objectionable current

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karl riley

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Bryan, interesting post, but I am still trying to get comments on the issues in my post. I don't deal with grounded conductors at all, only GECs. Any comments on the points in my post?

Karl
 
objectionable current

It would be good if we could clear up the meaning of the objectionable current section in the NEC. Can we do it?

Here is my take to start us off:
(I'm away and only have the '93 Code to look at, so make adjustments if any).
Title is Objectionable Current over Grounding Conductors.
Definitions says that Grounding Conductors means those going to grounding electrodes, so we are dealing with GECs.

(b) refers to alterations to stop the objectionable current when caused by use of multiple grounding connections.

OK, I interpret this to mean the multiple connections because of multiple grounding electrodes.

(b)(1) allows one to discontinue one or more but not all such grounding connections.

As I see it, when wiring a new building, the electrician connects to all available grounding electrodes. He has done his job correctly. Then the power is turned on and at some time it is discovered that a lot of current is flowing over the GEC to the water pipe. This is considered objectionable and the water pipe electrode is removed from use, making sure that the building has at least 2 electrodes left.

So any comments on the above will be welcome.

Now lets look at the current itself. Where does it come from? How does current get on a GEC other than fault current, which is not covered by this section? The only source I know of is neutral current from the neutral/ground bonding point in the main panel. Any other current on grounding conductors would be from a NEC violation or neutral/ground connection on the load side of the disconnect, but that is already covered by the Code. The only path which has low enough impedance to allow an abjectionable current (multiple amps) is the water pipe path (smaller amount on the CATV sheath). The reason of course is that the water pipe presents a parallel path for the neutral to return to the transformer.

So we can say that objectionable current will always be current going to the water system. Which means that that is what will have to be blocked to satisfy (b)(1).

OK. Let's try to nail this down so the Code committee members can look at it with fresh eyes and a logical mind.

Karl
 
In my opinion, the solution is simple. The majority of cases that may result in objectionable currents are due to NEC permitted reduced service/feeder grounded conductors coupled with the NEC's attempt to create the lowest resistance grounding system possible.

Even though the grounded conductor brought to service equipment is not permitted to be smaller than the required grounding electrode conductor, the connection to several electrodes of various earth resistances could result in excessive flow of normally operating current over the grounded parts.

In my opinion, I don't see a significant issue with the permitted reduction in the size of the grounded service conductor as long as it is determined the reduced conductor can carry both the calculated load demands and highest possible fault currents subjected to it when functioning as the effective fault path.

So this leaves us with the grounding system requirements that should probably be reconsidered. It is my opinion that no grounding system is need of a grounding electrode conductor in excess of #2 awg in the event of metal water pipe or building steel. The currently permitted conductor size for CEE and ground rod electrodes seems appropriate.

It is my opinion that only ONE electrode should be required for grounding the service, with all other metallic components of the structure only requiring bonding to the service. This conductor should not ever be required to be larger than #6 unless there is a branch circuit or feeder that is likely to energize that metallic component. In this case, the conductor should be sized based on the EGC supplied with the circuit.

Bonding is the key to solving 250.4(A)(1) issues, not grounding. I also have strong opinion about 250.58 and its contribution to objectionable currents on metallic and grounded parts.
 
Bryan, I re-read your post and understand it better. You are looking at the big picture and some of the reasoning behind the original requirement to use the water pipes as well as any other grounding electrode.

That gives the background. I was also hoping to get opinions on the NEC wording and its meaning.

First, does not the objectionable current section apply to GECs?

Second, is it not neutral return current that is the objectionable current?

Third, does not the allowed removal of one or more grounding connections refer to those going to grounding electrodes?

Hoping to get experienced opinion on this from Forum members so that this can be clarified once for all. Or is that impossible?

Karl
 
Karl,
The only thing that I can say, based on CMP5's statement when they rejected my proposal is that is their intent that all electrodes that are present remain in use. The only thing that can clear this up would be a proposal for the 2011 code that would force the panel to issue a statement.
Don
 
Don, I understand their attitude. But they still have to deal with the objectionable current section. You said they interpret multiple connections as referring to ??? multiple connections to one grounding electrode? That would not eliminate the neutral current to water pipe.

What multiple connections are they talking about? Can anyone give an illustration of objectionable current that is not already covered by an NEC rule, other than the flow of neutral to the water system?

Do you think their reluctance to detatch a grounding electrode is due to a true undeerstanding of the functions of grounding electrodes?

Karl
 
Please consider the following as food for thought, and not the statements of someone with deep experience in this particular area.

There are two separate safety issues here, parallel paths for the electrical system grounded conductor current, and equipotential bonding for current flow through the earth.

'Objectionable current' of the sort being discussed here occurs when you have multiple connections between the system grounded conductor and 'ground'. This quite simply introduces parallel paths for current to flow.

The NEC addresses this by permitting only a single connection between the system grounded conductor and 'ground'. (Exception for detached structures noted.)

The problem is that the NEC does not control the entire electrical system. Often the electrical system grounded conductor is grounded at the transformer, and then re-grounded at each and every house. Blammo: parallel paths. If each house has its own grounding electrode system, then this parallel path is relatively poor, 10's or 100's of ohms through soil, and very little current flow. But if the houses share a common metallic piping system, then you have a very low impedance parallel path and can see quite a bit of current over the GEC and this 'grounding electrode'.

Breaking the electrical connection to this common metal piping system will substantially reduce the current flowing over this 'parallel path'. There will still be some current, because the pipes are buried in soil, with grounding electrodes nearby, but it will reduce the current to the same magnitude as parallel paths through the soil alone.

However, breaking this electrical connection means that you now have grounding electrodes that are _not_ bonded together. In the even of earth currents, caused for example by _nearby_ lighting strikes or by multiply grounded primary distribution systems, you may see potential difference between these grounding electrodes, or you may see this 'foreign' current flowing on the grounded conductor between the grounding electrodes. The potential between one grounding electrode and another can be quite substantial in the even of a nearby lighting strike. Leaving even a stub of unbonded water pipe inside the home is a risk in the event of a nearby lighting strike. But as I said above, I don't have the experience to evaluate how much of a risk.

I believe that the safe solution would involve either running a four wire service with separate grounded and grounding conductors, treating each service entrance as a subpanel from a feeder (not likely to happen nor be permitted under current rules), or by adding dielectric sections to the water supply pipes _underground_ and away from the house, possibly bringing plastic pipe into the home.

-Jon
 
objectionable currents

objectionable currents

I object to any current that lasts more than several cycles and isn't great enough to trip a breaker
 
objectionable currents

objectionable currents

I also object to bonding the neutral to the ground in my service section when it's supplied by buss duct from the utility or any other metalic raceway.
Talk about a parallel path. But the head inspector in my town of 9 million syas the code states that the neutral shall be bonded in the service. Even though the utility also bonds the neutral 30' away and we have a grounded raceway to the service....
 
Jon, I follow your reasoning. As for equipotential bonding, even if the water pipe continuity between houses is interrupted, the houses that are all supplied by the same transformer are all "bonded" to that Tfrmer by their neutral conductors; thus the Tfrmr becomes the single grounding point for all the houses. So would this not modify any potentials between pipe stubs, since they are all connected to the same point at the Tfrmr?

Your preference for 4-conductor service entrances is currently practiced in Germany and other European countries. They have different variations. Since they were bombed out in WWII they could re-build using more contemporary thinking. They view the US power system as primitive.

And tm, if the utility provides service through bus ducts which provide parallel paths for neutral, I would think the neutral should not be bonded to ground in the entrance panel, which would be considered a subpanel??

This might be worth a thread of its ow. Try it out.

Karl
 
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