Finding earth ground impedence (ohms) at a residential electrical service panel???

[ActionDave]

Yeah, but I 'm up late looking to keep the dicussion going. Daylight Savings has me all messed up.

 

[K8MHZ]

Mine was just a "is it required" question. I see guys drive rods for all kinds of things, radio, phone, cable tv, etc. and not connect them to the grounding system. I Just roll my eyes and laugh at the fact they think they are doing good. I know what is required for "inner system bonding" but I wasn't sure about a ham radio and if it is able to run on a battery I would say it isn't required, for that aplication.

Ham radio has it's own set of rules. We don't need UL or an NRTL on our equipment as we can build our own. We don't operate on channels like all other (except military) services, we use frequencies. As far as grounding and safety, all we have as rules are 810.71 (A) through (C). (A) just says that a transmitter supplied by premises wiring must be enclosed by metal and all metallic parts must be connected to a grounding conductor. (B) states that metal handles and controls must be accessible and (C) requires interlocking for transmitters that operate at 350 volts or more (tube type). Receivers may or may not be grounded, but if they are, the grounding method has to follow 810.21 (A) through (K).

The 'premises wiring' is the tricky part. Obviously, a car battery is not premises wiring, but is a DC power supply meant to serve as the DC power the battery would considered 'premises wiring'? Sure, the supply is connected to premises wiring, but is the supply itself considered to be wiring?

What if there are no grounding conductors on the premises?

The subject of grounding is convoluted enough without the added ambiguity thrown at radio operators.

Take this installation, for instance. A 100 watt transceiver connected with two wires to a regulated 13.8 power supply. The power supply has a three prong plug, but since the house is only two wire, a GFCI is installed to provide a third hole. None of the antennas are outdoors.

Violation?

Now let's add an automatic antenna impedance matching device that get's its power from the radio. In order to function properly, the cases of the radio and the matching device have to be bonded together and there are lugs on the back of both for that purpose. The conductor making the bond would be an EGC, but would be doing NO grounding.

Violation?

The NEC goes into great detail about how grounding electrodes and conductors are supposed to be constructed and connected and for the most part are rather succinct about it. They aren't so clear on is what, exactly, needs to have an earth connection, let alone why.

In the above installation, a connection to the earth would serve no purpose whatsoever. Even if the transmitter was powered by 120 and fed with a three prong plug, so long as the neutral and ground were bonded, a connection to the earth, again, would serve no purpose.

So why the requirement?

That is subject to much conjecture. I think the main reason is that in some cases the connection to earth is useful and in others it's not (like in our sandy soil) and for brevity there exists a blanket requirement.

 

[johngary]

Sparkey 5

Sparkey 5

There are NEW meters on the market that will simply clip over the Grounding Electrode Conductor and directly read the ground resistance between the servicing utility ground rod at the transformer pole and your ground rod connection.

In our area the Ufer ground is required on all new foundations - meaning that other type of grounding is not accepted - if you miss the Ufer ground in the foundation, you then must install a ground ring around the building. EXPENSIVE DON'S MISS THE UFER GROUND.

We installed a Ufer ground, but did not get it inspected when the foundation was poured - the building contractor took pictures, but that is no longer an accepted way of proving the Ufer ground - pictures can be used again and again who know what job it was from so say the Inspectors.

So we used a NEW meter Cost about $1200.00 - to use it you must have access to the utility service. In our case the temporary power pole was still in tack - I meet the inspector on the job - I clip the meter over the #6 ground wiring going to the ground rod and read 13 ohms - GREAT -- I then disconnected the #6 wiring going to the ground rod and connected it to a #4 copper wire and ran it back to the main service on the NEW house - I attached it to the Neutral bar to which the UFER ground had been attached - I then clip the NEW meter over The #4 copper ground wire and it read 3 ohms - the inspector then said WOW - to show that the meter was reading accurately, I then went back to the temp power pole, reconnected the #6 ground wire to the ground rod and again clip the meter over the wire - LOOK again the meter read 13ohms.

What is the meter reading? - the ground resistance between the utilities ground at the Utility pole ground and the ground you have at the service. Why 25ohms to ground or less - I understand that the grounding will short to ground voltage in excess of 600V - meaning that the wires you are using are rated for only 600volts maximum - and that should a high voltage from lighting and/or a high voltage line short out to the neutral wiring of the incoming service drop that the grounding electrode conductor would take the major portion of the high voltage to ground thus saving the internal wiring within the building from being damaged.

 

[K8MHZ]

There are NEW meters on the market that will simply clip over the Grounding Electrode Conductor and directly read the ground resistance between the servicing utility ground rod at the transformer pole and your ground rod connection.

In our area the Ufer ground is required on all new foundations - meaning that other type of grounding is not accepted - if you miss the Ufer ground in the foundation, you then must install a ground ring around the building. EXPENSIVE DON'S MISS THE UFER GROUND.

We installed a Ufer ground, but did not get it inspected when the foundation was poured - the building contractor took pictures, but that is no longer an accepted way of proving the Ufer ground - pictures can be used again and again who know what job it was from so say the Inspectors.

So we used a NEW meter Cost about $1200.00 - to use it you must have access to the utility service. In our case the temporary power pole was still in tack - I meet the inspector on the job - I clip the meter over the #6 ground wiring going to the ground rod and read 13 ohms - GREAT -- I then disconnected the #6 wiring going to the ground rod and connected it to a #4 copper wire and ran it back to the main service on the NEW house - I attached it to the Neutral bar to which the UFER ground had been attached - I then clip the NEW meter over The #4 copper ground wire and it read 3 ohms - the inspector then said WOW - to show that the meter was reading accurately, I then went back to the temp power pole, reconnected the #6 ground wire to the ground rod and again clip the meter over the wire - LOOK again the meter read 13ohms.

What is the meter reading? - the ground resistance between the utilities ground at the Utility pole ground and the ground you have at the service. Why 25ohms to ground or less - I understand that the grounding will short to ground voltage in excess of 600V - meaning that the wires you are using are rated for only 600volts maximum - and that should a high voltage from lighting and/or a high voltage line short out to the neutral wiring of the incoming service drop that the grounding electrode conductor would take the major portion of the high voltage to ground thus saving the internal wiring within the building from being damaged.

I read the instructions for one of these clamp on meters and according to the mfg., a 'clamp only' reading will only give you ground loop resistance. In order to get earth ground resistance, at least one 'spike' must be used in conjunction with the clamp. You may want to get out your instruction book and make sure what you are measuring is earth resistance and not ground loop resistance.

 

[zog]

I read the instructions for one of these clamp on meters and according to the mfg., a 'clamp only' reading will only give you ground loop resistance. In order to get earth ground resistance, at least one 'spike' must be used in conjunction with the clamp. You may want to get out your instruction book and make sure what you are measuring is earth resistance and not ground loop resistance.

Correct, the post you quoted show a classic misuse of clamp on ground resistance testers. They are very limited in applications and are misused all the time. Most facilities do not even allow them to be used when the application is correct because there is no way to prove your results are accurate and are not being influenced by other things in the area.

 

[K8MHZ]

Correct, the post you quoted show a classic misuse of clamp on ground resistance testers. They are very limited in applications and are misused all the time. Most facilities do not even allow them to be used when the application is correct because there is no way to prove your results are accurate and are not being influenced by other things in the area.

I will be teaching a short class on the use of test equipment in a few weeks. This thread is very timely and will definitely be referred to in class.

 

[K8MHZ]

I understand that the grounding will short to ground voltage in excess of 600V

What does that mean and where did you come up with it?

 

[suemarkp]

Your test depends on the integrity of the connection of the neutral to the earth. If you were to snip the little #6 wire on the pole supporting the tranny feeding the panel you were taking these measurements from, your measured current would drop drastically. Do you not agree? The act of snipping the tranny ground does not change the resistance of the electrodes you are measuring, but it WILL change the current. Thus, you are not testing a connection to the earth, you are testing a connection back to the neutral terminal on the transformer.

Isn't the transformer grounded conductor the same on both the primary and secondary? Is it not also connected to each adjacent pole through the grounded conductor (low wire between the poles)? That appears to be how my utility system is wired. So if you snip the GEC at a power pole, there are many more on the other poles up and down the street. I think the wire resistance between the poles will still be less than the resistance of that one gorunded pole GEC you snipped.

I would think the utility grounding system would be about as well of a grounding comparison that you can measure against -- thousands of grounded poles all bonded together via the lower wire on the pole.

 

[K8MHZ]

Isn't the transformer grounded conductor the same on both the primary and secondary? Is it not also connected to each adjacent pole through the grounded conductor (low wire between the poles)? That appears to be how my utility system is wired. So if you snip the GEC at a power pole, there are many more on the other poles up and down the street. I think the wire resistance between the poles will still be less than the resistance of that one gorunded pole GEC you snipped.

I would think the utility grounding system would be about as well of a grounding comparison that you can measure against -- thousands of grounded poles all bonded together via the lower wire on the pole.

Not all poles are grounded. The ones with devices are and I think every 6th one is.

The idea of earth resistance testing is not to 'measure against' the utility grounding system. The purpose is to test the integrity of the connection of the electrode (or electrode system) on a premises to the earth. The utility grounding system has nothing to do with it.

You are correct, the fact that the POCO has thousands of electrodes connected together does make for a good ground. Even so, it doesn't protect against lightning or shock, and as inside wiremen, the integrity of the POCOs system means little to us until it fails (which it does) and presents stray currents on the earth.

The reading we get (in ohms) is not the resistance of the earth, although it does come into play. The reading we get is the resistance between the measured electrode and the earth.

If you look at how a fall of potential test is done it makes a bit more sense. That test compares the electrode being tested to other electrodes outside the sphere of influence of the first and takes both current and voltage readings. With a little math we can determine the resistance of the electrode being tested. A proper test is done with the test electrode NOT connected to the service or utility.

 

[LISHAJI]

A proper test is done with the test electrode NOT connected to the service or utility.

I ditto that! If you would plot a curve using the 3 rod fall of potential method, one can find out whether the utility ground connection was isolated or not from the local ground. See attached the curve the red represents when connection is not isolated.

 

[suemarkp]

So that was my point -- the utility system seems like a good earth ground reference to compare to instead of sinking your own remote rods to test against. Use the neutral (isolated from the panel) as your conductor to the remote rods. Use the rod you just sunk as the rod under test. Or just isolate that grounded neutral wire and use the utility ungruonded wire directly to the rod under test and see what kind of current flow you get. The only path the current can flow is from transformer, to rod under test, to earth, to utility rods, to utility GECs, to utility X0.

I realize the utility is not intended to be involved in ground rod testing. But it seems like a good remote system and you have a heavy wire to it in your service drop.

What has never made sense to me in the fall of potential test is how do you know if the remote rods have a decent connection to earth (is your local ground electrode poor, or the remote one, or both)? Seems like thousands of utility rods or a metal water utility pipe would be a well earthed remote electrode. Or does it somehow not matter?

 

[K8MHZ]

So that was my point -- the utility system seems like a good earth ground reference to compare to instead of sinking your own remote rods to test against. Use the neutral (isolated from the panel) as your conductor to the remote rods. Use the rod you just sunk as the rod under test. Or just isolate that grounded neutral wire and use the utility ungruonded wire directly to the rod under test and see what kind of current flow you get. The only path the current can flow is from transformer, to rod under test, to earth, to utility rods, to utility GECs, to utility X0.

I realize the utility is not intended to be involved in ground rod testing. But it seems like a good remote system and you have a heavy wire to it in your service drop.

What has never made sense to me in the fall of potential test is how do you know if the remote rods have a decent connection to earth (is your local ground electrode poor, or the remote one, or both)? Seems like thousands of utility rods or a metal water utility pipe would be a well earthed remote electrode. Or does it somehow not matter?

Again, we are not looking for anything related to the utility ground or the integrity of it's connection. The fall of potential test has specific requirements.

two auxiliary electrodes are driven into the soil at predetermined distances, per the testing specifications, in a straight line from the ground being tested.

The electrodes used for the test come with the tester and are designed to be used with that tester. The results of the test are then graphed and the final value is a result of a particular slope.

Remember, we are not measuring the conductivity of the earth, we are measuring the connection of an electrode to the earth. That's why the test purposely places all the electrodes outside of their respective spheres of influence. The sphere of influence presented by the utility's ground system is so large it may not even be possible to get away from it and still be close enough to perform the test.