First Time: 7.2 KV

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The JP

Member
Location
Grand Rapids, MI
:? Its my first time working with medium voltage.... 7200 Volts

I have an existing building with customer owned primary. It is an un-grounded system... Looking at 250 section X I am trying to figure out if I need to pull a grounding conductor with a new feeder I am installing. The run is piped 100% of the way with EMT from the switch to the new drycore 1500KVA transformer. Everything existing in the building has just the 3 cables pulled for each circuit, but this was also done back in the 60's.
Looking for some guidance.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
My recommendation would be to partner with someone that this is not their first go at MV. I'm all for on the job training, but it needs to be supervised by those knowledgeable on the subject.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
I don't see it as criticism, but more like well thought out advice.
And I agree, MV is not forgiving when mistakes happen.

To be quite honest, MV work is not something you try to learn on your own. Everything from the cables, testing, terminating, etc... are specialized tasks.

Roger
 

John120/240

Senior Member
Location
Olathe, Kansas
My recommendation would be to partner with someone that this is not their first go at MV. I'm all for on the job training, but it needs to be supervised by those knowledgeable on the subject.

I don't see it as criticism, but more like well thought out advice.

And I agree, MV is not forgiving when mistakes happen.

To be quite honest, MV work is not something you try to learn on your own. Everything from the cables, testing, terminating, etc... are specialized tasks.

Roger

JP Welcome to the Forum. Pay Attention: I doubt if any of the members know you personally but they do want You to go home in one piece. They do have your best interest at heart.
 

mkgrady

Senior Member
Location
Massachusetts
In an ungrounded system what purpose would a ground serve? Is this a three phase system with 7200 volts phase to phase? Around here 7200 would be single phase to ground on a grounded y system.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
In an ungrounded system what purpose would a ground serve? ...
The only difference in the bonding and grounding of grounded and ungrounded systems under the rules of the NEC is that the ungrounded system does not have a main bonding jumper.
 

mkgrady

Senior Member
Location
Massachusetts
The only difference in the bonding and grounding of grounded and ungrounded systems under the rules of the NEC is that the ungrounded system does not have a main bonding jumper.

Did not know that. It is so uncommon for me to work on ungrounded systems. I will ask though, and hopefully this will be relevant to the OP, what is the point of the egc in an ungrounded system?
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
Did not know that. It is so uncommon for me to work on ungrounded systems. I will ask though, and hopefully this will be relevant to the OP, what is the point of the egc in an ungrounded system?
To limit the voltage rise on the equipment in the event of a single ground fault, and to provide a fault clearing path in the event of two ground faults.
 

GoldDigger

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Location
Placerville, CA, USA
Occupation
Retired PV System Designer
To limit the voltage rise on the equipment in the event of a single ground fault,....
For those who are not already familiar with ungrounded systems, Don's point is that although the equipment metal will not serve to carry any current on a single fault in an ungrounded system, the voltage if the metal is left "hanging" can be offset from ground by more than the line to line voltage of the circuit. Not all the time, but it is possible and a serious safety hazard.
That could be mitigated some by a local earth ground, but that would not be NEC compliant.
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
OP said "just the three cables". What kind of cable? Are they single conductors? If it's normal UG MV cable, it's going to have a center conductor and a concentric shield. Three phase or single phase? Is the 7200 line to line (weird) or line to neutral (much more common)? If you mean line to neutral, but with the neutral ungrounded, that would also be pretty rare. I think a bit more info would help, but I agree with the others....7200 is very unforgiving stuff. Get an expert.
 
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