High current on grounding conductors?

[Shells]

Good Day
Recent maintenance testing on some grounding conductors (substation grounds and transformer grounds) revealed some 'high' currents on the grounding electrode conductors. The 'high' values were varying - in the range of hundreds of mA to as high as 14A. The test measurement was a clamp on ground resistance test by megger DET14C. I am going to have to do some more investigations because I want to be sure that the measurements were actually taken properly (considering the manufacturers recommendations - earth must be in path, must be loop resistance etc) and also I noticed that some of the higher readings were of transformer ducting. But my first question is how high is too high for leakage currents on a ground? Is there a way to calculate it or some rule of thumb based on feeder size? Also, am I correct t o assume this current is due to unbalanced load or an open neutral somewhere? Any advice is welcome.

PS. Facility has generators, compressors, pumps, transformers etc.

Thanks

 

[Ingenieur]

hard to estimate
voltage?
sec wye or delta?
if wye is the N grounded?
total MVA?

there are rules of thumb for estimating based on MVA, page 7 may help http://www.postglover.com/wp-conten...4/04/HR002-05_Hi_Resis_Grd_Conversion_Gde.pdf
eg, 4160 VAC ic = MVA x 1 A + no. suppressors x 1.5 A



most of our substations have 50 A ngr's set to trip at 20 A, so 14 A would not do it

 

[ATSman]

Rule of Thumb

Rule of Thumb

Good Day
Recent maintenance testing on some grounding conductors (substation grounds and transformer grounds) revealed some 'high' currents on the grounding electrode conductors. The 'high' values were varying - in the range of hundreds of mA to as high as 14A. The test measurement was a clamp on ground resistance test by megger DET14C. I am going to have to do some more investigations because I want to be sure that the measurements were actually taken properly (considering the manufacturers recommendations - earth must be in path, must be loop resistance etc) and also I noticed that some of the higher readings were of transformer ducting. But my first question is how high is too high for leakage currents on a ground? Is there a way to calculate it or some rule of thumb based on feeder size? Also, am I correct t o assume this current is due to unbalanced load or an open neutral somewhere? Any advice is welcome.

PS. Facility has generators, compressors, pumps, transformers etc.

Thanks

FWIW
Many years ago I remember working at a hospital with a 4000A 480/277v
switchboard. It had a panel AM that was fed from an in-line CT mounted between the neutral and EGC (Ufer.) I believe this was the grounding point on the Wye. As I recall the full scale on the AM was 100A and it was reading well under that value. I was working with an EE from GE at the time and I asked him if the wiring was correct, what would be the normal leakage current from neutral to ground on a system as old as this. He told me that GE's rule of thumb (due to older cable & bus insulation, system capacitance, etc) was 1% of the total load current. In this case the load current was a little under 3000A (4000A main) so that comes to just under 30A. I thought at the time that this made sense; the leakage current was directly proportional to the load current.
I would be interested in any comments on this subject and whether this rule would change on an MV or higher V system.