Amperage on the Ground?

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robrobertson

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I have a building that has 3ph, 120/240v system. This comes for the power company with no transformer so we use the power as is. Phase A has 6.4 amps,(Total Harmonic Distortion) THD voltage 3.7%, THD amperage 17.2%,(Crest Factor) CF 1.55. Phase B 93.6amps, THD voltage 5.2%, THD amperage 19.1%, CF 1.45. Phase C 80.3 amps, THD voltage 4.8%, THD amperage 12.8%, CF 1.46. Neutral to ground voltage is .02 volts, the amperage on the service is 5.2 amps and it is bonded to the water pipe with a ground rod. The question is where does all of all of the amperage on the ground come from? The building has about 150 computers, faxes, copiers, etc in the building so there is alot of non-linear load on the building, some computers are having problems and I think this might be some of it. Does anyone have any ideas??
 
Re: Amperage on the Ground?

Are you talking about the 5.2 amps that I assume you are saying is on the ground conductor going to the water pipe? That's not unusual. Consider that your ground and all the others like it in other buildings in the area essentially put the metallic water main in the street in parallel with the POCO neutral running down the street. There are going to be circulating currents through each ground that have little to do with the load you are drawing. Just for the heck of it I measured the current on the ground on my 120/240 single phase service and saw better than 2 amps.

-Hal
 
Re: Amperage on the Ground?

I wouldn't worry about the current on the GEC right now, it is proable normal. However you should be extremely concerned with:

A = 6.4-amps
B = 93-amps
C = 80-amps

Forget about harmonics and crest factor for now, get the imbalance taken care of and take another look.
 
Re: Amperage on the Ground?

This sounds like a high leg delta (4-wire delta)
With all most all line/neutral loads and the "A" phase is the high leg with 208 volts to neutral. If this is the case it would be hard to balance this load without installing a delta to 120/208 transformer.

But if you need to know if some of the load current is being drawn back through the grounding via neutral/ground bonds on the load side of the system you will need to calculate what the unbalance load should be, and see if this amount is what is on the neutral. if it is less than what you calculated some of it could be returning through the grounding. Since this is a high leg delta and this building should be the only load on the transformer, there should be no grounding current to another building as all the load current should be returning to source (transformer) But it could be a weak neutral between the service and transformer. this would show up in the load neutral balance calculation.

But allot depends on what type of problems the computers are developing as the answers can be very wide ranging from ground loops, to dirty power, to RFI surges. The list can go on forever.
 
Re: Amperage on the Ground?

Dereck or Charlie59
If I remember right if this transformer bank is made with just two transformers isn't there a limit as to how far they can be out of balance?
 
Re: Amperage on the Ground?

Wayne, I do not have much experience with high leg deltas, just plain old delta, wye, and single phase in my industry. But IMHO a large unbalance is a problem

After re-eading the post I have two comments.

1. The N-G voltage is very low, indicating either very little neutral current (not likely with the unbalance), The N-G bond point is very close, or a presense of a bootleg ground. robrobertson, can you expand on this.

2. Seeing how there is such a large non-linear load, and if the owner valued the equipment, I would look into either an isolation transformer or re-configure the service to single phase or wye and evenly distribute the load.
 
Re: Amperage on the Ground?

As far as I can find there is not a bootleg ground. The voltage on the high leg to neutral is 208vac. I was thinking of driving 2 or more ground rods and redoing the connection to the water pipes as the building is about 50 years old and not ment for this type of load. Does this sound like a good idea??
 
Re: Amperage on the Ground?

Originally posted by robrobertson:
I was thinking of driving 2 or more ground rods and redoing the connection to the water pipes as the building is about 50 years old and not ment for this type of load. Does this sound like a good idea??
Click to expand...
I doubt it will buy you anything, but it will not hurt anything. Why is the N-G voltage so low? Is the point you are measuring at near the N-G bond? What is the neutral current? I would expect to see about a full volt of N-G voltage downstream of the N-G bond with load current on the neutral.

Edited to ask ???

[ November 02, 2004, 02:58 PM: Message edited by: dereckbc ]
 
Re: Amperage on the Ground?

I was thinking of driving 2 or more ground rods...

Forget about the ground especially ground rods. Unless you have a problem with the neutral what you are measuring should be considered normal.

some computers are having problems and I think this might be some of it.

Computers always have problems. Unless you can find a smoking gun as far as power quality goes I would start looking elsewhere or probably chalk it up to the laws of probability due to the number you have.

-Hal
 
Re: Amperage on the Ground?

By measuring the amps on the neutral and then on the bond to the water pipe you can see what percentage may be taking the parallel path back to the transformer. If the service neutral is bad, it could be a high percentage.

What I do is put a flexible ammeter probe (AEMC) around the three phases and measure with an averaging ammeter for a few minutes. Then the same for the neutral. If the neutral has less amps, I would expect to find the missing amps on the water pipe.

If there are more neutral amps than the vector sum measured on the three phases, they may be incoming rather than outgoing, from another building's service.

Karl
 
Re: Amperage on the Ground?

Would you please clarify the correct voltages
of your system?
The voltages 120/240 V refer to a single-phase, 3-wire system.
The voltages 120/208 V (wye) refer to a 3-phase, 4-wire system.
 
Re: Amperage on the Ground?

saadt,

The single phase 3 wire system is 120/240.
The three phase 4 wire system is 240/120. Sometimes this sytem is called a wild-leg or high-leg, I have even heard it called a christmas tree.
 
Re: Amperage on the Ground?

120/240 also refers to a 4-wire delta that has a center tap between "A" and "C" phase that might be availble in your area this is also known as a "High-Leg" delta or a "Wild-Leg" Delta Because it will have about 208 volts from the "B" phase to neutral and ground.

Reread the post's and in his second post he even mention's it. (208 to ground)
 
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