Detached garage grounding

RAL
 
I've read a few different explanations of why two ground rods are a problem in the past but this is the first one that I actually understand. Rumor has it that two rods are a problem yet the local inspector insisted on it here and now I understand the reasoning. Two rods 8' away from each other and bonded together is no problem and offers more certain grounding for your equipment.
 
Thanks!
 
mikefamig said:
I'm not an electrician and I was repeating what my local inspector told me to do when I installed the service to my garage. Just as well though, two rods can't hurt anything. It is also important to isolate the neutral from the garage at the sub panel. The neutral should be grounded in only one point in the entire system which is usually at the load center in the main building (house).
Click to expand...
The more bonded rods the better... it only aids in lowering the impedance which is never constant anyway. Welding the ground wire to the rod is even better. And you're absolutely correct - the neutral should only be bonded in 1 place. That includes connecting a portable generator up to house wiring during a storm.
 
mikefamig said:
At any rate it is fine to ground a surge protector to an existing ground rod or to drive a new rod and ground to that. I can't see any difference as long as you maintain the less than 25 ohms rule.
 
Mike.
Click to expand...
Do you know if you're at 25 ohms or less? Really impossible without the proper equipment. Then again reaching that goal may not be easily feasible or even completely necessary.
 
An interesting email came today to my inbox from a magazine once know as Popular Electronics (Today it is Nuts and Volts). 
 
This piece was in the email.
 
Simple Earth Ground Tester
 
Question:
 
ARRL has published a circuit to test ground resistance. However, it involves the use of 110 volts AC. Not wanting to run around the yard with an extension cord, would you please suggest a battery-powered circuit that would economically measure ground resistance (in a three-electrode configuration) at 60 Hz to 200 Hz?
 
 
 
Yes, I have the circuit you request. But let me give the reader some background information on earth ground resistance before I spring it on them with no explanation of what in the world you're talking about. First, earth ground is used as a conductor in power distribution systems. Its purpose is to minimize the hazard of a lightning strike by grounding one leg of the AC line in soil. Second, radio waves use it too, as a ground plane to increase the effective radiation power of the antenna. In both cases, it's important to know the resistance of the soil and treat it with chemicals if the resistance is too low.
 
There are three ways to test soil resistance using two-, three-, or four-point measurment. The most popular is the three-point configuration (shown below). Its parameters are very well-documented, and it's easily implemented. You can think of it as a four-wire ohmmeter with two electrodes in common. The two outer electrodes establish a current in what can be considered an "earth" resistor.
 
QA_0703_EARTH-1.jpg

 
The voltage test point (P2) for three-point soil measurements is at 62 percent of the distance from the common probe (P1). The resistance is determined by using Ohm's Law (R = E/I), where I is the current flowing between P1 and P3, and E is the voltage between P1 and P2. When driving the stakes, it's essential that they be in a straight line — with P3 as far from P1 as practical (within limits — not the next county!). Unfortunately, I don't have room to discuss the effects of effective resistance zone overlap, but suffice it to say that the three-point method fairly compensates for it.
 
QA_0703_GND-resistance.jpg

 
As for the circuit above, it delivers an output equal to that found in most portable ground resistance testers — 26VAC (open) at 40 mA (shorted). A single 555 astable oscillator provides a 60-Hz squarewave that drives a 6VAC wall-wart transformer. The timing capacitor (0.1 µF, pin 2) determines the frequency — reduce it to .05 µF and the frequency increases to about 130 Hz. The rest is up to you and your Fluke (DMM).
 
Video321
 
Good point about the portable generator. I've seen people back-feed generators through an AC outlet with no idea that the neutral and ground are tied together at the generator completely defeating the safety provided by the ground wire.
 
So here's my public service announcement for anyone who is back-feeding a generator. When you do this you are creating a real potential for a really good shock. When you have the neutral and ground connected to each other at the load center in the house and also at the portable generator you are literally making the two separate conductors into one conductor with half the impedance. In other words you have no ground conductor, both wires become one neutral lead. The neutral and ground wire are both carrying current (being tied together at both ends). If a stray current occurs in the generator the metal frame can become energized without tripping a breaker and the generator can go on running as happy a clam until someone comes along and touches it. At this point that current can go through that person to the ground under their feet.
 
Ok that was my general service announcement, Mike.
 
EDIT
Not that I'm recommending back-feeding a generator but if you were to isolate the neutral from the ground in the generator you would alleviate this problem.
 
RAL said:
Does your garage have a subpanel, or just a single circuit with the breaker back in the house?
 - Yes, the detached garage has a sub panel and not just a single circuit.
 
Is that rod in the photo driven into the ground?  Looks like a piece of 1/2" rebar, which wouldn't meet code for a ground rod.
 - Yes. While I cannot verify, I believe this was a 10-12ft piece of rebar. The one pictured above is in a friend's home (again built 2014). My house was done the same way in 2008.
 
The NEC does allow using steel rebar of at least 20 feet in length as a grounding electrode when it is encased in concrete (e.g. a concrete floor).   But the copper wire connecting to it in the photo looks like it is #12 or #14, rather than #8.
- I doubt that rebar is 20-ft in length, but again have no way to confirm. I would agree that it's either 12 or 14; this was not something I previously inspected at all. I only have the picture(s) because I took pictures of every section of every wall in his home. In the case of my friend's home, the ground was placed approx. 10-ft away from the main panel, so the wiring runs up the wall, and then back down to the panel. I would guess that ground is 24-ft in length or so...
 
If I understand you correctly, this ground rod is at the house and not the garage, and is inside?
- Correct. The ground pictured is at the house, accessible from inside the attached garage. I am pretty certain that the rebar was placed after the concrete forms were in place, and the framers notched the bottom plate around it.
Click to expand...
 
Thanks for that, Pete.
Should make for a fun project in the spring.
 
Mike-
Not to beat a dead horse... I've talked to many people about gen safety right after Sandy and no one had any idea of it. The worst part is some of those people were electricians!
 
drvnbysound said:
Does your garage have a subpanel, or just a single circuit with the breaker back in the house?
 - Yes, the detached garage has a sub panel and not just a single circuit.
 
Is that rod in the photo driven into the ground?  Looks like a piece of 1/2" rebar, which wouldn't meet code for a ground rod.
 - Yes. While I cannot verify, I believe this was a 10-12ft piece of rebar. The one pictured above is in a friend's home (again built 2014). My house was done the same way in 2008.
 
The NEC does allow using steel rebar of at least 20 feet in length as a grounding electrode when it is encased in concrete (e.g. a concrete floor).   But the copper wire connecting to it in the photo looks like it is #12 or #14, rather than #8.
- I doubt that rebar is 20-ft in length, but again have no way to confirm. I would agree that it's either 12 or 14; this was not something I previously inspected at all. I only have the picture(s) because I took pictures of every section of every wall in his home. In the case of my friend's home, the ground was placed approx. 10-ft away from the main panel, so the wiring runs up the wall, and then back down to the panel. I would guess that ground is 24-ft in length or so...
 
If I understand you correctly, this ground rod is at the house and not the garage, and is inside?
- Correct. The ground pictured is at the house, accessible from inside the attached garage. I am pretty certain that the rebar was placed after the concrete forms were in place, and the framers notched the bottom plate around it.
Click to expand...
Click to expand...
 
I dug up the section of the NEC that specifies what the rebar must be. It doesn't have to be one continuous piece 20 feet in length, but can consist of shorter pieces tied together with tie wires.
 
"250.52(A)(3) Concrete-Encased Electrode. An electrode encased by at least 50 mm (2 in.) of concrete, located horizontally near the bottom or vertically, and within that portion of a concrete foundation or footing that is in direct contact with the earth, consisting of at least 6.0 m (20 ft) of one or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than 13 mm (1⁄2 in.) in diameter, or consisting of at least 6.0 m (20 ft) of bare copper conductor not smaller than 4 AWG. Reinforcing bars shall be permitted to be bonded together by the usual steel tie wires or other effective means. Where multiple concrete-encased electrodes are present at a building or structure, it shall be permissible to bond only one into the grounding electrode system."
 
video321 said:
Thanks for that, Pete.
Should make for a fun project in the spring.
 
Mike-
Not to beat a dead horse... I've talked to many people about gen safety right after Sandy and no one had any idea of it. The worst part is some of those people were electricians!
Click to expand...
video21
 
I have a question that you may be able to answer. Does a transfer switch somehow deal with this problem? If yes then how?
It seems to me now that even with a transfer switch there is a problem.
Mike.
 
mikefamig said:
video21
 
I have a question that you may be able to answer. Does a transfer switch somehow deal with this problem? If yes then how?
It seems to me now that even with a transfer switch there is a problem.
Mike.
Click to expand...
 
The answer depends on the transfer switch.  If the transfer switch has 2 poles and switches just the two hot conductors, then the bond between neutral and ground must be removed on the generator for the safety reasons you stated earlier.
 
But if the transfer switch has 3 poles and also switches the neutral, then the generator can have a bond between neutral and ground.  The extra pole in the switch will ensure that only one grounding bond is connected to the wiring at any time.
 
Mike-
RAL has it.
 
I don't use a transfer switch, but rather a breaker lockout (or whatever the hell it's actually called) with a generator that has a floating neutral. If I ever needed to bond it with the ground I can simply make up a jumper plug and insert it into the 240 receptacle.
 
Very interesting discussion, but I am still confused.   When I talked to my electrician about the measuring the impedance of the ground, he gave me a blank stare.   Has anybody actually worked with an electrician who could measure it?   Perhaps driving two ground rods is their answer to satisfy code without having to actually measure the impedance.
 
On the article that Pete provided, I am further confused.   I took a couple of electrical engineering courses 40 years ago, so I am a little foggy on the concepts.    However, it appears that in the article they have assumed that the 3 stakes have perfect contact with the soil, and they are measuring the resistivity (impedance) of the soil-- as opposed to the resistivity between the P1 electrode and the earth.    In my specific case, the soil is very thin (6 inches) with limestone below.   Therefore, driving 3-8 ft ground rods is not an option.   I dont think that burying 3 rods horizontally is a good option either, because the soil around the rods is disturbed and does not reflect the true earth resistance.
 
In the end, I just had my electrician tie the ground to the rebar in the foundation and hoped that gives me a low impedance ground.   (A Ufer ground)   However, I still dont know if it is less than 25 ohms.
 
rockinarmadillo said:
Very interesting discussion, but I am still confused.   When I talked to my electrician about the measuring the impedance of the ground, he gave me a blank stare.   Has anybody actually worked with an electrician who could measure it?   Perhaps driving two ground rods is their answer to satisfy code without having to actually measure the impedance.
 
On the article that Pete provided, I am further confused.   I took a couple of electrical engineering courses 40 years ago, so I am a little foggy on the concepts.    However, it appears that in the article they have assumed that the 3 stakes have perfect contact with the soil, and they are measuring the resistivity (impedance) of the soil-- as opposed to the resistivity between the P1 electrode and the earth.    In my specific case, the soil is very thin (6 inches) with limestone below.   Therefore, driving 3-8 ft ground rods is not an option.   I dont think that burying 3 rods horizontally is a good option either, because the soil around the rods is disturbed and does not reflect the true earth resistance.
 
In the end, I just had my electrician tie the ground to the rebar in the foundation and hoped that gives me a low impedance ground.   (A Ufer ground)   However, I still dont know if it is less than 25 ohms.
Click to expand...
 
I think that most electricians don't see the value of performing the test to be able to get away with a single ground rod, and would rather just drive a second rod and be done with it. No need to own the special equipment and it saves the time needed to make the measurement.
 
A Ufer ground, obtained by connecting the grounding conductor to the rebar in your foundation, should provide a good earth ground.
 
http://www.ecmag.com/section/codes-standards/what-ufer-ground
 
"Ufer developed the initial design for a concrete-encased grounding electrode that consisted of ½-inch, 20-foot-long reinforcing bars placed within and near the bottom of 2-foot-deep concrete footings for the ammunition storage buildings. Test readings over a 20-year period revealed steady resistance values of 2 to 5 ohms, which satisfied the specifications of the U.S. government at that time. This work eventually resulted in what we know today as the concrete-encased electrode in the NEC."
 
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