Ground Amps

[Brandon Loyd]

In regards to the amps flowing through the grounding electrode of a transformer, is there an acceptable amount of amperage allowed back to earth? Or is 16 amps on a 100 KVA transformer perfectly normal and acceptable?

 

[charlie b]

We need to clarify your use of a couple phrases, beginning with ?amperage allowed back to earth.? Current does not originate from planet Earth, and it does not return to planet Earth. Current originates from the secondary of the transformer, and that is the place to which it will return. If, by chance, current can find a path that includes planet Earth, a path that will bring it back to its source, then indeed current will flow through the dirt. In that event, however, you must also have a point within the circuit at which current leaves its normal path (i.e., the ungrounded and neutral wires and the internals of the equipment powered by the circuit), and finds its way into the dirt. That means there is a problem. You have a ground fault somewhere along the circuit. Perhaps a wire inside the equipment has broken free and has come into contact with the case of the equipment, which in turn is sitting on the garage?s concrete floor, which in turn provides a path for current to get into the dirt. If the equipment does not have a proper ?Equipment Grounding Conductor? to create a low impedance path back to the source, and thereby allow so much current that the breaker will trip, then you could have a current of 16 amps flowing through the dirt, back to the grounding electrode, up the grounding electrode conductor, and thereby back to the source.

The other term we need to clarify has to do with the point at which you measured the 16 amps. You said it was on the grounding electrode. That would be the rod that is buried deep into the ground. I presume you meant the grounding electrode conductor, which goes from the electrode to the transformer.

 

[Brandon Loyd]

It was measured on the GEC at 16 amps. Are you saying this amperage that was measure is not going to earth, but rather from the earth to the transformer?

 

[skeshesh]

I think that's exactly what Charlie is saying:

...then you could have a current of 16 amps flowing through the dirt, back to the grounding electrode, up the grounding electrode conductor, and thereby back to the source.

Brandon, is there another scenario you're thinking of to explain the current on the GEC?

 

[charlie b]

That is right. I would start looking for a ground fault in one of the circuits. I suggest turning off the circuits one by one, until the measured amps on the GEC goes to zero. Then inspect whatever is on that circuit. This is certainly not an acceptable situation.

 

[brian john]

You either have a grounded neutral or a neutral from your transformer tied into another transformer or the neutral from another transformer tied to your..

De-energize the transformer and see if the current is still there.

Lift the neutral ground bond and megger the neutral, if shorted to ground (and I bet it is Lift the branch circuit neutrals and find the shorted neutral.

Or you can do zero sequence measurements (Measure the branch circuits all phase conductors and the neutral simultaneously) on each branch circuit and find the one with a current reading, that is your problem,

 

[hurk27]

One thing comes to mind because I have had it happen.

Is this a utility transformer?

single phase or 3-phase?

What is the primary voltage?

I ask this because I had a situation where tenet's were getting shocked from the shower faucets in the basement apartments, after doing some voltage checks from the grounding to Earth and transformer to Earth I found I had about 56 volts between all the grounding and transformer to Earth.

This told me that the concentric neutral had burned off, or had lost connection at the pole, after the utility arrived they dug up the primary line and found almost all the exposed concentric neutral had dissolved, but this was caused by the loss of the C crimp to the MGN at the pole.

This was a very dangerous situation that was getting worse, the week before the water pipe kept getting holes in it so they replaced it with plastic, we found that the two ground rods at the service had almost completely dissolved, and the line man also found the one at the transformer was completely gone, the part above the ground just fell over, any more loss of this grounding electrode system would have applied the whole 7200 volts to the grounding of this apartment building.

With the loss of the concentric neutral the return path back to the power line was through the buildings grounding electrode system, and water pipe, when the water pipe was replaced with plastic it left nothing but the ground rods for this return path, as these rods dissolved the return path kept getting less and less resulting in higher and higher voltages to Earth which could have killed someone, if it would have been ignored to the point the last rod totally dissolved, it would have burned down, or someone would have been electrocuted.

I'm not saying this is what you have, but try measuring the voltage from the transformer case, to a small rod stuck in to the Earth a few feet away, if you have voltage above 10 volts call the utility immediately, let them know you have a lost primary neutral to a pad transformer that is causing voltage on the grounding.

 

[electrofelon]

Hurk,

Let me make sure I have it straight...

1. The primary neutral failed
2. Because it is a grounded wye system, the neutral current traveled into the dirt by the utility ground rod and back to the neutral somewhere else and/or back to the source
3. The utility connects together the primary neutral, secondary neutral, and utility grounds which is why the primary neutral was also going thru the house gec system, to dirt, then source.
4. If both the utility and customer GES failed completely, the premise GES and everything connected to it would have risen to a potential of 7200 volts very likely causing flash over and fire or electrocution

Right?

Does this make a good case for not connecting together the primary and secondary neutrals?

 

[hurk27]

Hurk,

Let me make sure I have it straight...

1. The primary neutral failed
2. Because it is a grounded wye system, the neutral current traveled into the dirt by the utility ground rod and back to the neutral somewhere else and/or back to the source
3. The utility connects together the primary neutral, secondary neutral, and utility grounds which is why the primary neutral was also going thru the house gec system, to dirt, then source.
4. If both the utility and customer GES failed completely, the premise GES and everything connected to it would have risen to a potential of 7200 volts very likely causing flash over and fire or electrocution

Right?

Does this make a good case for not connecting together the primary and secondary neutrals?

Yes you have it right, this is the same effect you have when you loose a neutral to a load, as you will have 120 volts on the load side of the neutral.

to answer the above in red, while yes it could be a case to not bond across a transformer, it is also a great case to bond across the transformer, the more paths back to the MGN the less likely that the voltage will rise to 7200 volts before someone notices it and calls for a repair.

As in my above example of a apartment building, while I'm not sure how long it was when the C crimp failed at the MGN and the concentric neutral dissolved but with the added back up of the water line and ground rods the voltage rise on the grounding only rose very little over several months, the sad thing was that the apartment management failed to seek repair at the first time it was reported, until the shocks became much more severe after the water line was repaired and the repairman also was shocked while working on it, the first contractor call kept trying to say there was a wire grounded some where and drywall would have to be removed spent 2 weeks trying to figure it out, then we were called in, I was there and immediately went to the grounding electrode system then to the pad mounted transformer checking the voltage to Earth, at which point when I realized it was neutral return voltage I cut the 800 amp main and called the utility and put in an emergency repair order, loosing the load reduced the voltage but since the transformer still has a load, there was about 13 volts still on the grounding, now of course we know if these last couple of ground rods would have dissolved they this would have been much worse, but since they did provide some time for detection and repair, it would back the argument of having the extra electrodes in place, now in a case of where the transformer cross bond was allowed to be removed, then I would say other safety measures could be implemented such as cutouts that could detect the loss of transformer return path loss and shut down the transformer, or some other system, but at who's cost to put these into place, the utility surely would fight this because of the added cost.

The above is for pad mounted transformers, pole mounted transformers would not share this danger if there is no primary to secondary neutral bond.

 

[kwired]

I'm with Hurk, first thing you need to do is figure out what is the source of the current. Is it from the system you are concerned with or from some other system that has developed an alternate grounded conductor current path and happens to be electrically connected to the system you are concerned with.