thoughts on grounding

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rgpickrell

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All the earth grounds in any given area are in effect paralleled with the power company's grounding system. Maybe they benefit more from this than does each individual installation. Any thoughts on this?

[ January 07, 2005, 08:46 PM: Message edited by: rgpickrell ]
 
Re: thoughts on grounding

There are at least two reasons for earthing.

The one I think matters most is that the grounding attempts to keep the earth potential at zero volts with respect to the voltage sourses.
 
Re: thoughts on grounding

rgpirkrell
There is little doubt that utility system ground resistance drops because of all the customer ground, but I wouldn't say that the utility benefitted more. The customer side would be a hazard in many instances without a customer grounding system, but the utility system would be just fine, although somewhat different.

Utilities benefit, but customers benefit much more.

Jim T
 
Re: thoughts on grounding

Maybe what I meant to say is that utilities benefit in a different way, with large emf and current, and long runs with lots of capacitance and inductance, grounding can affect performance and efficiency. And this huge grounding grid is provided by the customers.

As far as benefit to the customer, the grounding electrode, except maybe for lightning protection,
does little to handle fault current compared to the POCO grounded conductor, I think mainly because of the grounded conductor's connection to the XFMR secondary. And the POCO is better earthed because of their electrodes and
the collective customer "grid".

Any thoughts on this?

[ January 07, 2005, 11:59 PM: Message edited by: rgpickrell ]
 
Re: thoughts on grounding

I'm not sure how the potential of the earth is going to effect the impedence of the transmission lines.

If anything, grounding the neutral of the secondary of the service transformers is going to use more POCO power.

I think it's purely a safty consideration.

If you don't make it zero, what's it gonna be?
 
Re: thoughts on grounding

i have an underground service from my house to a power company pole on the property line. from this pole the power is supplied from a pole with a transformer on the opposite side of the street. this transformer feeds four homes. the secondary center tap of the transformer is grounded to the pole's ground that runs down to the bottom of the pole where they have a circle of solid copper wire stapled to the pole. the center tap also is connected to all four homes via a bare aluminum conductor and servse as the neutral. this neutral is connected to one ground rod (old code) at each(4) home's service.

many of us have worked on services that have lost their neutral connection from the power company. what we normally see is--lights dimming when something is turned on---or get bright when the stove burner is turned on. we know that we have a neutral problem--because both phase voltages are being varied by the load. the center point or zero is floating depending on which phase returns more current.

well, this tells you the high impedance between the ground rod at the service and the copper coil on the bottom of the transformer's power pole. if this resistance was low, the customer wouldn't even know the neutral on the service was "open" or "lost". if it was a good ground or zero resistance it would be a path in parallel with the power company's "open" neutral and maintain the home's neutral at a zero point, not allowing it to vary the voltage to circuits dependent on load.

the theory that the home's ground rod maintains a zero potential doesn't really make sense to me---zero in reference to what----static electricity? i have seen many homes that the lawn mower had taken the ground rod out of the picture for years---they never knew the difference. and lightning damaged homes don't even see the ground rod. the ground rod would have a meaningful function if the resistance back to the power company's zero potential point was low with good current carrying asspects!
 
Re: thoughts on grounding

Don't misunderstand Charlie, I mean in relation to the power co's transformers. Being in an electrified environment it's important to keep our feet at a known and safe potential. I don't know of anyone who expepts earthing (grounding)to do anything but keep dangerous voltages off of the earth or our metal in our structures.

You can have that and a fault path.
 
Re: thoughts on grounding

A utility (NESC regulated) uses earth as a conductor and a fault clearing path. The NEC forbids earth to be used this way...

[ January 08, 2005, 10:45 AM: Message edited by: dereckbc ]
 
Re: thoughts on grounding

This topic can vary as much as each separate location.
Dealing strictly with dwellings, the efficiency of the installation of ground rods varies tremendously.
In the southern NY area, ground rods offer very high resistance. The cold water electrode offers much lower resistance, but again is limited as far as ground faults are concerned.
In many years of observation, and talking to many electricians, very few have ever seen a service struck by lightning - I myself have never seen a service of any kind struck by lightning. (roofs, trees, siding yes many times)
On the other hand I have seen many services subjected to Utility high voltage due to different circumstances. (Just this past week, 12 homes were subjected to 13,800 volts ;) .

Pierre
 
Re: thoughts on grounding

I don't know if grounding really has anything to do with anything but safety as far as the POCO is concerned, but I have heard EE's talk about its role in managing line to ground capacitance. I guess this doesn't affect us in the trade much, and it's a pretty specialized area.

As far as on the customer side, there are many poor grounds, and sometimes the lack of them goes unnoticed. Maybe the grounding electrode's greatest importance is its
contribution to the "collective ground".

[ January 09, 2005, 05:50 PM: Message edited by: rgpickrell ]
 
Re: thoughts on grounding

I'm also curious about including capacitance in an application like the electrical layout of a building. I can imagine some impedence being added but my assumption is that it wouldn't amount to much in the end.
 
Re: thoughts on grounding

Charlie T., I'm thinking that you have a dislike for the earth ground because you've seen it fail as a means of fault clearing. With lower voltages it's not supposed to be relied on for fault clearing. For high voltage, If one line hits the ground and all you have is dirt or cement, although it's not a perfect meathod, I'd rather hope that this works than not have anything.

And there are still other usefull functions that earthing serves.
 
Re: thoughts on grounding

I agree with Charlie Tuna on this one. The code seems to be inconsistant. The NEC requires the grounding electrode system to be connected to the service grounded (neutral) conductor. It then goes on to require us to connect the other end to a suitable electrode, the lower the resistance the better. Yet at the same time, the NEC allows significant reduction of the grounded (neutral) conductor to effectively only the size required to carry the load served. It does go on to not permit the grounded (neutral) conductor to be smaller than the GEC, but it does permit it to be the same size.

Theory tells us that current will take all paths back to the source. The primary path will and should be the grounded (neutral) conductor, yet we connect a "low as possible" path right to it at the service. I believe I read that UL considers objectional current at 3.5 mA. This si easily acheived with resistance of less than 25-ohms at the service to ground.

The fact is that the lower the grounding electrode resistance, the more normal operating current that will flow over this path. This is fact. The other fact is that there is no evidence or fact that lighting or high voltage cross-over will be less destructive when imposed on the electrical system with a low resistance grounding electrode system.
 
Re: thoughts on grounding

The fact is that the lower the grounding electrode resistance, the more normal operating current that will flow over this path.
Click to expand...
That's a good point Bryan.
 
Re: thoughts on grounding

The fact is that the lower the grounding electrode resistance, the more normal operating current that will flow over this path.
Click to expand...
Very true.

And, in this particular scenario, the lower the grounding electrode resistance, the more current will flow through the unfortunate victim below.

This also illustrates the importance of the required grounding electrode at a sub-service in a separate building.

Ed

Safety3.gif
 
Re: thoughts on grounding

Ed,
This also illustrates the importance of the required grounding electrode at a sub-service in a separate building.
Click to expand...
I don't think that the addition of the grounding electrode at the panel helps the "victim" unless he is standing on or very close to the electrode. If he is away from the electrode he will be subjected to the full phase to ground voltage.
Don
 
Re: thoughts on grounding

If I were to try to draw an "equivalent circuit" of the resistances involved in my sketch above, what do you think the resistance of the earth itself should be?

I have heard some say that the resistance of the earth is virtually zero, and that the only resistances that should be considered are the resistances of the various connections to the earth.

I am interested in all opinions on this.

Ed
 
Re: thoughts on grounding

Ed,

Your graphics are excellent.
Don posted. . .unless he is standing on or very close to the electrode.
Click to expand...
That evokes quite an image for me. The low impedance path of the ungrounded conductors and the conductive metal of the Distribution & Load enclosure are small when compared to the resistance of the connection of the electrode out into the earth.

It is important to remember that the low resistance of the Distribution & Load enclosure continues to be low through the conductor that connects to the grounding electrode. . . and continues to be low all the way through the electrode to the surface in contact with the earth.

99% of the voltage drop in the equivalent circuit is in the earth surrounding the electrode.
 
Re: thoughts on grounding

Ed,
i don't think that the actual earth resistance really makes any difference unless it is low enough to cause the overcurrent protective device to operate. If the OCPD does not open the circuit, the victim sees the same voltage as does the grounding electrode. He is another parallel path for the current. As Al said most of the voltage drop in the earth return path takes place very close to the grounding electrode. I don't have my IEEE Green book right now, but I think that it says 80% of the voltage drop in the earth return path takes place within 18" to 30" of the grounding electrode. If the victim is standing within this sphere of influence, then he will not receive the full voltage, but if he is away from the electrode, he will be subjected to the full fault voltage.
Don
 
Re: thoughts on grounding

Originally posted by Ed MacLaren:
I am interested in all opinions on this.
Click to expand...
I think a good place to start is by looking at the two extremes.

One; lets assume the panel is completely isolated, like it is magically floating in air, absolutely no return paths availible. If that were the case, then full voltage would be applied to the enclosure, no OCPD would operate. The victim would be the only return path and considered a normal load.

Second is to assume the ground or fault impedance to be zero. The OCPD would operate. The case would remain at zero potential even during a fault. The victim would be in parallel with 0 ohm?s, and would not receive a shock.

The reality is neither is likely to be true, and the answer lies somewhere between the two extremes. Even if they?re no GE connections, there is likely to be some unplanned return path like a raceway, building steel, concrete, etc. This would form a simple voltage divider circuit. The enclosure would go up to some potential for the victim to be exposed too. How much, who knows, too many variables but IMHO, it is closer to the first scenario.

Edit:
Now for something realistic. Lets say the EGC is in place, the EGC is the same size as the ungrounded conductors, and there is no supplemental GE connection. During a fault, a simple voltage divider is formed by the ungrounded conductor and EGC. Since both of these conductors take the same route, same size, and same length, it is reasonable to assume the voltage on the enclosure would be approximately 1/2 the supply voltage. That voltage would remain on the enclosure for the time it takes the OCPD to operate. The victim would receive a shock until the OCPD operates.

[ January 10, 2005, 05:15 PM: Message edited by: dereckbc ]
 
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