Grounding Rods

[Redskinsdb21]

If you have 3 grounding rods....

rod one has resistance of 30 ohms....rod 4 has resistace of 60 ohms...rod 5 has resistance of 40 ohms...what is the true resistance of the grounding rods? what does true resistance mean?

 

[DK PE]

I can't say I've seen this but I would assume these look like three resistors in parallel with one terminal connected to "true ground". I'll let you do the parallel resistor math.

 

[Redskinsdb21]

I cant say I have either and its confusing the heck out of me.....but it is out there....and I am not sure treating as if its parallel is the solution, hoping someone comes along that understand it

 

[robertplant22]

Have not seen this problem, but here is an idea, not sure if this is correct:

Call the rods R1, R2, and R3 (R for resistance). If they are all connected in parallel, once you add two of the resistances in parallel (say R3llR2), the equivalent resistance is in series with remaining resistance (R1).

The problem is which two resistances do you add in parallel? is there a diagram along with this question?

 

[CntrSnr2001]

theyre just looking for total resistance of the 3 groud rods combined. IIRC, true resistance just refers to the total system ground resistance.

the three ground rods act like resistors in parallel wen they're jumpered together.

if there is no jumper, then i have no idea what the question is asking lol..... any other verbiage from the problem?

 

[Redskinsdb21]

That is the verbage, and adding the total resistance wasnt one of the choices

 

[DK PE]

So is the answer the obvious and 3 resistors in parallel and 13.3 ohms?

 

[Audienceof1]

Seems like you guys have an answer, but for what it's worth, NEC section 250.53(A)(2) states that if the first one doesn't get you to less than 25 ohms, drive the second one at greater than six feet away from the first and call it a day. And by the way, who stole section 250.56 out of my 2011 NEC? I just noticed it missing! In the 2008 version, there was a section titled Resistance of Rod, Pipe, and Plate Electrodes (250.56). Sounds like someone thought it would make a nice problem, but it doesn't make sense to ask when the code addresses it directly.

 

[CntrSnr2001]

FWIW, per ieee green book, ground rods should be located the length of the ground rod away from each other to avoid overlap of grounding and achieve max grounding capability, i.e. - if you use 10 ft ground rods, then separate them by 10 feet.

 

[nmh0408]

FWIW, per ieee green book, ground rods should be located the length of the ground rod away from each other to avoid overlap of grounding and achieve max grounding capability, i.e. - if you use 10 ft ground rods, then separate them by 10 feet.

That's not what NEC requires!

 

[CntrSnr2001]

true - its not what nec requires, but i'm just stating it based on ieee. good point to emphasize, though.

 

[wes21]

FWIW - I just attended a telecommunication grounding and bonding class.

As mentioned by a previous poster the NEC states min 6' distance between rods, was discussed in class. But as the instructor pointed out, the NEC is a life safety code - not "a best practices manual" -(the Code also states that you can have up to 13 recpts/20 A ckt, how often do you design that?) and the minimum 6' distance is there to prevent an electrician from being in contact with both ground rods simutaneously - think about a six foot arm-span. If the two rods were at different potentials and the electrican was touching both at the same time, he could complete an electrical circuit.

Also covered in the class was: Three ways to reduce grounding resistance.

1. Increase driven rod length.

2. Increase the quantity of ground rods

3 Chemical enhancement

The preferred method is #2 - because you can take advantage of the "law of parallel resistors"

I hope this helps.

 

[teazer]

answer to this?

 

[cupojoe PE PMP]

FWIW, per ieee green book, ground rods should be located the length of the ground rod away from each other to avoid overlap of grounding and achieve max grounding capability, i.e. - if you use 10 ft ground rods, then separate them by 10 feet.

That's not what NEC requires!

NEC is a guide of bare minimum requirements, engineering judgement is required to figure out the right thing to do, even if that may be in excess of the NEC bare minimum. IEEE Standards (and other industry standards) are great places to get information to make these engineering judgments.

 

[KatyLied P.E.]

FWIW - I just attended a telecommunication grounding and bonding class.

As mentioned by a previous poster the NEC states min 6' distance between rods, was discussed in class. But as the instructor pointed out, the NEC is a life safety code - not "a best practices manual" -(the Code also states that you can have up to 13 recpts/20 A ckt, how often do you design that?) and the minimum 6' distance is there to prevent an electrician from being in contact with both ground rods simutaneously - think about a six foot arm-span. If the two rods were at different potentials and the electrican was touching both at the same time, he could complete an electrical circuit.

Also covered in the class was: Three ways to reduce grounding resistance.

1. Increase driven rod length.

2. Increase the quantity of ground rods

3 Chemical enhancement

The preferred method is #2 - because you can take advantage of the "law of parallel resistors"

I hope this helps.

On a somewhat side note, "Chemical enhancement" reminds me of how it used to be a common utility practice to dump loads of salt in the ground to reduce grounding resistance and in doing so, reduce stray voltage. For all I know, that may still be a practice.

 

[ali1361]

I would consider the average as the resistance of one rod. But if you tie them together then you would need to use green book recommendation to find out the overall resistance.