parallel path for neutral current created by multiple grounding electrode connections

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pegggu

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hi all, hope you guys can provide some insight on this as i'm stumped. i know it's been discussed a few times, i've read many of the threads but am still a bit confused.

why is it bad and what happens when you bond the neutral to multiple grounding electrodes/rods?
see the attached picture. i'm just confused as to why this is a problem and why it is better to only have 1 neutral to ground bond at the supply/transformer as opposed to having multiple points of ground paths?
 

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Other than the earth between the utility transformer and the service disconnect I do not see any parallel path for neutral current in the drawing posted.
 
iwire said:
Other than the earth between the utility transformer and the service disconnect I do not see any parallel path for neutral current in the drawing posted.
Click to expand...
There are multiple paths through earth between the service disconnect and the service transformer, but that does not result in objectionable current anywhere.
I might however question whether the portions of EGC which are now also serving as either GECs or bonding jumpers are correctly sized.
 
Here?s the problem with your image.

  • You correctly show yellow arrows pointing to the right along the black wire.
  • You correctly show yellow arrows pointing to the left along the white wire, up to the point where the current reaches the service disconnect.
  • From that point, you are incorrectly showing current moving to the right along the green wire.
  • Also, at each building disconnecting means, including at the service disconnect and the utility transformer, you are incorrectly showing current flowing to, and through, the dirt.

The driving force that is causing current to flow is the transformer. When current arrives (along the white wire) at the service disconnect, it will only flow to the left, back to the transformer. There is no driving force that would push it to the right (along the green wire) or downwards (along the GEC into the dirt). The center point of the transformer pulls the current back into, and through, the transformer, and sends it once again to the right (along the black wire). That is the only current path. There is no parallel path related to the GECs.
 
I disagree. There is a voltage difference between the bond point and the transformer caused by the normal current and the impedance of the service neutral.
The resistance of all of the parallel paths through earth is far higher than the neutral resistance, but that does not mean that no current will flow. Just no significant current.
If you open the neutral, then the other currents will become significant, and objectionable.

I much prefer the discussion of voltage differences to pushing and pulling, if for no other reason that our experience of physical forces does not match what happens when the wires curve.
If you go all the way to the water and pipe analogy, you will see water flow on the earth paths.
 
If you eliminate the thought of the earth as a parallel path for the neutral current the "problem" goes away.
 
Now let me address your question. Let?s revise your image in the following ways:

  • Erase all the GECs and the dirt. For my discussion, they are not needed.
  • Replace the words ?Service Disconnect? with ?Main Service Panel.?
  • Replace the words ?Building Disconnecting Means? with, from left to right, ?Subpanel 1,? then ?Subpanel 2,? and finally ?Subpanel 3.?
  • My version is therefore all about a single building with four panels.
  • At the far right panel (Subpanel 3), run a jumper between the neutral and ground bus bars.
  • We just placed the white and green wires in parallel. Note that they are connected to each other both at the Main Service Panel and at Subpanel 3.

Here is the impact of this new configuration:

  • When current flows to the right along the black wire, it will return to the transformer via both the white wire and the green wire.
  • But the green wire, the equipment grounding conductor, could be a metal conduit, instead of a separate wire. That would mean that current is flowing along the surface of the conduit, and that a person touching that conduit could get a shock.
  • Also, since the enclosures of the panelboards, the conduits, and all nearby piping are all bonded together, every piece of metal in the building will be energized.
 
infinity said:
If you eliminate the thought of the earth as a parallel path for the neutral current the "problem" goes away.
Click to expand...
The critical thing is that it is a parallel path for the service neutral, just not for the building neutrals. When you make a single bond to a single electrode you are providing a parallel path to the service neutral. But the NEC does not have a problem with that!
 
GoldDigger said:
The critical thing is that it is a parallel path for the service neutral, just not for the building neutrals. When you make a single bond to a single electrode you are providing a parallel path to the service neutral. But the NEC does not have a problem with that!
Click to expand...

My point exactly. ;)
 
Why is one of the primary conductors connected to the secondary neutral?

I know utilities use grounded conductors for the primary sometimes, but I didn't think this connection was made.
 
iwire said:
Let's stick with the image shown, there is only one parallel path for neutral current and it's impedance will be high. :)
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No. There are as many parallel paths as there are ground electrodes, and the impedances of all of them are high. Being mostly the electrode to earth impedance. If you have 5 identical electrodes the impedance of that parallel part of the path will be 1/5 of that of an individual electrode.
But all will be in series with the utility earth electrode(s).
 
The current in the diagram is flowing in the EGC and the multiple GECs and ground rods. There is no question of multiple ground neutral bonds.
And the disconnects are electrically in parallel, even though the physical layout is a daisy chain rather than a fan of home run feeders. That physical layout has no effect on the existence of parallel paths, just on the shape of those paths.
Nobody has suggested putting a bond at each panel. That would indeed be against code and would lead to objectionable current.
What we are looking at is the effect of a distributed GES versus a single point connection between the GES and the EGC mesh.
For various reasons Mike Holt argues that to minimize stray voltages and their associated currents it is ideal to have a single point of connection between GES and EGCs. But that is not required by code and is not the same as having only one ground to neutral bond.
 
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GoldDigger said:
What we are looking at is the effect of a distributed GES versus a single point connection between the GES and the EGC mesh.
For various reasons Mike Holt argues that to minimize stray voltages and their associated currents it is ideal to have a single point of connection between GES and EGCs. But that is not required by code and is not the same as having only one ground to neutral bond.
Click to expand...

Can you please elaborate on this topic and/or reply with a link to more reading on this issue. I am working on a new work shop at our facility and the design engineer is requesting that all electrical equipment be individually connected to the grounding electrode system with a bonding jumper. What I am used to seeing and installing is a single bonding jumper between the grounding electrode system and a single ground bus bar located at or very near the source, and an equipment grounding conductor installed from each piece of equipment back to that main ground bus bar. In other words, what the engineer is requesting is almost exactly what is in the picture at the top of this thread, except replace the sub panels with equipment like HVAC units, 480 volt welder receptacles, etc.
 
dbhinds said:
Can you please elaborate on this topic and/or reply with a link to more reading on this issue. I am working on a new work shop at our facility and the design engineer is requesting that all electrical equipment be individually connected to the grounding electrode system with a bonding jumper. What I am used to seeing and installing is a single bonding jumper between the grounding electrode system and a single ground bus bar located at or very near the source, and an equipment grounding conductor installed from each piece of equipment back to that main ground bus bar. In other words, what the engineer is requesting is almost exactly what is in the picture at the top of this thread, except replace the sub panels with equipment like HVAC units, 480 volt welder receptacles, etc.
Click to expand...

You are certainly correct that normally the egc in the branch circuit and feeders will connect the equipment to the grounding electrode system and earth. I dont see what a "better" or "dedicated" conductor back the the GES would accomplish. I would ask for a justification and what he hopes to accomplish. Sounds like the ancient mythical quest for the "better ground".
 
What's missing for me in the illustration is that each disconnect would have to be in a separate structure from the others to be required to have it's own ground rod. In that case, there is a parallel path, but I can't image the voltage gradient between the utility rods and the structures' rods being dangerous in anything other that a failed neutral situation between the utility transformer and the service disconnect. It's the same situation that exists all over the place between any two utility poles with ground rods driven and connected to the primary neutral.
 
dbhinds said:
Can you please elaborate on this topic and/or reply with a link to more reading on this issue. I am working on a new work shop at our facility and the design engineer is requesting that all electrical equipment be individually connected to the grounding electrode system with a bonding jumper. What I am used to seeing and installing is a single bonding jumper between the grounding electrode system and a single ground bus bar located at or very near the source, and an equipment grounding conductor installed from each piece of equipment back to that main ground bus bar. In other words, what the engineer is requesting is almost exactly what is in the picture at the top of this thread, except replace the sub panels with equipment like HVAC units, 480 volt welder receptacles, etc.
Click to expand...
As long as each circuit has an EGC run with the circuit conductors back to the source, the addition of a connection to the grounding electrode system is not a code violation. It doesn't really serve any purpose, but it is not a violation, and is a very common requirement in industrial electrical construction specifications.
Now if there is not an EGC run with the circuit conductors, that is code violation and could result in a serious shock hazard in the case of a ground fault.
 
As to the original drawing. This is one structure and each space is divided by a two hour firewall which in Washington can now be considered four separate buildings and have a service for each one. They will all have a main breaker panel and bond the neutral to the can. They will all connect to the common UFER ground in their space. For this situation all four meters are in one enclosure on the end of the structure and have no equipment ground but rigid metal conduit. Wouldn't this be a parallel path, but accepted per the code.
 
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