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hungrycat

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if i have hh boxes strung out in the ground for 3 blocks,which is better?
an equipment grounding conductor throughout the entire system or a 3/4 cu. ground rod in every hh box?

isnt the intent of a ground conductor to get to earth as soon as possible?wouldnt this expedite an overload on the overcurrent device?
 
hungrycat said:
isnt the intent of a ground conductor to get to earth as soon as possible?
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Not if your talking about the EGC.
wouldnt this expedite an overload on the overcurrent device?
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Even if your talking about the GEC, as Bob says, a GEC attached to a earthed rod has nothing to do with opening an OCPD.

Roger
 
Hungrycat,

Even with that much distance, the resistance in the equipment grounding conductor is going to be much less than the resistance between the ground rod and earth.

The current to the load along your circuit starts at the transformer (or other source) and returns to the transformer. The lower the resistance in the return path, the more fault current, the quicker the overcurrent will trip.

A ground rod will, if lucky, give 25 ohms. Look up the ohms per 1000 feet, in the Table 8 Chapter 9, for your EGC and it'll be a lot less for the distance you're considering.
 
Question back at you.
What do you think the purpose of a ground rod is?

There are five correct answers, lets see if you know one of them.
 
being a contractor and not an engineer,i never really thought of the real purpose of a ground rod other than re-routing fault currents to ground.
all i really do in commercial wiring is install designed wiring.i do very little design work because of the cost size of my contracts.you would think after 35+years some of this would rub off on me.guess i`ve been at this too long.
 
Here are the four NEC reasons for connecting an electrical system to earth, I don't know the fifth reason Dereck mentioned

1) Limit the voltage imposed by lightning,

2) Limit the voltage imposed by line surges

3) Limit the voltage imposed by unintentional contact with higher-voltage lines

4) Stabilize the voltage to earth during normal operation.

I agree with one and three.

In my opinion four is not possible

I have no idea how a connection to earth can limit the voltage imposed on by a surge on an ungrounded conductor.
 
iwire said:
4) Stabilize the voltage to earth during normal operation.
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iwire said:

In my opinion four is not possible
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It is more than possible, it is certain. But you have to step into the realm of physics to understand why.

I know you know that ?voltage? is also called ?potential difference.? That phrase means ?difference in ?potential,? from one point to another point.? But what is ?potential??

?Potential? has to do with the amount of energy (measured in units of ?joules?) it would take to move an amount of charge (measured in units of ?coulombs?), in the presence of an electric field that is either helping you push the charge (i.e., opposite charges attract) or fighting you as you try to push the charge (i.e., like charges repel) FROM the one point you are considering TO the other end of creation (i.e., so far away that there is nothing nearby and the charge will just coast along on its own from there).

So, if it takes 20 joules to move a charge of one coulomb from Point A to ?infinity,? and if it takes 5 joules to move a charge of one coulomb from Point B to ?infinity,? then the ?difference in potential? from Point A to Point B is 15 volts (i.e., a ?volt? is ?one joule per coulomb?). But what does that tell us about the difference between either Point A to Point C, or between Point B and Point C? Nothing, that?s what.

In an ungrounded, singe phase, 120 volt AC system, the difference in potential between the ?hot leg? and the ?neutral leg? is 120 volts. But which of the following is true:

(1) The ?potential difference? between the hot leg and planet Earth is 120, and the ?potential difference? between the neutral leg and planet Earth is 0?

(2) The ?potential difference? between the hot leg and planet Earth is 125, and the ?potential difference? between the neutral leg and planet Earth is 5?

(3) The ?potential difference? between the hot leg and planet Earth is 240, and the ?potential difference? between the neutral leg and planet Earth is 120?

(4) The ?potential difference? between the hot leg and planet Earth is 1200, and the ?potential difference? between the neutral leg and planet Earth is 1080?

The correct answer is ?no one knows, and it changes from moment to moment anyway.?

But if you connect the neutral point to a ground rod, the answer would become item (1), and it would stay that way forever. That is what is meant by stabilizing the voltage to earth during normal operation.
 
Charlie, good answer. I would imagine that #4 could easily exist near the end of a primary distribution line run unless ground rods were installed periodically. The neutral voltage would be a function of the neutral impedance returning to the substation, no?

Karl
 
Thank you for you complete answer. I have known the text book anwer " to stabalize the voltage ". But that information is almost meaningless without your pratical real world discription,. Thanks again.
 
After I posted that physics lesson, a simple analogy occurred to me. Let?s talk about distances. Let?s consider a group of cities that are all located on the same road. The road runs in a straight line from ?somewhere far West of here? to ?somewhere far East of here.?

? Someone tells you that the distance between the city of Appleton to ?that place somewhere far East of here? is 420 miles.
? That same someone tells you that the distance between the city of Nashville to ?that place somewhere far East of here? is 300 miles.
? For now, let?s believe what that someone said.

Here are some questions.

1. How far is it from Appleton to Nashville?
2. Did you need to know the name of ?that place somewhere far East of here,? or the exact location of that place, in order to figure out the answer to #1?
3. How far is it from Appleton to the city of Grover?
4. How far is it from Nashville to the city of Grover?
5. Did you need to know the answers to #3 and #4, in order to figure out the answer to #1?

Finally, make the following substitutions, and read through the material above one more time.

? Substitute ?potential? for ?the distance from a given city to that mystical place somewhere East of here.?
? Substitute ?potential difference,? or ?voltage,? for the distance between any two cities.
? Substitute ?one volt? for ?one mile of distance between two cities.?
? Substitute ?Phase A? for ?Appleton.?
? Substitute ?Neutral? for ?Nashville.?
? Substitute ?Ground,? meaning ?planet Earth,? for ?Grover.?

I don?t know about you, but I get an answer of ?120 volts? for question #1.
 
Pretty good Bob and Charlie. However here is my more simplistic approach and real life answers

1.Lightning Protection: Provides a planned low impedance path for lightning current to flow when lightning strikes.

2.High Voltage Protection: In the event of accidental contact with higher voltage systems like primary to secondary circuits on a distribution system.

3.Stabilize System Voltages: In large ac ungrounded systems like a 3-wire delta, if a line-to-ground short should occur, up to 25 amps of steady state AC leakage current could flow through the leakage capacitance without causing the OCPD to operate. The current represents a hazard if personnel get into the path. More importantly part of the steady state leakage current flows through conductor insulation, speeding up deterioration and thus shortening useful life of the system. If you ground the system, you short out the capacitance, thus eliminating the problem.

4.Switching Surges: As explained in #3, when a system is grounded, its leakage capacitance to ground is shorted out. Since the capacitance is shorted, over voltage conditions due to resonances between capacitance and distributed inductance are avoided. This reduces the magnitudes of transient over voltages, thus increasing the lifetime of the system and components.

5.Static and ESD: When a system is grounded, electrostatic charges cannot be stored in equipment frames chassis, or open overhead lines.

How did I do??
 
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hungrycat said:
being a contractor and not an engineer,i never really thought of the real purpose of a ground rod other than re-routing fault currents to ground..
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This is the biggest missconception of all grounding myths. Earth or a ground rod plays no part or has anything to do with clearing a fault in a low voltage system. The NEC forbids earth to have anything to do with clearing faults. That is the job of the EGC and other bonding conductors. Earth is just a referenece point. Ever seen a plane, spaceship, train or auto grounded?
 
charlie b said:
It is more than possible, it is certain. But you have to step into the realm of physics to understand why. ......
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Charlie,

I read your explanation and I (believe) understand what you said. But to what Bob said, it seems like the voltage stabilization you described is just in reference to earth, and it does not stabilize the actual voltages between the conductors of the power system.

True or no?
 
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