Objectionable Current?

[shortcircuit2]

Is there any engineering practice relative to the spacing of a grounding electrode system of a structure with utility service and the grounding electrode system of the outside pad mount transformer that feeds the structure?

For example, if an outside utility pad mount transformer with 480/277 volt wye secondary with its typical grounding electrode system ground ring and corner ground rods were say within 4 feet of the structures grounding electrode system...would there be a parallel path in certain conditions through the earth for neutral current to flow on the structure grounding electrode system to the transformer grounding electrode system back to the transformer?

 

[mopowr steve]

Of course electricity takes all paths.

 

[don_resqcapt19]

There will always be a path in a code compliant system. The utility grounding system will be physically connected to the building grounding system via the service grounded conductor, the main bonding jumper and the building grounding electrode conductor.

 

[Haji]

Is there any engineering practice relative to the spacing of a grounding electrode system of a structure with utility service and the grounding electrode system of the outside pad mount transformer that feeds the structure?

For example, if an outside utility pad mount transformer with 480/277 volt wye secondary with its typical grounding electrode system ground ring and corner ground rods were say within 4 feet of the structures grounding electrode system...would there be a parallel path in certain conditions through the earth for neutral current to flow on the structure grounding electrode system to the transformer grounding electrode system back to the transformer?

Some Engineering practice, outside US, is to connect all grounds together if the ground resistance is low and to keep individual grounds separate if the ground resistance is high.

In US, the practice is to connect all grounds together, irrespective of individual ground resistance in an effort to achieve a 'single ground'.

 

[shortcircuit2]

OK, i was thinking that the possible flow of current on the grounding electrode system and essentially all metal connected to it would be what 250.6 was intending to avoid. So are we saying that some stray/circulating current is OK and unavoidable in the USA electrical system?

250.30(A)(1) ex#2 discusses this parallel path, but does say the earth shall not be considered to make this path. But if the 2 grounding electrode systems are close together in the earth and you have low resistance in certain conditions, it will allow some current flow.

In seeing this, I was just asking if there were engineering practice to design such an installation with some specified earth separation between the 2 grounding electrode systems to minimize this Objectionable Current?

 

[KundaliniZero]

If you can, see IEEE 80.

I have read somewhere that if you have two grounding system, the reality is they are join. If you put to much effort to separate them the result is always the same they will join but in the design process you should join them because in fault condition a hight current of kA will flow throught ground wires and you should avoid a diference of voltage between both grounding systems. If ypu dont do that a person can put one foot in one grounding system and the other foot in the another. The result could not be safe for the person that can touch both system with one hand and another hand.

So i will recomend you to join both grounding system to produce a safer design.

Enviado desde mi GT-I9195 mediante Tapatalk

 

[romex jockey]

Well it depends where the EE is from>>>

That said , if of the TNC-S system , we have>

250.6(B) Alterations to Stop Objectionable Current. If the
use of multiple grounding connections results in objection-
able current, one or more of the following alterations shall
be permitted to be made, provided that the requirements of
2S0.4(A)(S) or (B)( 4) are met:
(1) Discontinue one or more but not all of such grounding
connections.
(2) Change the locations of the grounding connections.
(3) Interrupt the continuity of the conductor or conductive
path causing the objectionable current.
(4) Take other suitable remedial and approved action.


Of course this would be predicated on someone 'objecting' , which in my experience would be sensitive equipment power quality needs

~RJ~

 

[romex jockey]

If you can, see IEEE 80.

I have read somewhere that if you have two grounding system, the reality is they are join. If you put to much effort to separate them the result is always the same they will join but in the design process you should join them because in fault condition a hight current of kA will flow throught ground wires and you should avoid a diference of voltage between both grounding systems. If ypu dont do that a person can put one foot in one grounding system and the other foot in the another. The result could not be safe for the person that can touch both system with one hand and another hand.

So i will recomend you to join both grounding system to produce a safer design.

Enviado desde mi GT-I9195 mediante Tapatalk

One could reach both Xformer and meter on some of my installs with stretched out arms

But to join GEC's together would mean another solid neutral .....

~RJ~

 

[shortcircuit2]

Well it depends where the EE is from>>>

That said , if of the TNC-S system , we have>

250.6(B) Alterations to Stop Objectionable Current. If the
use of multiple grounding connections results in objection-
able current, one or more of the following alterations shall
be permitted to be made, provided that the requirements of
2S0.4(A)(S) or (B)( 4) are met:
(1) Discontinue one or more but not all of such grounding
connections.
(2) Change the locations of the grounding connections.
(3) Interrupt the continuity of the conductor or conductive
path causing the objectionable current.
(4) Take other suitable remedial and approved action.


Of course this would be predicated on someone 'objecting' , which in my experience would be sensitive equipment power quality needs

~RJ~

#2 would make sense...

I found this other long discussion on here similar to my question that has shed some light for me...
http://forums.mikeholt.com/showthread.php?t=166962&page=7

 

[don_resqcapt19]

... But if the 2 grounding electrode systems are close together in the earth and you have low resistance in certain conditions, it will allow some current flow.
...

But given that the two grounding electrode are physically connected together in a code compliant installation, the only voltage that is available to drive current between the two grounding electrodes is the voltage drop on the conductors that connect the two grounding electrodes. This drop will be very small and unable to drive much current.

 

[iwire]

OK, i was thinking that the possible flow of current on the grounding electrode system and essentially all metal connected to it would be what 250.6 was intending to avoid. So are we saying that some stray/circulating current is OK and unavoidable in the USA electrical system?

250.30(A)(1) ex#2 discusses this parallel path, but does say the earth shall not be considered to make this path. But if the 2 grounding electrode systems are close together in the earth and you have low resistance in certain conditions, it will allow some current flow.

In seeing this, I was just asking if there were engineering practice to design such an installation with some specified earth separation between the 2 grounding electrode systems to minimize this Objectionable Current?

Unless the USA moves away from MGN distribution (Multigrounded Neutral Systems) there will be current flow through the Earth.

In my opinion you are worrying about something that you cannot control.

With every service in the US there are two grounding electrode systems, the utilities and the one we install per the NEC. There will always be some current flow between them do to the power companies use of one conductor for the EGC and the grounded circuit conductor.

Add a feeder from your house to a detached garage and you add another GES and current flow through the earth between those GES.


Its not ideal, especially if you are a four legged creature standing in slop. In that case you might well feel current flow though your body.

It would be tough, if not impossible for an electrician to comply with code and eliminate this current flow through the dirt.

Check out '547.10 Equipotential Planes and Bonding of Equipotential
Planes. The installation and bonding of equipotential
planes shall comply with 547.10(A) and (B). For the purposes
of this section, the term livestock shall not include poultry.' That section is an attempt at by the NEC to address current flow through the dirt and notice they are not using 250.6 to do it.

 

[mbrooke]

OK, i was thinking that the possible flow of current on the grounding electrode system and essentially all metal connected to it would be what 250.6 was intending to avoid. So are we saying that some stray/circulating current is OK and unavoidable in the USA electrical system?

It is unavoidable. Current takes all paths available. The utility transformer neutral is connected to earth at the pole/vault/padmount and again at the building so some current will detour through the grounding electrode system to get back to the utility transformer. This is present in almost every single utility service in the US to some degree or another and any time the grounded conductor is also used for enclosure/disconnect/equipment grounding.

250.30(A)(1) ex#2 discusses this parallel path, but does say the earth shall not be considered to make this path. But if the 2 grounding electrode systems are close together in the earth and you have low resistance in certain conditions, it will allow some current flow.

The earth is generally a poor conductor compared to pipe, conduit or data cables.

In seeing this, I was just asking if there were engineering practice to design such an installation with some specified earth separation between the 2 grounding electrode systems to minimize this Objectionable Current?

By far the biggest parallel current path would be the water main. Where the concern is great a dielectic union is inserted about 10 feet from where the pipe leaves the building's foundation underground.

But, if I may, why are you concerned about these parallel current paths? There is no wrong answer, just curious

 

[GoldDigger]

The earth is generally a poor conductor compared to pipe, conduit or data cables.

Just to be pedantically clear, the earth is a really great conductor (because of its bulk). The earth path usually has a high resistance, but only because of the high contact resistance between ground electrode and earth at each end.
The resistance between two POCO ground electrodes a mile apart is not significantly greater than the resistance of the same electrodes 100 feet apart.

 

[mbrooke]

Just to be pedantically clear, the earth is a really great conductor (because of its bulk). The earth path usually has a high resistance, but only because of the high contact resistance between ground electrode and earth at each end.
The resistance between two POCO ground electrodes a mile apart is not significantly greater than the resistance of the same electrodes 100 feet apart.

Perhaps (though you are actually more correct then me), but how far does on have to drive a ground rod in a desert to obtain 1 ohm?

 

[GoldDigger]

Perhaps (though you are actually more correct then me), but how far does on have to drive a ground rod in a desert to obtain 1 ohm?

How many licks does it take to get to the center of a Tootsie Pop?

It could take a few more feet than the depth of the water table. And it would often be more productive to use a different type of electrode, like a CEE.

You are absolutely correct that the earth path is unlikely to be able to trip OCPD at domestic voltages. But the reason is just not the resistance of the earth itself.

 

[shortcircuit2]

Add a feeder from your house to a detached garage and you add another GES and current flow through the earth between those GES.

But there is not a neutral to ground bond at the garage. So the unbalanced system current from the garage loads wouldn't seek a path back to the source on the GES at the garage.?

What current would flow? Is it just there in the earth because of the utility MGN?

 

[iwire]

What current would flow? Is it just there in the earth because of the utility MGN?

Yes, because that new GES at the detached garage is connected to the MGN some current will flow on it.

 

[shortcircuit2]

But, if I may, why are you concerned about these parallel current paths? There is no wrong answer, just curious

I was wondering if there would be any touch or step potential at the case of the padmount utility transformer? Also would there be any circulating currents in the grounding system that would affect sensitive electronic equipment in the building. There's my curious questions.

 

[shortcircuit2]

Yes, because that new GES at the detached garage is connected to the MGN some current will flow on it.

Ok i get it. But it isn't objectionable current to concern us.

I was just curious in my example, if the lower resistance of the grounding electrode systems being a mere few feet apart, it would be more objectionable...to a level of concern. But all here imply it is normal.

 

[iwire]

I was just curious in my example, if the lower resistance of the grounding electrode systems being a mere few feet apart, it would be more objectionable...to a level of concern. But all here imply it is normal.

Not only normal but almost always unavoidable due to NEC requirements and the the utilities use of the MGN distribution system.