Earths or grounds power flow. - Page 1
Forum section: electronics / general

author: G7PSK
author_rank: Super Contributor
timestamp: May 17, 2012, 08:38:52 pm
content: Dave's video on earthing faults with oscilloscopes got me thinking about how much power can be transmitted through the earth.What came to my mind was what if I got a long lead and connected to the mains live here in the UK and then traveled over to Australia and then connected that end to earth, what level of power would flow and would it blow the wire, for the purpose of this mental exercise I am ignoring the lead impedance.

author: T4P
author_rank: Super Contributor
timestamp: May 17, 2012, 09:00:48 pm
content: Many amps but at that kind of distance ( 16983.04 km = 16,983,040 meters )... AWG 10 ( IF it's a 2.5mm² wire ) is 3.276392 per km is 55643.09639168ohms impedance ...don't even think about passing 1 amp ... the voltage will just collapse instantly

author: siliconmix
author_rank: Frequent Contributor
timestamp: May 18, 2012, 07:45:15 am
content: rf seems to like it.

author: G7PSK
author_rank: Super Contributor
timestamp: May 18, 2012, 08:36:09 am
content: Rf is quite happy with false grounds as well. But what is the current carrying capacity of the earth. The overall resistance is quite high.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 18, 2012, 02:27:10 pm
content: Well if you ignore the impedance of the wire you wont get any power loss in the wire, so you wont blow the wire.Is that what you mean?But as far as the impedance model of the earth from one side to the other, I have no idea, but some electrical distribution systems use "single wire earth return", so it can't be too high.

author: G7PSK
author_rank: Super Contributor
timestamp: May 18, 2012, 02:49:21 pm
content: The only time I have seen the earth used as a return is in old style telegraph equipment where the current requirement is in the milli amp range. I suspect that the earth points on power plants would not sustain a large current for very long. The earth you see at home or in the work place is bonded all the way back to the center tap of the sub station secondary, They are usually wound as delta / star with the primary being delta and secondary the star winding this will be bonded to earth giving your neutral then every time a termination is made for supply another bonding to earth is made and as it is usual to run both earth and neutral side by side here in the uk there is copper all the way back to the center tap of the transformer. So if you make a connection to earth on equipment it is copper all the way but if that connection is to the actual ground the current is going to be limited by the earthing rod. What sort of uninterrupted current will a standard earth rod carry to ground are we talking about the water in the ground boiling or what.I have just been out and measured the earth hesitance. The test leads on my meter are one meter long each so I stretched them out in opposite directions and pushed rods half a meter long into the ground which is still wet from earlier rain. 161.4 K ohm is the reading.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 18, 2012, 03:30:58 pm
content: I am surprised your soil resistivity is so high.http://en.wikipedia.org/wiki/Single-wire_earth_returnI know we still have some of these systems in Tasmania, although only in remote places.But I didn't realise they used them in the Basslink cable which attaches our grid to the mainland.

author: SeanB
author_rank: Super Contributor
timestamp: May 18, 2012, 04:11:30 pm
content: Mains earthing resistance is required to be below 1 ohm, measured at the earthing rod. The earthing mat here by me is, thanks to MTN putting in a 5m by 5m earth mat with multiple grounding rods, well below this. All bonded to everything metallic, cabling, racking, coax, aircons, satellite dishes, waste piping, mains incoming frame with a 25mm cable with protective grease on each connection. It will probably survive a direct lightning hit, even though it is not designed to do so, more there for static and near misses.A funny thing is actual lightning earth systems are allowed to be up to 8 ohms. This can make a difference, as the 100kA current will flash over to mains wiring nearby.

author: G7PSK
author_rank: Super Contributor
timestamp: May 18, 2012, 04:17:53 pm
content: I have seen horses electrocuted by stepping onto ground where a cable is leaking 240 volts and the other year several horses were killed at a race course where a cable was leaking this is due to a horse being a better conductor than ground.

author: SeanB
author_rank: Super Contributor
timestamp: May 18, 2012, 06:01:19 pm
content: Will say that we stopped people from using our wall as a urinal. The sheet frame was poorly earthed there, so we wired up an earth spike nearby, and ran a buried cable into the shop. Connected it to one lead of the megger, and the other lead to the sheet wall. Waited for the first " test subject" and then turned the handle when appropriate. Word soon got around............We also had a chair with a CDI ignition system in it, with a coil and a small relay wired as a buzzer, with the HT wired to wire threaded through the chair back. A 12V battery underneath, and a pressure switch underneath completed the setup. You could sit in it, but if you leant back far enough it would bite.

author: IanB
author_rank: Super Contributor
timestamp: May 18, 2012, 06:22:58 pm
content: Quote from: G7PSK on May 18, 2012, 02:49:21 pmQuote from: HackedFridgeMagnet on May 18, 2012, 03:30:58 pmThis would not necessarily be surprising.Rainwater is effectively an insulator, and soil resistance is only lowered by dissolved minerals from the soil being present in the water. Some soils such as peat or loose sand may not have much in the way of minerals to lower the conductivity. Dry, desert like soils will also have low conductivity of course.Earthing systems try to overcome this problem by having a larger buried surface area, and by going deeper below the surface.Digging thin short rods into the soil gives you both a very small surface area and a very shallow penetration. Dip your meter probes into a bowl of tap water and compare readings. I bet they will be similar.Note that earth return power distribution systems use 10's of kV and only a few amps. They would not work at low voltages like 240 V.

author: SeanB
author_rank: Super Contributor
timestamp: May 18, 2012, 06:42:06 pm
content: The Cahora Bassa powerline is SWER.http://en.wikipedia.org/wiki/Cahora_Bassa_%28HVDC%29Very strange to see a power line consisting of one wire only stringing itself across the country on very long glass insulators.

author: IanB
author_rank: Super Contributor
timestamp: May 18, 2012, 06:51:56 pm
content: Quote from: SeanB on May 18, 2012, 06:42:06 pmI'm sure the enormous capacitance of the earth is a great help in enabling these systems to work.From what I've read the DC (as opposed to AC) single wire systems cross sections of sea (where the sea being salt water has a very high conductivity compared to the land).

author: SeanB
author_rank: Super Contributor
timestamp: May 18, 2012, 07:07:40 pm
content: From the diagrams on wiki the line is modelled as a 0.2uF capacitor. 533kV at 1800A per line.

author: IanB
author_rank: Super Contributor
timestamp: May 18, 2012, 09:01:10 pm
content: Quote from: SeanB on May 18, 2012, 07:07:40 pmWhich line, the Cahora Bassa line?It was not in fact the line capacitance that I referred to, it was the capacitance of planet Earth. Since the Earth is moderately large it has a capacitance that might reasonably be approximated as infinite.In this example the Cahora Bassa line is not SWER, it is bipolar. All the current is carried in the suspended conductors. It is also DC, meaning that ground capacitance wouldn't help with the earth return (capacitors have infinite impedance to DC).The 1800 A is carried in 4520 mm² of conductor, working out at a current density of 0.4 A/mm². Earth return is not used except in exceptional circumstances, and then the capacity is somewhat reduced.

author: G7PSK
author_rank: Super Contributor
timestamp: May 18, 2012, 09:15:31 pm
content: I have just found this on line The earths capacitance is 712 micro farads, I understood that the earths capacitance was one farad as calculated by Faraday.http://www.rheintech.com/blog/archives/1025

author: SeanB
author_rank: Super Contributor
timestamp: May 18, 2012, 09:18:50 pm
content: It may be earth reference when operating but with the capability to run a SWER in case of fault on a single line ( Thanks RENAMO for blowing it up so often) it did have a lot of use when it actually was intact.As far as I know all SWER systems are DC, as the capacitance between the wire and the surrounding water/land provides a lot of energy storage. A 3 phase system only has the capacitance between the wires, a much smaller area in comparison.The Farad is defined by a set of plates separated from each other, and is determined by the area and the separation, as well as the insulating medium between the plates. At first approximation the larger the area the bigger the capacitance. likewise the closer the plates the bigger the capacitance. Any material other than a pure vacuum increases the capacitance.

author: G7PSK
author_rank: Super Contributor
timestamp: May 18, 2012, 09:27:51 pm
content: Just found this it gives details on power earthing systems.http://www.iea.lth.se/publications/Reports/LTH-IEA-7216.pdf

author: Zero999
author_rank: Super Contributor
timestamp: May 24, 2012, 12:29:31 am
content: Quote from: G7PSK on May 17, 2012, 08:38:52 pmThe answer is there's theoretically no limit to the amount of power that can be transmitted though the earth because power is dependant on the voltage.What I think you mean is using the earth to carry current, without too higher voltage drop. The answer is, it depends on the conductivity of the soil and the area of the buried electrodes. A large electrode in damp earth can provide a very low resistance path to another similarly large electrode, a fair distance away.QuoteAs others have mentioned, I think the cable would be more of a problem than the ground resistance: the sea is a very good conductor.