3 phase delta 240 V
- Thread starter domnic
- Start date
- Status
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: 3 phase delta 240 V
Not many But there can be stray voltages develop between these wires and Earth and care must be exercised when working live or dead. Also lightning can induce very high voltages when a power supply has no reference to Earth so this is another factor when deciding which way to go. But if you have a mission critical operation that will cost if you have unexpected shut down because of a ground fault then this is the way to go just don't feed any electronics or PLC's with out going through a step down transformer with a bonded secondary. This way these circuits will have some protection against lightning.
And most of all use a ground fault detector.
Not many But there can be stray voltages develop between these wires and Earth and care must be exercised when working live or dead. Also lightning can induce very high voltages when a power supply has no reference to Earth so this is another factor when deciding which way to go. But if you have a mission critical operation that will cost if you have unexpected shut down because of a ground fault then this is the way to go just don't feed any electronics or PLC's with out going through a step down transformer with a bonded secondary. This way these circuits will have some protection against lightning.
And most of all use a ground fault detector.
Re: 3 phase delta 240 V
If you visualize 3 ungrounded line conductors feeding various loads and it stays that way every thing is fine. Now, one of those lines happens to go to ground due to a motor failure of whatever, it is still not a big issue as there is not complete circuit back to the source.
The big danger comes should a second line go to ground especially in some other location in the facility. Then visualize the path that the fault current will take from the point where the first line is grounded to where the second is grounded.
There is no telling which piece of building steel it will travel through, machine frame, and conductive item that may between and in the path of that fault current. It's pot luck.
As the other poster stated a ground detection system is a must in order to detect and annunciate that a line has gone to ground. These systems are commonly used where a ground fault in a grounded system would shut down a continuous process which would be quite costly. With a ground detector system, the qualified personal would be notified so that the grounded line can be isolated and repaired of to give one time enough to shut down the process in an orderly fashion and then locate and make the repair.
Please correct me if I'm wrong but I don't believe you can use an ungrounded system unless the are qualified people available to supervise the system if required.
If you visualize 3 ungrounded line conductors feeding various loads and it stays that way every thing is fine. Now, one of those lines happens to go to ground due to a motor failure of whatever, it is still not a big issue as there is not complete circuit back to the source.
The big danger comes should a second line go to ground especially in some other location in the facility. Then visualize the path that the fault current will take from the point where the first line is grounded to where the second is grounded.
There is no telling which piece of building steel it will travel through, machine frame, and conductive item that may between and in the path of that fault current. It's pot luck.
As the other poster stated a ground detection system is a must in order to detect and annunciate that a line has gone to ground. These systems are commonly used where a ground fault in a grounded system would shut down a continuous process which would be quite costly. With a ground detector system, the qualified personal would be notified so that the grounded line can be isolated and repaired of to give one time enough to shut down the process in an orderly fashion and then locate and make the repair.
Please correct me if I'm wrong but I don't believe you can use an ungrounded system unless the are qualified people available to supervise the system if required.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: 3 phase delta 240 V
250.36 High-Impedance Grounded Neutral Systems.
High-impedance grounded neutral systems in which a grounding impedance, usually a resistor, limits the ground-fault current to a low value shall be permitted for 3-phase ac systems of 480 volts to 1000 volts where all the following conditions are met:
(1) The conditions of maintenance and supervision ensure that only qualified persons service the installation.
(2) Continuity of power is required.
(3) Ground detectors are installed on the system.
(4) Line-to-neutral loads are not served.
High-impedance grounded neutral systems shall comply with the provisions of 250.36(A) through (G).
I couldn't find anything in the NEC that would require this But it should be added to 250.36 Which does require this:
250.36 High-Impedance Grounded Neutral Systems.
High-impedance grounded neutral systems in which a grounding impedance, usually a resistor, limits the ground-fault current to a low value shall be permitted for 3-phase ac systems of 480 volts to 1000 volts where all the following conditions are met:
(1) The conditions of maintenance and supervision ensure that only qualified persons service the installation.
(2) Continuity of power is required.
(3) Ground detectors are installed on the system.
(4) Line-to-neutral loads are not served.
High-impedance grounded neutral systems shall comply with the provisions of 250.36(A) through (G).
Re: 3 phase delta 240 V
Thanks to pointed that out. Yes, high resistance grounding is not the same as ungrounded but some of the statements made in the article could refer to an ungrounded delta system. If it is not part of the 2002 code then do does the code panel feel strongly enought to include a reference to ungrounded deltas in the 2005 NEC? It may be simply an oversight.
It seems to me that it should not be take lightly and over looked and should be included in the NEC.
Moderators??
Thanks to pointed that out. Yes, high resistance grounding is not the same as ungrounded but some of the statements made in the article could refer to an ungrounded delta system. If it is not part of the 2002 code then do does the code panel feel strongly enought to include a reference to ungrounded deltas in the 2005 NEC? It may be simply an oversight.
It seems to me that it should not be take lightly and over looked and should be included in the NEC.
Moderators??
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
Re: 3 phase delta 240 V
I am always a bit nervous about parts of the code where it says "only qualified people" can service something. That implies that the code accepts the idea that unqualified people are allowed to service anything else. Somehow that has always bothered me.
I am always a bit nervous about parts of the code where it says "only qualified people" can service something. That implies that the code accepts the idea that unqualified people are allowed to service anything else. Somehow that has always bothered me.
Re: 3 phase delta 240 V
Maybe the reference to "only qualified people can service" should be that said facility must be "supervised by qualified people." The subject facility would by responsible to assure that their facility is monitored at all times by an individual who is trained and understands that distribution system and is able to address any distribution issues that could arise.
Such a distribution is much more complex that those which most of us are more familiar with and, as illustrated by the questions posted regarding ungrounded deltas, is a mystery for quite a few.
Maybe the reference to "only qualified people can service" should be that said facility must be "supervised by qualified people." The subject facility would by responsible to assure that their facility is monitored at all times by an individual who is trained and understands that distribution system and is able to address any distribution issues that could arise.
Such a distribution is much more complex that those which most of us are more familiar with and, as illustrated by the questions posted regarding ungrounded deltas, is a mystery for quite a few.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: 3 phase delta 240 V
Yes as long as you include a equipment grounding conductor (EGC) in the PVC that has the branch circuit conductors in. Remember an ungrounded delta just don't have a grounded circuit conductor (neutral) It still has to have a EGC that will be bonded to a grounding electrode at the service. This is there for where two or more ungrounded conductors fault to ground (EGC not Earth ) And also for the protection somewhat from lightning.
Yes as long as you include a equipment grounding conductor (EGC) in the PVC that has the branch circuit conductors in. Remember an ungrounded delta just don't have a grounded circuit conductor (neutral) It still has to have a EGC that will be bonded to a grounding electrode at the service. This is there for where two or more ungrounded conductors fault to ground (EGC not Earth ) And also for the protection somewhat from lightning.
- Status