What is the ground rod for?

[enigma-2]

Mr

Mr

Interesting replies. I'll try to add to the confusion from a different prospectivve.

It's always been my understanding that the purpose of grounding an electrical system was to limit the maximum circuit voltage-to-ground.

In a 240 volt system, the voltage measured between the current-carrying conductors, is 240 volts. This is measured relative to the earth, the same potential as we are assumed to be at.

Now, if the system wasn't grounded, the actual voltage-to-ground would be floating independent to the earth; and while we still measure 240 volts between the two current-carrying conductors, we could now have as much as 600 volts-to-ground (or more) between a phase conductor and the earth.

Changing from what was once a painful, but survivable 240 volt shock, to a deadly 600 volt one.

So, the purpose of grounding a system through a ground electrode, is to maintain the maximum voltage of the system, relative to the earth, to 240 volts (or whatever the system was designed for).

Bonding is for a different reason, as you learned. It's purpose is to provide a low resistance path to ground in order to draw sufficient current during a fault, to trip the overcurrent protection.

 

[bphgravity]

While the potential of voltage rise on the secondary system is limited by the grounding of one circuit conductor, this action actually increases the likely hood of shock from the normal operating voltages.

But there is really no such thing or no real way to absolutely positively prevent the unintentional grounding of a circuit conductor. So we ground the electrical system which creates a greater potential of shock from the normal voltages present but decreases the potential of high voltages occurring on the system which could lead to fire or shock.

 

[coulter]

... if the system wasn't grounded, the actual voltage-to-ground would be floating independent to the earth; and while we still measure 240 volts between the two current-carrying conductors, we could now have as much as 600 volts-to-ground (or more) between a phase conductor and the earth. ...

enigma -
Tell me how you came up with this "600 volts-to-ground (or more)".

Just so you know, I'm a believer in ungrounded systems are safer than grounded systems.

carl

 

[bphgravity]

...Tell me how you came up with this "600 volts-to-ground (or more)"...

l

In the event the transformer primary faults to the secondary or higher voltage line drops onto the secondary conductors.... All highly unlikely.

 

[hillbilly]

What does a ground rod do?....

The way I see it....

Think of a line of cars going down a mountain side ....all going the speed limit in a nice orderly fashion (like normal current flowing on electrical circuits)

....then...

Along (behind) comes a huge, heavily loaded truck going way too fast and no way to stop (lightning strike).

If there's a "run-a-way" truck ramp (Grounding electrode) the truck will attempt to turn onto it, and succeeds.....but because of it's speed and inertia, a part of the truck hits the last car in the line and causes a chain reaction fender bender (voltage spike) as the truck plows into the ramp (earth).

So....the truck (lightning strike) has diverted to the truck ramp (grounding electrode) and mitigated to total damage to the line of cars (circuit wiring) because most of it's (kinetic) energy has plowed up the "run-away" ramp (earth).

In the example, some part of the truck will always hit the line of cars just as electricity will always follow all available paths...but...the bulk of the energy (current) will follow the path of least resistence.

If you remove the truck ramp (grounding electrode) from the story,
the line of cars (electrical circuit) must absorb all of the energy of the truck (lightning strike), and a lot more damage ($) will occur.

Just my theory.

A question for the experts.
Does electrical current have inertia?
Does it possess kinetic energy?
Am I way off base?

steve

steve

 

[coulter]

Inertia is analogous(sp) to inductance

Working on a counterpart to kinetic energy
Humm .... Ek = (1/2)mv^2

well, capacitance maybe:

E = (1/2)CV^2

not sure the model fits that good

As for being off base, yes, but so what

carl

 

[George Stolz]

Steve, I don't know if your theory is sound (truthfully don't know) but I did enjoy the analogy.

 

[mgd]

"a magnetic field can be considered the storage of kinetic energy (a current is a moving charge).... an electric field can be considered the storage of potential energy.... in either case the energy is stored and is available to do work at a later time. stored energy can only be dissipated in heat, sound, light, mechanical work, or radiation."

 

[coulter]

I didn't phrase my question right. Should have said how do you come up with this 600V shock on an ungrounded system that isn't there with a grounded system?

In the event the transformer primary faults to the secondary or higher voltage line drops onto the secondary conductors.... All highly unlikely.

Okay, that works. I've seen this once about 20years ago. A 138kV line dropped on the 13.8kv headed out to my house. I suspect that pulled the 7900V (1ph) up to 79kv, which pulled my 120V up to 1200V (all referenced to ground). Ruined a few plug strips and a fax machine.

So, here is the question: How did the ground rod with it mystical connection to the earth do anything to mitigate this?

I'm thinking an ungrounded system changes to a deadly above 600V shock, just like a grounded system.

carl

 

[vhay]

Standards are implemented to protect equipment and personnel from the most likely happenings. Components are manufactured to withstand a particular system voltage (line to line, line to neutral, and line to ground. By bonding the neutral to ground, the local system voltage with respect to ground is stablized. If something catastrophic occurs, such as 138kV line falling on a lower voltage line, it is just that, catastrophic. If for example, there was a lightning strike closeby, or a fault in the switchyard next to the power plant you are working in, the potential from ground where you are to ground where the feeders originate can be significantly different causing extremely high voltages on the local system with respect to ground if the local system is left "floating". I had to replace many of the flourescant light ballasts in a facility because the neutral had been left ungrounded. There are always cases where the seat belt has claimed a life but in most cases it is the opposite.
Vern

 

[iwire]

By bonding the neutral to ground, the local system voltage with respect to ground is stablized.

Vern, how do you stabilize the voltage by bonding it to a huge resister?

I had to replace many of the flourescant light ballasts in a facility because the neutral had been left ungrounded.

Why would a ballast 'care' what the voltage potential between earth and neutral is?

 

[vhay]

When you bond the neutral to ground, the line to neutral voltage will be the same as line to ground voltage. If the ground potential where you are standing changes with respect to ground somewhere else on the huge resistor, you will not know or care. All electrical components will only withstand "X" amount of voltage before failing to ground. "X" is a variable. The point is, if you bond the neurtal to ground, your local installation voltage will remain stable with respect to local earth.
Vern

 

[al hildenbrand]

Carl,

It occurs to me that understanding "line surge" might take a little wider field of view. Reposting your query:

NEC 250.4A1 discusses limiting the voltage imposed by "... lightning, line surge, or incidental contact with higher-voltage lines ...". NEC 250.4A1 also alludes to a grounded system helping to mitigate this voltage.

From this NEC article, I infer the NEC considers voltage anomalies from "lightning", separate from "line surge", separate from "incidental contact with higher-voltage lines"

I think I understand voltage anomalies caused by lightning. I think I understand voltage anomalies caused by incidental contact with HV lines.

********* Please listen to this piece *************
I don't know what NEC 250.4A1 is referring to when it uses the term "line surge"

So, the questions are:
What is line surge (as used in NEC 250.4A1)?
What are some causes of line surge?
How would a grounded system mitigate the effects of a line surge?

Alternate question only if the first three are not answered:
How would one measure this undefined line surge?

**** Hope you got clear to here ***************

This is not just a debate thing for me. I really am interested in what the NEC is trying to save us from.

Back when I was spending most of my time sitting in various classrooms, I well recall the following. The explainer was a power electrical engineering professor who was trying to bridge the mathematical theory we'd been studying to the non-theoretical complexity of real world macro systems.

The professor used the example of a generator, on the Earth, that is supplying power across a transmission line to a load miles away. This is an early (late 1800s) system, built in a part of the country that is not "electrified" at all, by comparison to our present milieu.

He explained that installations that were single-wire-earth-return, or ungrounded deltas, or similar transmission configurations, chosen for the minimum of material and labor to construct, that these installations had unstable voltages at the load end.

The line to ground voltage at the load was not the same as at the generator.

By adding an additional conductor to the transmission line, a conductor that was reconnected to earth at regular intervals, along the transmission line route, and was connected to ground at the generator and at the load, any fluctuations in the Earth's local potential, relative to the distant generator ground reference, would effectively by shorted out.

He explained that the Earth, being simultaneously an infinite source and sink of charge and current, was the culprit. He stated that the fluctuations could be in the thousand of volts range.

This, intuitively, made sense to me, as the atmospheric events that accumulate and build to a lightning release of charge are not the singular manifestation of such a charge occurring. Localized charge buildup between a finite volume of atmosphere, depending upon atmospheric conditions, and the Earth below it happens without lightning.

I submit that these charges are the "line surges".

 

[LarryFine]

By adding an additional conductor to the transmission line, a conductor that was reconnected to earth at regular intervals, along the transmission line route, and was connected to ground at the generator and at the load, any fluctuations in the Earth's local potential, relative to the distant generator ground reference, would effectively by shorted out.

It sounds like the bonding strip in AC/BX cable. It's not there for end-to-end conductivity, but to short out turn-to-turn impedance along the cable.

 

[3wire]

celtic answered question to the point

celtic answered question to the point

Did the house burn down...anyone killed?

need to read the basic premise of what nec is for.

administered by national fire protection association...

reasonable operation, safe to mammals in general, and NO fires.

doesnt guarantee system connected devices to perform, jsut be

S A F E...

 

[Pierre C Belarge]

Maybe some help on LINE SURGES

Maybe some help on LINE SURGES

Carl
I am now more interested than before. You see, you have taken my mind from its oblivous state to one now where the old saying makes alot of sense "the more I know, the less I understand"

So I tried to google this topic. My first google led me right to this thread , that was not much help.
I tried other methods of googling and came to this PDF...maybe it will help.

http://library.abb.com/GLOBAL/SCOT/SCOT245.NSF/VerityDisplay/D29AB4F8266DBFECC1256FCE00259CA6/$File/1998%20Zagreb%20Cigre.pdf


Let us know.

 

[1793]

...BLUEGRASS MUSIC RULES!!!

Thank God for Sam Bush !!!

 

[RayS]

Okay, that works. I've seen this once about 20years ago. A 138kV line dropped on the 13.8kv headed out to my house. I suspect that pulled the 7900V (1ph) up to 79kv, which pulled my 120V up to 1200V (all referenced to ground). Ruined a few plug strips and a fax machine.

So, here is the question: How did the ground rod with it mystical connection to the earth do anything to mitigate this?

carl

2 ways-
- helps trip the poco's breakers (with lots of distributed ground rods, it becomes a low enough impedance to help)

- by diverting some of the current, reduces the applied voltage (the spike across your normal system will be in parallel with the grounding system)

 

[coulter]

... helps trip the poco's breakers (with lots of distributed ground rods, it becomes a low enough impedance to help)...

As I understand that's true. But what does that have to do with the above 600V shock enigma-2 says we can get from an un-grounded system as opposed to a grounded system? My question refers speciffically to posts 41, 43, 44, 49.

...by diverting some of the current, reduces the applied voltage (the spike across your normal system will be in parallel with the grounding system)

You may have to draw me a picture on this one. Through what parallel path is any current diverted from the hot wire to to the grounding system? I'm not seeing the science on your explanation.

carl

 

[scott thompson]

. But what does that have to do with the above 600V shock enigma-2 says we can get from an un-grounded system as opposed to a grounded system? My question refers speciffically to posts 41, 43, 44, 49.

Capacitive Coupling effect.

The high Voltage to Ground (from Capacitive Coupling) on an Ungrounded AC Power System, stresses the Insulation of Conductors to the point that leakages may be produced - in the form of Arcing.

Voltages to Ground on Ungrounded Systems may range from as low as 10 Volts, to as high as 1000 Volts.

Measure Voltage to Ground on an Ungrounded AC System , using a High Input Impedance type Voltage Meter (like a DVM). You will get various results.
Do the same test on the same System with a Low Input Impedance Voltage Meter (like a "Wiggy"), and the results will be "0" Volts.

Scott