A customer of mine has a manufacturing facility. There are two 200 amp 120/208 services beside each other. The AHJ said I could use the same set of ground rods for both services. I added a 400 amp single phase service at the other end of the building for some new equipment. The AHJ had me bond the 2 grounding electrode systems together. As a precaution I took an amp reading on the #2 bare cu that ran between the services. Amps=0. I then checked the #6 conductors from the 3 phase services that ran to the pair of ground rods. Amps=15. If I separate the #6 conductors from the shared rods there is no voltage between them. How can you get current flow without voltage present. I omitted the faulty test equipment by using 2 different meters and amp probes tested elsewhere. I believe the current flow is from introducing a parallel path between the 2 neutrals with the #6 conductors terminating together at the shared rods, but the lack of a voltage when I separate them is confusing to me.
CURRENT FLOW ON GROUNDING ELECTRODE CONDUCTOR
- Thread starter pat04607
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It's possible it could be an E = IxR current flow with a very low R because the path is bolted copper.
It can also be caused in a circuit where there's a lossless current flow due to capacitive charging between systems and / or impedance mismatch between systems where relative excess capacitance in one system is trying to balance with relative excess Inductance in the other.
In that case the load supplied would be zero, no loss or power out, except for the I2R loss of the conductor itself.
It could be normal and benign system noise or it could in fact be a miswired and hazardous "objectionable" parallel path for neutral current over the system earth ground conductor system.
More investigation would be necessary to see if a downstream second neutral to ground connection has been made.
just the cowboy
Inactive, Email Never Verified
- Location
- newburgh,ny
Sounds like double bonding
Sounds like double bonding
Sounds like the neutral is bonded at both ends, panel and xformer. What happens to the current on the netural when you lift the ground, does it go up?
Sounds like double bonding
Sounds like the neutral is bonded at both ends, panel and xformer. What happens to the current on the netural when you lift the ground, does it go up?
I'm not measuring any voltage, yet I measure 9 to 15 amps when they're in production. Less than 1 amp when they shut down. They have about everything to produce harmonic current flow. Can you have measureable harmonic current flow without voltage present?
The utility has been involved and can't put a finger on it.
The relationship of volts, current, and impedance is immutable. You have to have voltage for there to be any current whatsoever. The fact you measure none is likely because what _dan said... the impedance is quite low.
That said, if the current on the grounding electrode conductor varies proportionately with the system load, then you have a line-to-ground fault somewhere... or perhaps a resistive grounded neutral connection and the grounded neutral current of one system is using the grounding of the other system as a return path.
Let's say you have two 3/0 in parallel with 100 amps on each. You open one conductor and the other has 200 amps on it, so, within it's heating limit. What is the voltage across the open that forces the 100 to flow when closed. Calculable but not much, and certainly very low if the path length is under 25 ft.
With a GEC or EGC, in a steel framed building with conduit, the other parallel path for the 15 amps when you open the circuit is all the conductive steel mass of the building, the frame, the conduits, the machinery, the other utility piping.
You are not opening the circuit, only opening one path among many like swatting one flea on horse's butt. The grounded steel mass of the building is a shunt for the current flow, independent of the source.
Proper design philosophy is to provide an intended dedicated (compliant) path for this flow, which the inspector is looking for.
If you have lots of very large drives or UPS, the front end caps may be internally ground referenced and that grounded reference can flow a reactive lossless current, noticeable on large systems. Could also be leakage to ground and a miswired downstream neutral to ground connection, introducing neutral current over the grounding path.
JFletcher
Senior Member
- Location
- Williamsburg, VA
Between what two points are you measuring for voltage?
I didnt think one building could have 3 services. That aside tho, you have a G-N bond upstream of where you think/expect it is and have created an unintentional parallel path.
Was the reading 0.0V?
Is there another set of rods or another grounding conductor connected upstream (disco, xfmr) of where you took your measurements?
oldsparky52
Senior Member
- Location
- Wilmington, NC USA
I'm just a home schooled non union electrician so if my thoughts are dumb, just tell me.
1st thing I thought of is you have a parallel path from each service at the building back to the transformers feeding those services. I assume they are fed from the same PoCo distribution circuits. I wonder if there is enough of a current path from all of the bonding and grounding (both PoCo and building premises) for that parallel path to be a low impedance path between the 2 supply transformers? (I hope my thoughts are conveyed well).
2nd thing I thought of was how much voltage would it take to push 15 amps through that #2. Let's assume it's 500' of conductor (a guess since I don't know how far it actually is). #2 has a resistance of 0.000194 ohms/ft. VD=IR, so I = 15 and R = .097 (500*0.000194). VD=1.46. So all it takes is 1.46 volts to push 15-amps through that #2, so if you disconnected the #2 in one spot, you should read about 1.5 volts. If the distance is 250', then your voltage reading should be about 0.73 volts. Not much.
How much neutral current are on the services when you read the 15-amps on the #2? Have you established that there is a relationship between the amount of neutral current and the current on the #2? Could it just be a parallel path for neutral currents back to the PoCo transformers? IDK.
1st thing I thought of is you have a parallel path from each service at the building back to the transformers feeding those services. I assume they are fed from the same PoCo distribution circuits. I wonder if there is enough of a current path from all of the bonding and grounding (both PoCo and building premises) for that parallel path to be a low impedance path between the 2 supply transformers? (I hope my thoughts are conveyed well).
2nd thing I thought of was how much voltage would it take to push 15 amps through that #2. Let's assume it's 500' of conductor (a guess since I don't know how far it actually is). #2 has a resistance of 0.000194 ohms/ft. VD=IR, so I = 15 and R = .097 (500*0.000194). VD=1.46. So all it takes is 1.46 volts to push 15-amps through that #2, so if you disconnected the #2 in one spot, you should read about 1.5 volts. If the distance is 250', then your voltage reading should be about 0.73 volts. Not much.
How much neutral current are on the services when you read the 15-amps on the #2? Have you established that there is a relationship between the amount of neutral current and the current on the #2? Could it just be a parallel path for neutral currents back to the PoCo transformers? IDK.
oldsparky52
Senior Member
- Location
- Wilmington, NC USA
After thinking about this some more, I realize if the #2 wire resistance is a series resistance that when disconnected all of the available voltage will be read between the 2 points and may be higher than the calculated VD on the conductor.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
If one system is 208/120 three phase and the other is 120/240 single phase then they are separate systems, but they both are grounded and inherently bonded through the POCO's MGN as well as through GEC and EGC on your premises. Neutral current from either system will have some current flowing through all possible paths, the bulk of the current will be through the least resistance path, which is likely the grounded service conductor unless it has compromised connections.
oldsparky52
Senior Member
- Location
- Wilmington, NC USA
I think it would help if we knew what the neutral currents were on the services when the OP was reading the 15 amps.
Also, at what amperage (on that GEC jumper) do you start looking for problems?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
230.2(D) allows another service for the single phase service because it has different characteristics then the three phase service.
The three phase is likely one service with two disconnecting means.
Each allowed service must have all of it's disconnecting means grouped together, but there is no requirement to group multiple services together.
230.2(E) does require you have a permanent plaque or directory to tell you where the other service disconnecting mean are located.
Hi, Thank You all for all your help. Problem resolved. Putting in the jumper was introducing a very low impedance path. I went back to the basics, Ohms Law. I used a wire resistance calculator (link below) and found the voltage to be .06. There was voltage but too low for my meter to pick up. Thanks Again. Pat.
https://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-table
https://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-table
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