3 phase 3 wire service

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lfloyd

Member
On a typical service the grounded service conductor is connected by the bond jumper to provide a return path for ground faults.
I saw a 3 phase 3 wire service with no service conductors other than the 3 phase conductors.
3 service conductors in and 3 load conductors out from the service main breaker and a connection to an electrode system is all they have in the customers service. No others conductors were provided by the utility company.
Is this OK?
What acts as the ground fault return path to the source?
 

pete m.

Senior Member
Location
Ohio
Sounds correct to me. I wonder if Ground Detectors were required for the original install.

You won't see "sparks" if one phase goes to ground but when the second phase joins the first... 4th of July.

Pete
 

lfloyd

Member
If a single phase conductor energizes a metal enclosure, is it not possible that there will not be enough current to trip the breaker and the enclosure and all connected metalic systems would remain energized?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
If that is the only service at the building then it is not compliant. Every building must ghave a neutral brought to it. (250.24(C))
 

augie47

Moderator
Staff member
Location
Tennessee
If that is the only service at the building then it is not compliant. Every building must ghave a neutral brought to it. (250.24(C))
(Back on "the job" two days and I already get to "disagree" ? Life is good :D )

Dennis, The way I read 250.24(C) a neutral is only required to be brought to the building if the service is grounded at any point. If the POCO supply is 3 phase 3 wire (only) would the install not be compliant ?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
(Back on "the job" two days and I already get to "disagree" ? Life is good :D )

Dennis, The way I read 250.24(C) a neutral is only required to be brought to the building if the service is grounded at any point. If the POCO supply is 3 phase 3 wire (only) would the install not be compliant ?
Well isn't the 3 phase trany grounded somewhere. It mentions ac system at an point-- are they talking the distribution lines as well or just the service.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
If that is the only service at the building then it is not compliant. Every building must ghave a neutral brought to it. (250.24(C))
Only if the utility provides a neutral. Completley ungrounded services may be code compliant.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
If a single phase conductor energizes a metal enclosure, is it not possible that there will not be enough current to trip the breaker and the enclosure and all connected metalic systems would remain energized?
Assuming an ungrounded system, a single fault to ground creates a grounded system. There will be very little current when this fault occurs, and the voltage of this phase to ground will drop to zero. The metallic system will not have a voltage above ground.
 

kwired

Electron manager
Location
NE Nebraska
If the supply is ungrounded then there is no grounded conductor to run. (This would be an ungrounded system) All metal raceways, enclosures, etc must still have equipment grounding conductors to ensure they are all at same potential and must still be connected to a grounding electrode system. Ground fault detection systems are also required.

lfloyd: you may also want to make sure you don't have a corner grounded system. If so one of the phases is grounded, and should be bonded to enclosure at service equipment, it can not have a fuse installed in the grounded phase but is acceptable to pass through a circuit breaker that opens all conductors when operated.
 

lfloyd

Member
This is a 1200 amp main for a 480 volt 3 phase 3 wire.
Isn't ground fault protection only required for solidly grounded systems?
There is no shock hazard here with grounding only connected to an electrode system?
 

kwired

Electron manager
Location
NE Nebraska
This is a 1200 amp main for a 480 volt 3 phase 3 wire.
Isn't ground fault protection only required for solidly grounded systems?
There is no shock hazard here with grounding only connected to an electrode system?
If it is ungrounded you need ground fault "detection" not ground fault protection. They are two different things.
 

wirenut1980

Senior Member
Location
Plainfield, IN
This is a 1200 amp main for a 480 volt 3 phase 3 wire.
Isn't ground fault protection only required for solidly grounded systems?
There is no shock hazard here with grounding only connected to an electrode system?
On an ungrounded system, no there is no shock hazard if one phase goes to ground. I know, it seems completely counter-intuitive. The reason being is that the so called "ground fault current path" has high impedance and only a small amount of current flows due to there not being a main bonding jumper at the service entrance.

The thing to be sure of with an ungrounded system is there still have to be equipment grounding conductors run from the equipment back to the service entrance. If EGC's were not run there would be the possibility of a shock hazard if there is a phase to ground fault at one machine, and a phase to ground fault of a different phase at another machine next to it and someone contacted the case of both machines. That person would then be in the path of a phase to phase fault.
 

LISHAJI

Member
Location
Albany, NY
If it is ungrounded you need ground fault "detection" not ground fault protection. They are two different things.
One simple way to detect a single line to ground fault is to measure the RMS value of line to ground voltages: A significant increase in voltage unbalance will signify a
ground fault condition!!
 

PEDRO ESCOVILLA

Senior Member
Location
south texas
question to the op. what is the service intended for? how and where will they derive as neutral if one is needed? how and where will they ground an initial inside transformer, if single pahse power is neede. they (whoever is using this system) won't get any kind of good single pahse power from this system without an initial point of " intentionally grounded service conductor" ie service neutral. so, no dennis you may not be wrong at all. it probably should be a 3 phase 4 wire service
 
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lfloyd

Member
This for an industrial facility with 3 phase 480volt motor loads and a few 208 volt transformers for lighting and receptacles. All lighting is 120 volt. No neutral loads on the 480 volt.
 
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