Apparent leakage current from phase bus bars to ground bus bar

D

Dale001289

Senior Member
Location
Georgia
I have a scenario where a bus bar enclosure containing 3, copper bus bars, A,B and C and 1 copper ground bus bar. The feeder from the upstream switchgear is 480V with an 800A for the feeder so there are two parallel 1/C, x 3 - 500 kCMIL per phase attached to the 3 phases.
When I put a meter across each phase to ground I'm getting readings of leakage current (inductance?) from each phase bus bar to ground, 15V, 30V and 15V respectively. I noticed the parallel feeders are NOT bundled together in triad shape in the cable tray above - they are laying flat. Could this be the cause?
 
Ar you using a high-impedance meter? You could easily be reading capacitive-coupled voltage from somewhere. (Use a low-impedance meter to see if there really is something there.)

If you're really curious, megger each lead.

Is this actually a problem or just seeming wrong?
 
Its a low-impedance meter, Fluke.
if the cable meggers ok, then could this mean capacitive coupled voltage be due to the conductors lying flat in the cable tray instead of being bundled to cancel out inductance?
thanks for your input
 
Dale001289 said:
I have a scenario where a bus bar enclosure containing 3, copper bus bars, A,B and C and 1 copper ground bus bar. The feeder from the upstream switchgear is 480V with an 800A for the feeder so there are two parallel 1/C, x 3 - 500 kCMIL per phase attached to the 3 phases.
When I put a meter across each phase to ground I'm getting readings of leakage current (inductance?) from each phase bus bar to ground, 15V, 30V and 15V respectively. I noticed the parallel feeders are NOT bundled together in triad shape in the cable tray above - they are laying flat. Could this be the cause?
Click to expand...
Your readings don't make sense to me. If you're worried about leakage current, why are you measuring voltage differences to ground? Also, I would think you would have more like 277 to ground, not 15 - 30.
 
gadfly56 said:
Your readings don't make sense to me. If you're worried about leakage current, why are you measuring voltage differences to ground? Also, I would think you would have more like 277 to ground, not 15 - 30.
Click to expand...

The OP is probably measuring 15V to grd bus because its not energized


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Dale001289 said:
We have not disconnected the load side yet. The run is about 200 feet - all in cable tray


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Click to expand...

Could be coupling.

Did you measure between the phases or just Phase to ground?

Voltage to ground on a 3 wire service can be misleading.

I would also see if you can get frequency or phase angle of the voltages between each phase. Like A to B vs B to C vs C to A.
 
Dale001289 said:
if the cable meggers ok, then could this mean capacitive coupled voltage be due to the conductors lying flat in the cable tray instead of being bundled to cancel out inductance?
Click to expand...
It could, but why does it matter? (Other than solving the puzzle, that is.)

Dale001289 said:
3Ph, 3W system so there is no line to neutral voltage
Click to expand...
How is the system grounded? Or is it? Voltages to ground on an ungrounded system can be all over the place, and I'd expect the fields to be, too.

(Which fluke are you using?)
 
Elect117 said:
Could be coupling.

Did you measure between the phases or just Phase to ground?

Voltage to ground on a 3 wire service can be misleading.

I would also see if you can get frequency or phase angle of the voltages between each phase. Like A to B vs B to C vs C to A.
Click to expand...

I plan to measure phase to phase tomorrow.
If it is capacitive coupling what would be the solution?


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zbang said:
It could, but why does it matter? (Other than solving the puzzle, that is.)


How is the system grounded? Or is it? Voltages to ground on an ungrounded system can be all over the place, and I'd expect the fields to be, too.

(Which fluke are you using?)
Click to expand...

I’m using a T5-1000. The system is not ungrounded. Its delta wye system, solidly grounded with a 13.8kV-480V upstream XFMR-
but there is no line to neutral voltage
It’s just 3 wire with an EGC


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Dale001289 said:
I’m using a T5-1000. The system is not ungrounded. Its delta wye system, solidly grounded with a 13.8kV-480V upstream XFMR-
but there is no line to neutral voltage
It’s just 3 wire with an EGC


Sent from my iPhone using Tapatalk
Click to expand...

The transformer is solidly grounded on the 480V side how? You should have a neutral at the service then right?

Are you saying the feeder you are measuring is 3 wire or the service is 3 wire (not grounded / no neutral) ?
 
Dale001289 said:
I’m using a T5-1000. The system is not ungrounded. Its delta wye system, solidly grounded with a 13.8kV-480V upstream XFMR-
but there is no line to neutral voltage
It’s just 3 wire with an EGC


Sent from my iPhone using Tapatalk
Click to expand...
Sounds like the egc is not connected if it’s a grounded wye, that or the bond at the transformer is not connected.
 
Elect117 said:
The transformer is solidly grounded on the 480V side how? You should have a neutral at the service then right?

Are you saying the feeder you are measuring is 3 wire or the service is 3 wire (not grounded / no neutral) ?
Click to expand...

Actually there are no line to neutral loads the switchgear so there was no need to have a grounded conductor (grey) from the XFMR center tap. There is however a green grounding conductor (basically the SSBJ) that ties to the neutral to the ground buss in the swgr


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Dale001289 said:
Actually there are no line to neutral loads the switchgear so there was no need to have a grounded conductor (grey) from the XFMR center tap. There is however a green grounding conductor (basically the SSBJ) that ties to the neutral to the ground buss in the swgr
Click to expand...
Proper terminology is important in this discussion.

Who owns the transformer?
If it belongs to the utility the secondary is considered a service. The NEC requires the wye point of a service transformer to be brought into the service equipment.
If it belongs to the customer the secondary is considered a separatly derived system. The NEC does not require the wye point of a separatly derived system to be brought to secondary overcurrent protective device(s).
 
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