Current on main ground

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rfazeli55

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Hi,

I would like to know if you have 277/480V, 3 phase system at 2000A. What is the maximum current you can have on your main ground system? I know it should be zero (ideal case) but my understanding is all large buildings there will be few amps on the ground. I was unable to find this value anywhere and wanted to see if there is a rule of thumb or code that I am not aware of indicating the maximum? please let me know if you have any questions or if I am not explaining this properly.

Thanks
 
Of interest would be the voltage drop from ground system to remote earth. The current is a possible indication of a compromised POCO or feeder neutral wire.

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I should have added hard to say what a typical building is but in Washington DC 12 story office building we have measured 2-12 amps.

If you have multiple services some of the current is due to multiple ground connections.
 
MechEdetour said:
Can you elaborate?

Thanks
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There are two potential sources of current in the GES:

1. Actual leakage, which would probably trip a ground fault breaker if one was present. This should generally be tracked down to faulty equipment and corrected.
2. Current which is returning to the main panel (where the ground/neutral bond is located) but is for some reason returning to the POCO secondary neutral through the earth path instead of through the POCO wire neutral.
This will always occur because the GES to earth to POCO ground path is in parallel with the wire neutral. There will be a current divider effect which is related to the ratio between the earth path resistance and the wire neutral resistance. Anything more than maybe 1% of the total neutral current would seem to me to suggest that the wire neutral resistance is higher than it should be. However if there is a water pipe ground electrode and a metallic water main system the "earth" path resistance may be of the same order of magnitude as the wire path resistance.
 
so the system on one switchboard rises to 304A on ground wire from Fluke meter readings and drops to 26A. I am trying to see if I should even worry about the 26A on ground? This is a 13 floor, million square feet office building. there is no IEEE or any document indicating a percentage that could be on ground? like harmonics have certain threshold percentage. Thanks in advance to all who respond.
 
rfazeli55 said:
so the system on one switchboard rises to 304A on ground wire from Fluke meter readings and drops to 26A. I am trying to see if I should even worry about the 26A on ground? This is a 13 floor, million square feet office building. there is no IEEE or any document indicating a percentage that could be on ground? like harmonics have certain threshold percentage. Thanks in advance to all who respond.
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Exactly what point of the system are you measuring? The grounded service conductor, a grounding electrode conductor, an equipment grounding conductor?

Those first two will vary depending on load conditions and the resistance of the grounding electrode and can be "normal".

The last one I mentioned is likely a result of improper neutral to ground bonds somewhere in the building.
 
kwired said:
Exactly what point of the system are you measuring? The grounded service conductor, a grounding electrode conductor, an equipment grounding conductor?

Those first two will vary depending on load conditions and the resistance of the grounding electrode and can be "normal".

The last one I mentioned is likely a result of improper neutral to ground bonds somewhere in the building.
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good point, so from local Utility 25kV switchboard it goes to 2 transformers that are 2400kVA to bring down 25kV to 480V. the secondary of each transformer goes to a main-tie-main switchboard rated at 2000A. we put a fluke meter on Phases A, B, C and Netural and ground bar of each side of the main switchboard and on one side we are reading those measurements. Please note that the tie breaker is open.
 
rfazeli55 said:
good point, so from local Utility 25kV switchboard it goes to 2 transformers that are 2400kVA to bring down 25kV to 480V. the secondary of each transformer goes to a main-tie-main switchboard rated at 2000A. we put a fluke meter on Phases A, B, C and Netural and ground bar of each side of the main switchboard and on one side we are reading those measurements. Please note that the tie breaker is open.
Click to expand...

Do you have 277 volt loads being supplied? And if so where is neutral bonded? If bonded in more then one place you will have stray neutral currents on every possible parallel path, including metal raceways, equipment enclosures, etc.
 
You need to take current readings in multiple locations and if possible zero sequence readings on the main and/or all feeders. If you measure current with Zero Sequence on the main, you need to do some searching.


And as noted above if indeed you have a large zero sequence on a GFPE protected switchboard you could have tripping of the GFPE protected distribution system. Additionally, verify you have no ground connection connections on the neutral in the switchboard downstream from the Neutral Ground disconnect link and GFPE Sensor (CT).
 
rfazeli55 said:
so the system on one switchboard rises to 304A on ground wire from Fluke meter readings and drops to 26A. I am trying to see if I should even worry about the 26A on ground? This is a 13 floor, million square feet office building. there is no IEEE or any document indicating a percentage that could be on ground? like harmonics have certain threshold percentage. Thanks in advance to all who respond.
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you read 300 A on the ground (bonded to xfmr X0)?
26 A steady state on only one side only?
imo you have an issue

here's some rules of thumb for charging current
I would expect no more than 3-4 A total with 5 mva 1-2 A per xfmr
https://www.megaresistors.com/products/neutral-grounding-resistor/capacitance-to-ground/
the numbers are from an old GE paper

on a mining system with ngr 1 mva 480/3 we see <1 A
these are long cables laid on wet earth
8-10 ckts
each with 125 mA gf protection
 
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