Danilo The earth should not play any role at all in operating overcurrent devices on either ungrounded or grounded systems.
In the case of a second fault on an opposing phase in an ungrounded system very little current if any will flow through the earth. If any current flows through the earth it will only be because the earth ended up in parallel with your EGC via incidental contact.
The majority of current will flow through the equipment grounding conductor and / or metal enclosures.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: Ungrounded System
This would be the same if you were to take two phase conductors and connected them together which would cause a short circuit and open the OCPD device. As said before, the reason we wire a service like this is for redundancy to provide time to repair the first fault without requiring a system shutdown. some operations on a production lines could be dangerous if power was lost in an uncontrolled manner, with the proper alarm to tell you when the first fault occurred gives you time to schedule a controlled shutdown to repair the fault.
The first fault connects one of the ungrounded conductors to the equipment grounding system (conduit,EGC,GEC) this is now the return path back to source but no OCPD's open just yet as the fault circuit has not been completed, which happens when the second fault to the grounding system occurs. You now have a complete path from one ungrounded conductor to the second ungrounded conductor and the path back to source is complete.
This would be the same if you were to take two phase conductors and connected them together which would cause a short circuit and open the OCPD device. As said before, the reason we wire a service like this is for redundancy to provide time to repair the first fault without requiring a system shutdown. some operations on a production lines could be dangerous if power was lost in an uncontrolled manner, with the proper alarm to tell you when the first fault occurred gives you time to schedule a controlled shutdown to repair the fault.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: Ungrounded System
Do you need the redundancy of having an ungrounded system? Do you need controlled shut down of any equipment that a grounded system would not provide? If not then why not just ground the "B" phase? I would think the POCO would have to do the same at the transformer but why a high impedance system? Does this system have any GFP installed at the service?
What kind of work is done at this plant? there are many questions that need to be answered before you can decide upon which is right for you. (that sounded like a TV commercial
)
The operation of the plant being the major one.
Do you need the redundancy of having an ungrounded system? Do you need controlled shut down of any equipment that a grounded system would not provide? If not then why not just ground the "B" phase? I would think the POCO would have to do the same at the transformer but why a high impedance system? Does this system have any GFP installed at the service?
What kind of work is done at this plant? there are many questions that need to be answered before you can decide upon which is right for you. (that sounded like a TV commercial
) The operation of the plant being the major one.
- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
Re: Ungrounded System
In the event of a ground fault in an ungrounded system, is there potential on all metallic equipment until the fault is repaired?
In the event of a ground fault in an ungrounded system, is there potential on all metallic equipment until the fault is repaired?
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Re: Ungrounded System
Good question. Think corner grounded delta for your answer.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Re: Ungrounded System
If you mean between the metallic components and earth, I'd say "no", because grounding and bonding is still done, and for the same reasons: minimazing potential difference between different surfaces.
When a ground fault occurs on one phase on an otherwise-ungrounded system, the voltage on the other phases suddenly snaps to the phase-to-phase voltage above ground, as if intentionally grounded.
Then you have what is effectively a corner-grounded delta system, and the previously-ungrounded system would respond to a second phase ground fault just as a corner-grounded system would to one.
In the meantime, any non-current-carrying parts should have a low-impedance conductive pathway to all the usual grounding points: earth, building metal, pipe systems, anything "likely to become energized", etc.
Here's a slightly-similar experience I had as a helper, many moons ago: we installed a 3-phase 480/277-Y to 208/120-Y transformer and panel for a warehouse, along with fluorescent lighting.
The mechanic I was working with hadn't grounded the panel neutral before we energized the system. The lights all worked fine, until we noticed that one fixture cut off when the bonding jumper sparked.
I immediately deduced that one of the fixture's ballast output wires was pinched to the fixture housing. The floating panel neutral had a potential to ground equal to the ballast output.
There was no shock hazard because the transformer frame and panel enclosure were grounded, even though the neutral was "hot" until bonded. That's why neutrals are insulated just like hot conductors.
Grounding a neutral forces the hot wires to be a set voltage to earth. Sure, there's potential (sorry!) for shock, but it minimizes higher voltages during accidental primary-to-secondary faults.
George, to expand upon the two previous responses, I would first ask you "potential between what two points or surfaces?"
If you mean between the metallic components and earth, I'd say "no", because grounding and bonding is still done, and for the same reasons: minimazing potential difference between different surfaces.
When a ground fault occurs on one phase on an otherwise-ungrounded system, the voltage on the other phases suddenly snaps to the phase-to-phase voltage above ground, as if intentionally grounded.
Then you have what is effectively a corner-grounded delta system, and the previously-ungrounded system would respond to a second phase ground fault just as a corner-grounded system would to one.
In the meantime, any non-current-carrying parts should have a low-impedance conductive pathway to all the usual grounding points: earth, building metal, pipe systems, anything "likely to become energized", etc.
Here's a slightly-similar experience I had as a helper, many moons ago: we installed a 3-phase 480/277-Y to 208/120-Y transformer and panel for a warehouse, along with fluorescent lighting.
The mechanic I was working with hadn't grounded the panel neutral before we energized the system. The lights all worked fine, until we noticed that one fixture cut off when the bonding jumper sparked.
I immediately deduced that one of the fixture's ballast output wires was pinched to the fixture housing. The floating panel neutral had a potential to ground equal to the ballast output.
There was no shock hazard because the transformer frame and panel enclosure were grounded, even though the neutral was "hot" until bonded. That's why neutrals are insulated just like hot conductors.
Grounding a neutral forces the hot wires to be a set voltage to earth. Sure, there's potential (sorry!) for shock, but it minimizes higher voltages during accidental primary-to-secondary faults.
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