Single wire earth return - Page 1
Forum section: electronics / beginners

author: xibalban
author_rank: Regular Contributor
timestamp: June 26, 2020, 05:05:02 am
content: This one is more electrical than electronics, but I reckon it is ok to ask it here.In a single wire transmission system, the earth acts as the return path for current to "flow back to source". Would this work for anywhere on earth, say, if we transmit current on a single wire from Paris to Los Angeles (the cities are irrelevant here, obviously. I really meant, on two different sides of the globe, this earth), ignoring the voltage drop on the long transmission wire.

author: sibeen
author_rank: Frequent Contributor
timestamp: June 26, 2020, 05:42:39 am
content: Sure, it'd work. If you ignore the voltage drop on the cable why wouldn't it. The return current can travel through the earth and water and molten lava if it has to.

author: gbaddeley
author_rank: Regular Contributor
timestamp: June 26, 2020, 06:29:37 am
content: Actually the voltage drop through the earth would be greater than the wire, but if you are transmitting at a few thousand volts and reasonably low current, the losses are not significant.

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 26, 2020, 09:10:04 am
content: I cant find exact values but as I remember the Earth resistance is about several times larger then metal (Alu, Fe) wire, and of cause depends on actual minerals presented.

author: xibalban
author_rank: Regular Contributor
timestamp: June 26, 2020, 10:59:45 am
content: Brings me back to the original question, that bothered me. How does single wire earth return actually work, with that high earth resistance?? I sure have read quite a bit about it, but nothing seems to get through my thick skullAnd, it seems that I am not the only one, after all, who is confused (made me feel better about myself when I read the first answer).Quote from: sibeen on June 26, 2020, 05:42:39 amThis is what I thought too, but no my friend, the truth is deeper than you think it is.

author: KMoffett
author_rank: Regular Contributor
timestamp: June 26, 2020, 11:55:30 am
content: My start in electronics. Old, rural telephone networks in the USA were single wire on poles with earth return. Very low current, so no great ground resistance IR losses. I found that I could tap into the network to listen. Took a hand crank telephone receiver and connect one pin to a nail in the ground and connect the other pin to a bare wire thrown over the wire between telephone poles. Worked anywhere along network.As the current and distance in a earth return system increase, so do the losses.

author: rstofer
author_rank: Super Contributor
timestamp: June 26, 2020, 12:53:16 pm
content: The ground return resistance may be on the order of 5 to 10 Ohms. If the circuit was 12,000V and 10A load (120 kW), the earth return would drop just 100V out of the 12,000V. Not too significant!Quotehttps://en.wikipedia.org/wiki/Single-wire_earth_return#Operating_principleThe trick is to use a fairly high voltage at low current. A #1 AWG conductor has a resistance of about 0.8 Ohms per mile. For a 10A circuit, this drop is just 8V, not a big part of the 12,000V line voltage.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 26, 2020, 02:06:53 pm
content: You may want to have a look at "HVDC". (High Voltage Direct Current).This is used to transport large amount of electricity over long distances, and also for under see lines, where capacitive coupling makes AC unusuable.HVDC usually consist of 2 wires. One wire with a positive high voltage, and the other with a negative high voltage. (800KV is no eception, I think nowaday's it's even over 1MV on some lines)These systems additionally tend to have GND electrodes which is used as a "backup", and makes it possible to do maintenance work on one of the wires. The GND electrodes need special care to limit damage by electrolysis.

author: xibalban
author_rank: Regular Contributor
timestamp: June 26, 2020, 04:18:13 pm
content: Quote from: Doctorandus_P on June 26, 2020, 02:06:53 pmThanks, it sure sounds interesting. I'll check that out.Quote from: KMoffett on June 26, 2020, 11:55:30 amTelephone lines, I believe aren't really high voltage, and hence you could hook up your "tap" telephone without getting yourself zapped, right? But, as others have pointed out, "high voltage" is required for such a single wire transmission. The two statements don't add up well.Quote from: rstofer on June 26, 2020, 12:53:16 pmThe big question is, for how long a distance is this resistance valid for?QuoteSo, say we got two 6 m stakes, one in Australia and the other in England. So, are you saying that the resistance between them would be between 5 to 10 ohms, despite the distance?

author: Zero999
author_rank: Super Contributor
timestamp: June 26, 2020, 06:34:25 pm
content: The earth has a huge volume for the current to flow through, so the surface area and the earth which immediately surrounds it, dominates the return voltage drop. The distance between the electrodes doesn't afect the voltage drop too much.

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 26, 2020, 07:58:04 pm
content: The earth works good as ground wire especially for thre-phase system. It is usually good enough ballanced at high voltages so neutral wire doesn't have as large current as phase ones (up to 10% maybe in normal mode).That's why (because of large comparative resistance) we don't use earth as wire in one-phase power system.

author: Monkeh
author_rank: Super Contributor
timestamp: June 26, 2020, 08:47:47 pm
content: Quote from: Vovk_Z on June 26, 2020, 07:58:04 pmUhm.. except for the systems the topic is titled for? SWER is quite common.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 26, 2020, 09:19:49 pm
content: Quote from: xibalban on June 26, 2020, 04:18:13 pmI would expect it to not vary much and for the resistance to be dominated by the neighborhood of the ground connections. For instance, in 2D films point-to-point resistance only increases as the log of the separation between the electrodes since the farther they get apart the more width the current can spread out over. In 3D there is even more space for the current to spread out over. Earth isn't a uniform conductor so I assume you actually have to take into account the geology of the land, but in normal circumstances the current density and therefore voltage drop far from either end should be extremely low.

author: Manul
author_rank: Super Contributor
timestamp: June 26, 2020, 10:05:51 pm
content: I wonder what influence does a capacitance have. For example if one takes an ideal wire (ignoring it's resistance and impedance for simplicity) and runs this wire to the Moon, it should be possible to transfer AC power just because of Moon (and Earth) self capacitance (even if Moon to Earth capacitance would be zero, which is not). I mean it should be possible to send some power over one wire without DC or even AC return path. Wikipedia writes that Earth self capacitance is around 710uF. That is a lot even for 50Hz (around 4.5 Ohms impedance).

author: ejeffrey
author_rank: Super Contributor
timestamp: June 26, 2020, 11:00:03 pm
content: Quote from: Manul on June 26, 2020, 10:05:51 pmWell, a capacitor *is* an AC return path. For electrostatics the self capacitance of the two bodies just add in series if you neglect the mutual capacitance (which is only a small correction in this case). However, this isn't electrostatics as the earth-moon distance is nearly 100 wavelengths at 50 Hz. So you have to look at this as a transmission line + antenna problem and I think this falls apart pretty badly. I'll admit I haven't thought a lot about what happens here, but I would guess that even if you manage to launch some power into the impedance presented by the wire it would almost all be radiated rather than make it to the moon.

author: hamster_nz
author_rank: Super Contributor
timestamp: June 26, 2020, 11:05:52 pm
content: I used to cycle in the area around a HVDC line & it's terminal.... you can see the construction of the earth return electrode here:https://youtu.be/a9zdz780l-U?t=1191Currently (ahem) rated at 2400 A, and 0.122 Ohm.https://en.wikipedia.org/wiki/HVDC_Inter-Island#Earth_electrode_stations

author: Manul
author_rank: Super Contributor
timestamp: June 27, 2020, 12:18:33 am
content: Quote from: ejeffrey on June 26, 2020, 11:00:03 pmYes, of course capacitor is an AC return path, I was not trying to say otherwise. But my point is that it should work without any return also. If there is an RF wave going througth open space (and open space I believe is also a transmittion line) it can obviously transfer power with no return. For example if there is antenna on the Moon surface (ground plane).In case of one wire telephone signals (higher frequencies), impedance of self capacitance of Earth may be dominating, not the full ground return path impedance. But thats just my guess (I have no clear idea how to calculate).

author: wizard69
author_rank: Super Contributor
timestamp: June 27, 2020, 06:45:26 am
content: Quote from: Vovk_Z on June 26, 2020, 09:10:04 amI suspect that it is highly variable. Elecricians/Electrical engineers can run into building grounding problems based upon the building location and the earth it sits on. If it sits on earth at all, bed rock has a different conductivity that wet earth or clay.Probably a better question is why is this question being asked because relying upon the earth for a circuit return is bad voodoo.

author: TomS_
author_rank: Frequent Contributor
timestamp: June 27, 2020, 07:07:41 am
content: Quote from: rstofer on June 26, 2020, 12:53:16 pmIs there really such a 6m rule of thumb? I recall when working for a wireless telecommunications company we would take a ground resistance measurement which would tell us how long the rods needed to be.IIRC, and it was some years ago now, in one location we had to drive 10 or so meters into the ground. It was very sandy soil in that location.At tower locations we would drive one rod for each leg of the tower + one for the electrical systems in the hut, and run a ring of copper strap linking them all together. Unfortunately in some cases we had vandals come by and nick anything that wasn't buried. 😒

author: Zero999
author_rank: Super Contributor
timestamp: June 27, 2020, 08:55:56 am
content: Quote from: ejeffrey on June 26, 2020, 09:19:49 pmYes, the distance doesn't matter much. The earth has a huge cross-sectional area. It's the contact resistance between the earth and the electrode, which dominates the resistance. Once the current is flowing through the earth, it can spread out over a huge volume.Suppose we take a plastic cube shaped box, with a volume of 1m3, glue two 1m2copper electrodes to the inside of each side and fill it with the local soil and measure the resistance, which is around 1k. Now imagine the current flowing through the soil between two large electrodes, placed 100km apart, spreads out over a an area of of 10km2, the distance is 100km but cross-sectional area is now 10km2, so the resistance is 100k*1k/10k2= 1Ohm.In reality, the above example is flawed. The current can take a much larger cross-section than 10km2and the electrodes won't be that big. The further the electrodes are apart, the more the current can spread out, so the resistance of the earth over most of the path, will be insignificant, compared to the earth immediately surrounding the electrodes.Quote from: Manul on June 26, 2020, 10:05:51 pmThat's irrelevant for this discussion, as the earth is acting as a return current path, but you seem to be a bit confused about self-capacitance.Normally when we talk about capacitance, we really mean mutual capacitance. A large capacitor from a power supply might have 100 000µF printed on the label, which is 0.1F, but that's the mutual capacitance measured between its plates. The self-capacitance will be several orders of magnitude lower, around 100pF. When we talk about charging a capacitor, electrons from the cathode are moved to the anode. The net charge never changes, because the number of electrons inside it, remains the same! This means that if we measure the capacitance between the terminals, it will be 0.1F, but between the earth and the capacitor it will only be 100pF. The earth has such a large self-capacitance, compared to the capacitor, it can be ignored. Now if we take two of those capacitors, place them far enough apart that the mutual capacitance is insignificant and measure the capacitance between them, it will be 50pF, equivalent to them both connected in series.Quote from: Manul on June 27, 2020, 12:18:33 amWhen a circuit is connected to an antenna, electrons are moved to and fro, between the circuit and the antenna. At very high frequencies, the self-capacitance of the circuit itself, is high enough not to matter, lower frequencies require a large ground plane to increase the self-capacitance of the circuit and act as a source/sink for the electrons, much lower frequencies require a physical connection to the earth, otherwise the ground plane would have to be enormous to increase the self-capacitance to the required level.Old telegraph systems used the earth as a return. The self-capacitance of a cable will be tiny, compared to the earth, so no, the self-capacitance of the cable will dominate that of the earth.

author: KMoffett
author_rank: Regular Contributor
timestamp: June 27, 2020, 12:08:47 pm
content: ...Quote from: KMoffett on June 26, 2020, 11:55:30 am"Telephone lines, I believe aren't really high voltage, and hence you could hook up your "tap" telephone without getting yourself zapped, right? But, as others have pointed out, "high voltage" is required for such a single wire transmission. The two statements don't add up well....."I forgot to mention that there is a brief high voltage on the old single wire telephone systems. It happens when one of the users cranks the magneto in the phone to cause the other users phones to "ring". Do any of you remember using one of these magnetos to drive worms out of the ground?