# Ground Earth Resistance explained in electrical system

I have several questions about the electrical systems used in the field and the grounding mechanisms.

So, as far as I understand there are multiple systems that can be used to distribute electrical current in the field: TT, TNS, TN-C, TN-C-S. In a single phase system it will be the live wire, ground and the protective ground (aka PE).

The TNS, TN-C and TN-C-S systems are relatively easy to understand, there is a clear path through wires for all of the signals (live, ground and PE), but for the TT system the PE will be connected separately using the grounding/bonding/earting (see last question) rod buried in the soil. And the current is supposed to flow through the ground back to the transformer in case of a ground fault. But here is my first question: How is this useful, the soil has a big resistance/impedance and that means it will cause a really small current to flow through it and maybe the breaker won't break. Also it is somehow confusing because searching the web, the actual resistance needs to be smaller then 200 ohm (TT system), maybe there is something that I don't understand, but having the transformer 100 meters away with a soil resistivity of 40 ohm*m (got it from random table found online for the best conductor soil) will give you at least 40 * 100 ohm (clearly bigger then 200?). Still looking on some YouTube videos some guys are measuring the Earth Ground Resistance and it's about 2.1 ohm ? Video for loop impedance.

The second question is about the Earth Clamp Resistance Meters. How are those doing their job ? As far as I understand they will induce a current in the loop and measure the current and using the Ohm law will deduce the resistance. But how is the loop formed, through the ground back to the transformer and then through the ground back to the device ? Can't really see the loop here.

And the last question is about the terms used, I need some explanation for the actual meaning, because when searching the web, people tend to interchange them whenever they want in a wrong way.

Grounding vs earthing vs bonding

PS: English is not my primary language. There is a very high chance that I've got everything wrong. These questions have haunted me for a long time, any online resources to further understand are highly appreciated.

• Ah, your question about earthing is a little bit too long and a little bit confusing. Let me wiki "Earthing System" and see if I can understand you better. (1) "Earthing system - Wikipedia": en.wikipedia.org/wiki/Earthing_system. Cheers. Aug 9, 2020 at 14:03
• I have read the Wikipedia column and now come back to your questions. I see that you have three questions: (1) For TT, how come the soil ground resistance is so small? (2) How does the Earth Clamp Resistance Meters work? (3) How come the terms are so confusing? I think it is better to answer your question in this sequence (3) Clarify the confusing terms, (2) Find out why the earth resistance is so small, (2) How the Earth Clamp Resistance Meter is doing it job. / to continue, ... Aug 9, 2020 at 14:21
• @tlfong01 I am fine with any order you consider, as long as I understand them. Aug 9, 2020 at 14:24
• Now for your third question about confusing terms. I think the above Wikipedia does clarify many things. I have no problem understand the seemingly confusing terms. Please let us know if there are any sections in that article you still find confusing. After we clarify things, then we can move to your first question. Ah, bed time, see you tomorrow. Sorry for all the typo errors. Cheers. Aug 9, 2020 at 14:25
• OK. so let us start with the above Wiki article. Please let us know if there are any sections you find confusing. It is OK to just have a rough idea for now first, perhaps things become clearer when we discussion the question: How come the "earth resistance" is so small? Cheers. Aug 9, 2020 at 14:30

In a TT system, the current that will flow in a dead short from the live to the protective earth may not be enough to blow a fuse or trip a circuit breaker. That's why TT systems should always use an RCD (or GFCI in American terminology; they are different names for the same thing). An RCD will trip if the leakage current is only a few tens or hundreds of milliamps.

The Earth may be a rather poor conductor, but it's an extremely fat one - about 12742km at its widest. As a result, pretty much all the resistance between the house eath rod and the supply transformer earth rod is in the soil around the rods themselves. How far apart they are makes little difference.

Grounding and earthing are pretty much the same thing. Earthing is more British English, while grounding is more American. Bonding is connecting two things together to ensure that they have the same voltage in the event of a fault. You get electrocuted if you touch two things at different voltages at the same time, so bonding different things together can reduce the chances of electrocution.

• Your explanation is very clear. Cheers. Aug 10, 2020 at 1:19
• Hey, your explanation is relatively satisfying and I agree that the earth is much alike a infinite grid resistor (a fat resistor). I even saw that in Australia there are some power lines that only have line wire and using ground as a ground in order to make them really cheap. I guess it is just too non-intuitive/hard for me to accept that there is actually a very small resistance over a large distance... Do you have any paper/documents/resources that are describing the soil resistance. Aug 10, 2020 at 10:42
• Also if the ground is really good, I could put a battery with a rod and an led with a rod and power the led using the ground/soil as a current return path, right ? Aug 10, 2020 at 10:43

Ground is a term strictly used everywhere to define what is meant by 0V reference. Choosing earth for a protection path return is lossy , yes, but <200 Ohms for dist. Transformer, DT to home and 2 Ohms for a transmission tower is expected so the ground rods are different for home and tower.

Ground impedance is tested by injecting a current and measuring voltage injected in the path from Neutral to PE.

When Neutral is earth bonded, they mean they define 0V at the transformer or tower where the connection is made between Neutral and Earth ,since Earth cannot be the same voltage all across the planet (eg transatlantic has thousands of Amps conducted between continents due to V/R of earth and transatlantic communication cables with many conductors. I digress. (Hope you are using Google translator)

So when line current does not match Neutral current due to any significant leakage current in common mode differential relay (RCD or GFCI) due to humidity or ground fault (severe) the TT system is protected only by the IR rise of ground current , local surge protectors (line filters and MOV’s , TVS) which is better than a million volts with no protection but not perfect. ( my farm buddy recently went AWOL and lost his Modem /router/Wifi recently due to Lightning transient but he was protected by TT earthing to house. But somewhere along the line the comm wire transient destroyed the modem.

“The term “grounding” has a wide range of meaning in the electrical industry. The term is used for three broad areas:”

• earth reference voltage for ground rods
• Continuous loop for surge faults
• sufficient current to trip surge protective devices upstream

Since the last item varies with the feed current limits, the ground rod resistance specs do vary according to IEEE 82 test methods.

• You explanation is very clear, I mean only to the EE professionals. Me newbie knows what are RCD, GFCI, MOV, TVS, modem, router, wifi, because they are real things. But the theory stuff is too hard for me. I guess I need to google and wiki for some newbie tutorials and perhaps spend one locking down weekend to get a better picture, or scratching the surface. Cheers. Aug 10, 2020 at 1:35
• So from the first paragraph you are defining two impedances, one for the transmission tower rod and for the house/local rod. Why should we defined it as this ? This is like we are ignoring the ground/soil between those and consider them to be 0 ohm and I don't think this is the case. I am not denying your information, as I searched the web found a lot of formulas that are actually calculating only that (using the length of the rod and the soil resistivity), but it's not intuitive/common sense. In the case of the transient protection I strongly agree. Thanks. Aug 10, 2020 at 10:51
• The Ground resistance must be low enough to trip upstream circuit protection thresholds and no less. Aug 10, 2020 at 15:48