A contractor informed us that our main electrical panel is not actually connected to ground. He proposes to install a copper tube to earth below the panel and then connect that to the ground wire input on the panel. My questions are:
1) If the panel is actually connected to some other ground in the building, is there any harm in connecting to a real-earth ground in addition?
2) Once the ground wire is attached, what would be a safe way to test the installation to make sure that a randomly selected circuit fed by that panel has its ground wire attached to the real-earth ground?
Did he check the meter can? Was this ever inspected? There is no problem with multiple ground electrodes anyway. The more the better.
There is no way for a homeowner to really test the ground electrode system but the code says if you are not sure, drive 2 rods. The circuit tester will check the ground paths inside the house to be sure each outlet is connected to the ground bus.
No, there is no harm. Originally panels were bonded to cold water pipes for a ground. Then later a ground rod (5/8"x8') was driven in as a supplemental (backup) ground.
You can't test it without the correct equipment. But then again, it's not up to you to test it, that's why you call an electrician who is trained to do this. If you're questioning the word of the contractor that was there, then A don't use him, or B call someone else for a second opinion.
Unless your electrical contractor is planning to install a second ground rod in addition to the first he/she has to conduct a ground impedance test using rather expensive equipment. The test result has to show that the impedance to earth of the first rod is twenty five ohms or less. If the tested impedance is greater than twenty five ohms then a second electrode is required. If he/she is going the two electrode route instead of testing you should ask them to sink the second rod at least twice the length of the rods being used apart. The code does allow a separation of only six feet but that is to allow for urban installations were a greater separation cannot be achieved. Best practice is to separate driven rods by twice their length and to separate them from underground structures such as foundation walls by at least their own length. With eight foot rods that means eight feet out from the foundation wall were possible and then another sixteen feet to the second rod were feasible. That provides separate and complete shells of earth for each rod to dissipate any fault current resulting from a lightning strike to the lines or a high voltage power cross caused by line damage. The more of that fault energy that is successfully dissipated the less of it will enter your home on the service conductors.
What types of electricians would be likely to have proper ground testing equipment? I'm thinking someone who installs high voltage panels or generally works with high voltage equipment is likely to be well versed in grounding, but how to find such a specialist?
When I have seen this they hired an engineering company and got back stamped certifications. I assumed it was more of a legal thing than a technical thing.
The last time I looked that NEC was rather tolerant of what most would consider to be a "bad" ground.
I believe the old rules said that the "ground resistance" of THE ground rodshould be 25 ohms or less and if it didn't meet that requirement a second ground would have to be installed.
Rather than screw around, most electricians simply installed a second ground rod.
The "real world" solution to your problem is to install 2 ground rods (8' long, etc) and connect them to the panel ground.
There are some arcane rules about how connections between the grounding wires should be made (yeah, wire nuts aren't gud enuf.)
It's useful to speculate on what is the most "dangerous" condition.
Most would say that it would be a broken neutral between the power company transformer and you entrance. If your loads are unbalanced, you can end up with your neutral being 120 volts from ground and each and every "ground" pin on each outlet 120 volts from ground.
In such a situation, the main thing your "ground rod" does is reduce the voltage gradient between "real" ground and "local" ground.
That, alone, is sufficient justification to install your two ground rods and bond them to the panel and local neutral.
In these situations, the "more the merrier" rule applies. It certainly wouldn't hurt to run a copper cable around the house and connect several ground rods to that cable and, of course, bond that cable to your two rod "official" ground which, BTW, is also used (or should be used) by the cable company, the telephone company, and dish/tv antenna installers.
The better your "local ground" is the lower the potential difference between "power company ground" and your "local ground" can be.
It went largely unnoticed by a lot of people but the NEC now says "if available" steel in the footer shall be incorporated into the grounding electrode system. A lot of building departments are saying you will make it available and you will install the Ufer ground in any construction that gets a footer. It is part of the footer inspection now most places..
I agreed with most of the ideas replied back to you, I do see a few options, such as:
-Checking with your city representatives in the electrical department and they will guide you in the proper direction with permits.
-You can also look at popular electrical installers, perhaps people with a long history of work through home electrical wiring, perhaps they can send someone out to give you an overview of what needs to be done, then they will guide you in the direction of proper permits which need to be obtained.
-You can do alot of reading and research and know your stuff, so when you call someone to do the work you will have an idea of what needs to be done. (not the most popular if you don't like to do alot of research).
-Assume that your electrician knows what he is doing is correct and kick-back and relax. (not recommended).
That is - the building system "ground" is 120V from earth.
I am not fond of the multiple meanings and functions of "ground" and "ground wire". It contributes to the difficulty reading the NEC chapter on grounding.
You weren't looking at fault current, but adding fault current to the "local" earth:
Suppose you take a simple example of a parking lot light with only a ground rod (no ground wires) and a near miraculous resistance to earth of 10 ohms. There is a short from hot (120V, 20A circuit) to the metal pole. No worry - we have a ground rod. The current to earth is 120V/10 ohms = 12A. If the breaker has the allowed continuous load of 16A, that gives a 28A fault current. Looking at the time-trip curves for a SquareD breaker, it will trip in 30-200 seconds. If the circuit is loaded lighter the breaker may trip - never.
But the rod drags up the local earth potential? As a rule of thumb, 70% of the voltage drop in the earth away from the rod is in the first 3 feet. From the earth over 3 ft from the rod to the pole the voltage will be 85V or more. (This is also affected by the resistance to earth at the utility transformer.)
Now extend that to a building.
For fault current you might get better results from a metal underground water system - but you are working with the resistance to earthing at the utility transformer too. With a typical municipal metal water supply the path is likely through the water pipe, to an adjacent building, through the neutral-ground bond at that building, thorough the neutral service wire for that building, back to the supply transformer.
The NEC requires an effective path for return of fault current. "The earth shall not be considered as an effective fault-current path."
I thought "concrete encased electrode" was a recent addition to the code. I was surprised that it appears as far back as I looked, 1984, with language "if available on the premises..." IMHO it is not "available" unless a connection is brought out - would be interesting what a court would say....
But the language starting 2005 is electrodes "that are present at each building" In most buildings 20 ft of rebar, which may be tie-wired together, is "present". And it is perhaps clearer in the exception that the code wants the electrode for new construction.
I don't remember when they started requiring a Ufer ground here - I think it was with the new language of 2005. It wasn't flagged in the code change book I read. But Ufer replaces ground rods - a major plus.
Am I reading right that your inspection is by the footer inspector, not an electrical inspector?
Yes that is true in 1&2 family. The Ufer will be verified by the structural inspection when they look at the footer. (although the electrical inspector and the structural inspector are usually the same guy these days) Typically it is just a piece of rebar that is turned up into the wall and the 4ga copper connection is made to that later.
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If somebody screws up and grouts the cell with the Ufer in it, they will be chipping out concrete to make it available again. Hence the green paint.
In commercial they may have a separate electrical inspection for the Ufer. I know I have driven 150 miles to look at an acorn clamp and 10' of copper wire. I finally started telling them to take a picture and Email it to me if it was 2 counties away..
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