Earth Resistance - Doubt

Your question doesn't seem right the way you ask it. Can you clarify a bit?

For example, a car battery certainly looks electrolytic, but it won't ever charge or discharge if you have a high resistance (that is, corroded) terminal. So, I'm not sure how you mean to separate the two effects.

Please give more details.

No! I am not talking connections here! Even an individual earth pit without making any external connections to it - we do worry about the earth pit resistance. Isn't it? If I rememebr right, NEC (US) recommends a maximum value of 25 Ohms per earth pit or per earth electrode.

Do we really have to bother about ohmic resistance of the earth? If that would be the case, we might as well fill up the earth pit with some metal dust! Why additives like Salt, Charcoal, etc.?

I am still not certain what you mean. When refueling aircraft on the flightline we were reqired to use grounding points that had been tested to 10'000 ohms. If the resistance exceeded that the grounding point would have to be improved. Sometimes the resistance changed depending on the time of year and moisture in the soil. If the ground couldn't be improved by replacing the ground rod they might have to install a ground bed using several grounding rods connected together.

If this is still not what you are asking, please clarify further.

Drew

The earthpit resistance is the resistance faced by our earthing electrode with respect to what ?

Of course the other end of the current loop - It may be the distribution transformer or the Power transformer or the Generator as the case may be.

There is a very good possibility that the two ends of my conductors (or the earth electrodes) completing the circuit are hundreds of mile apart. But still we will like this to have minimum resistance between them.

Then the question is how to achieve it?

The soil has its resitivity. despite all the efforts we can not change that and bring to zero value, though ideally we will like to have. So we do the next best thing- Let us have the resistivity, try to reduce it, and decrease the resistance any way.

To do this, we crate the high conductance area and simultaneously increase the surface area aroound the electrode.

Keep wet. Add various conductive materials. Add highly ionic chemicals (salt) that will penetrate through the wet soil and will increase the surface area to hundredes of times that had it been the conductor (electrode) by itself.

The metallic dust will only increase the area corresponding to the area you have filled and will only again work through conduction - ie if the metallic particles get mixed with soil, you may lose the advantage.

The activated charcoal in addition will retain the salt and release slowly thereby keeping the area conductive, and assuming a bit of wetness is maintained, the salt concentration will remain and keep conducting.

I still don't get what you are trying to do. Are you simply grounding electrical parts so a short circuit will flow to ground or are you trying to push electricity through the ground?

If you are simply grounding, it doesn't matter how far apart your grounds are, they are not conducting to each other. The reason we use so many in grounding equipment is to proved a faster path to earth than traveling along the equipment. If the ground did fail a test on high ohms the problem would be increased risk to our equipment. If you have too much resistance and a short or lightning strike occurs the ground and wires could overheat and start a fire or damage equipment.

If you are doing something like cathodic protection of underground metal structures, yes you do want the earth to be more conductive, or you place your anode closer to the cathode. In some places our pipeline travels through the ground is very dry and not conductive, we have a hard time keeping the potential high enough to protect the pipe. We do not worry too much because if the soil is un-conductive, it will inhibit stray current oxidization too.

What exactly is your earthing electrode doing? Is it protecting equipment or part of a cathodic protection system, or is it part of a lightning protection system?

Drew

It is not what his electrode is doing .

As far i can guess- he is asking a theoritical question -

a) Why the useage of charcoal and salt in pit.

b) Why earth resistance (and not capacitance)

Typically, in the military, if our ground was bad we would increase surface area of the groundbed instead of trying to increase the conductivity of the soil because that would only be a localized effect. Many of our grounding systems are designed to handle lightning or high voltage pump motor shorts or high static potential from aircraft.

It is logical to deduce that adding conductivity to the soil around your electrode would increase the surface area to the soil you didn't sweeten so it should help reduce the resistance of a ground.

As for the capacitance, the charge has to come from somewhere. You can have areas of positive and negative charge depending on local electrolitic condition and atmospheric potential (lightning storms). But that charge would probably be transitory due to changing conditions of the soil and atmosphere.

You might not think it, but they do build up a big charge while flying. I hated being the first guy to touch the aircraft when it came in for fuel because if I connected the ground with my hand too close I might get hit by the static charge. The few times I got hit it was much more intense than any foot scuffing on the carpet can produce!

I am sure there are people out here who can give the theory behind this, most of my information comes from the practical application of cathodic protection and grounding and bonding of fuel systems. Perhaps one of the electrical guru's will chime in soon.

Drew

I am supposed to be one based on the definition of CR4, on a sabbatical (not logging in).

Else like any other except some who don't think so I am a student and learning still after 30 odd years of leaving the college.

The charge bulid up and the static is common and doesn't need a Guru.

And the earth as a capacitance we learnt in good old days (Capacitance of a sphere- We had to calculate this value in our second or third year of Engineering not sure, might be even pre-engineering physics.)

And the capacitance though supply the charge (or current, whatever way you like it), the lead has to be well connected else you land up in the ohmic loss. It is that simple. And for that we will like to have the resistance of soil to be minimised and that's all.

As far as your flying shocks, as I said is a expected phenomenon.

However I ehard some reports (Truth not verified) there had been quite a few explosions in Petrol (Or gas Or Gasoline- whatever name you like it ) when some of the passemgers got down from the car, and shorted the static build up and generated the all important spark in the combustible vapour environment.

I don't vouch for it and infact can not really understand (and hence rather disbelieve it) since the motor vehicles are not likely to get so charged up and then again the vapour concentration must exist and the spark must be at the correct place (the filling stations are well ventillated at least the one I have seen , and the codes will not allow a nice closed filling station)

I am still only in my second year along the journey of becoming a Mechanical Engineer and have not encountered capacitance of a sphere yet...but now am looking forward to finding out more when I do!

As I said, my experience with grounding and bonding is practical, and in some cases actual. Almost every time I get out of my truck I get hit with a static charge as I close the door. It is so consistent that I intentionally hit the door with my forearm to take the shock in a less sensitive place. If I were to leave my door open, slide out and happen to grab the fuel handle and get it close to my fuel cap before getting rid of that charge it could make the spark right there at the open fuel port. That is the condition that usually starts fires at gas stations. We have been provided videos showing fires and all I saw indicated the initial spark was at the fuel handle when placed in contact with the vehicle.

I understand how static charge can build in a vehicle, dust and particles contacting while in motion, but I tend to suspect in my case it is sliding off the seat that makes my charge. Regardless of how the charge accumulates it is a very good idea to grab metal as you exit your vehicle and Never to get back in after fueling has started. I dont even let passengers enter or exit my vehicle while fueling. One of the videos did show a woman reenter her vehicle after starting fueling, get out and reach for the handle, then we saw the flash of the vapors exiting the tank.

You are correct, due to ventilation at and around petrol stations, the chance of fire is limited to spills and right at the fuel port on your vehicle. When you put fuel in, it pushes highly concentrated vapors out which mix with the atmosphere to create the combustible mixture. On more than one occasion I have spoken to individuals that I see getting in and out of vehicles while I am filling up mine.

Drew

Whoa, hoss. Slow down a mite. Answer me a coupla questions so I know how to answer you.

Are you asking about the NEC ground resistance standard?

Are you familiar with IEEE grounding standards and procedures for testing?

Is there a specific situation involved here?

You cannot measure an earth pit resistance without making connections to it.

I was just referring NEC as an example. I am familiar with IEEE (Wenner'e Four Probe, et al.). I am also aware that without making connections you cannot measure the earth resistance. But, when I said "no connections" I meant not connected to the earthing system of the plant. That is mesauring the resistance of the earth pit in isolation.

I think, I should make my question clearer. You see, I have read in IS 3043 (Indian Standard Code of Practice for Earthing) that the conduction thro earth is NOT ohmic, but electrolytic. And, that's why we add additives and water to the earth pit to have an electrolyte. If this is true, that is, if the current conduction thro earth is not ohmic, why should we bother to have a low earth resistance? Why do the authorities impose it? Am I clear?

I think you're having two different confusions.

Just because you measure resistance and/or use the unit ohms doesn't make it ohmic. Ohmic means the voltage-current curve is linear.

The earthing resistance consists of (at least) three things: resistance of the rod, resistance from the rod to the soil, resistance through the soil. Putting chemicals in the pit can particularly affect the last of these.

But, you've got to get large amounts of current out into that pit without raising the voltage excessively. What better measure than ohms? I'm missing your objection.

Good conversation, dont let it get hostile though I am enjoying learning new things about grounding and look forward to reading this through to the end.

I'm not hostile. Only grumpy.

Thanks for your informative replies! But, do you mean that even in electrolysis, the ohmic resistance of the electrolyte does matter?

You're missing an important idea: resistive and ohmic are two different things. Electrolytes are not ohmic. They will have an impedance which, of course, includes a resistive term, but they are not ohmic.

The electrolysis ( created by salts) is the thing that matters. What we are trying to do is adding the electrolytes to reduce the soil resistivity.

I think there is a confusion about electrolyte Vs Eletrolytic capacitance.

I will just add a good handbook, go through it

http://www.rammeter.com/bulletins/GettingDownToEarth.pdf

Page 8 and 15 onwards (if you really don't find it interesting enough)

Also see this

http://www.lightningman.com.au/Earthing.pdf

Page 38 (7.1)

Low soil resistivity is to some extent tied to an electrolytic mechanism with such characteristics as - chemicals composition, soil ionisation, homogenous grain size and even distribution, playing a large determinant due to the effect on the retention of soil moisture and packing density in contact with the electrode.

So, Is it the electrolyte that conducts with impedance or would the soil be conductive without electrolyte?

If it IS the electrolyte's impedance, why do I measure in ohms the effectiveness of my fuel tanks ground. We have a rod with a gizmo on the end that has a porous tip on it and we have to scuff or dig down to moist soil, then we put one end of the multimeter on the tank, and the other on our rod with the gizmo and measure the ohms between the gizmo in the soil and the tank.

If you need it, I will try to look up the gizmo. We use something similar but slightly different to determine the effectiveness of our cathodic protection system.

Drew

Read the attachments- there the studies were done with and without electrolyte. And also the megger handbook about earth resistance testing.

As you go away and away from the earthing electrode, the surface area is increasing, so after some radial distance, it becomes so large that even if you have high resistivity, resistance (= resistivity x cross section area) becomes low enough to be insignificant.

What you need to do it reduce the resistivity where it matters- that is near the electrode where the surface area is still small enough that the R is affected by the resistivity.

You know the best example is distilled water- pure water is one of the best insulators around, but mix the tiniest amount of electrolyte and the ions act not only to meke it conductive, but also helps in releasing the H₂ and O₂.

Then though the water (salted ) is electrolyte, we still bother about its resistance.

Let me give you another electrolyte where you are measuring the resistance- the cells (battery) - we measure the EMF and the internal resistance - and the chemical ones are electrolytic?

(BTW- the unit of impedance is ohms too as is the one of resistance ) - in fact in our old college days we used to interpret as the resistance to flow of electricity - it may be ohmic resistance, non-linear (eletrolytic), Inductance (wL), capacitance (1/wC) or any other type.

Again with your gizmo, you are trying to measure the eaarth resistance of the tank earth (the method might be similar to page 9 of the megger hand book, or may be one of the more modern versions, However for our factory earth, we still rely on the good old one.

BTW: form air craft you have gone over to tanks?

Bulk storage fuel tanks for a cross country pipeline, most vehicles are not grounding during fuel transfer or refueling (except through the nozzle if equipped with a ground wire inside the hose or using a metal coil inside a hard hose).

I was working for a pipeline company and got to help our CP (cathodic protection) guy find a broken wire. You would be surprised how fast the wire going to the anode will degrade if the insulation is comprimised by a small cut.

I think the gizmo we used was just a specialized electrode. It was slightly thicker than my thumb with a rubber cap over a porous tip. The whole thing was no more than 8 inches long (about 20 cm). It was attached to a pole so we could push it into the ground without bending over. He had two, one for testing ground, and the other for testing CP potential had a GPS and multimeter attached to the pole so he could record the location he performed the test (important when your pipeline goes from Louisiana to Virginia).

Which attachments are you referring to? I will look up the Megger handbook at the library on campus.

Drew

It is the one on #23, getting down to earth. that is the megger hand book. Click on the link and and down load.

There may be an ambiguity in the posed question. By "earth resistance" are we discussing the planet as a conductor, or do we mean the resistance of wires/cables in a grounding system? If the latter, low resistance is important for safety, proper operation of fault-trip devices, bleed-off of static accumulation, etc.

"The Soares Book on Grounding and Bonding" is one reference that comes to mind.

The earth is a theoritically infinite electrolytic capacitor. However what we are doing is connecting the capacitors terminal with the earthing electrode. The connection is obviously considered ohmic- since the current flow is through the ions as in any other case (electrons/ holes)- and here the earth resistance come in picture the resistance that is between the theoritical capacitor and the electrode terminal.

It is very similar to the semiconducting material- though they have capacitance, but what we bother about is their resistance.

Cool, my first good answer.

And I forgot to mention Nikola Tesla and his idea of providing electricity to people by putting a charge in the ground. Unfortunately I don't remember the details, but I am sure google can. I would add a link, but I got to run. Happy googling!

Drew

Too early for you. I had to wait for a 100 odd to get my first one. Of course after that it maintained a better ratio one in 25 or so in the end.

Earth resistance must be low to allow efficient operation of circuit protection devices( ELCBs, etc) and discharge to earth leakage currents within shortest time possible before voltage rises to lethal level causing shocks , electrocution to persons or animals, damage to equipment and fires.