Telluric Currents
MMMMMMMMMM

Ground Return

Almost as soon as people discovered electric current, they found the current could flow through the Earth. Early telegraph, telephone and power systems all routinely used the earth as one of the conductors in their systems.

While it is less common to use the earth as an intentional electrical conductor today, some applications still do. If you wish to learn more, try researching the topic of single-wire earth return (SWER). This describes some AC (50-60 Hz) power distribution systems. They are typically in rural areas, and supply modest amounts of power - such as enough to power a single home or farm.

Earth return is also used in some monopolar high voltage direct current (HVDC) systems. This is particularly true for submarine cables, where there is easy access to a highly conductive salt water return path.

Earth return is also available for back-up or emergency use on HVDC lines such as the Pacific DC Intertie, also known as Path 65.

Ground return for substantial amounts of electrical power is generally discouraged, if not out right prohibited, by standards such as the National Electric Code. The problems include:

  • Hazardous step potential. The earth has a non-zero resistance. If a large amount of current flows close to the surface, there may be a significant voltage difference over a span of a meter or less. The difference in voltage between the two feet of someone standing on the ground may be high enough to create a hazard, as the current may find it easier to flow through their body, than through the ground. They may be safe if their feet are next to each other, but suffer injury when they take a step.
  • Hazardous touch potential. This is similar to step potential, but involves someone touching a large object. A person standing on the ground will be at one voltage. They then touch a large object that is electrically insulated from the ground on the side close to them, but attached to the earth at a distant point. This can create an even larger voltage difference than in the case of a step - as the distance between the person’s feet and the distant point at which an object is grounded is greater.
  • Corrosion of pipelines, communication lines, etc. Long distance structures, such as pipelines, may become the unwitting conductors in earth return systems. This can cause problems with corrosion at their joints, or where they current enters and/or leaves the object.
  • Saturation of power transformers. The earth currents may inadvertently enter AC power distribution systems, and supply a DC current through transformers. This can raise the magnetic field density in the core to a point that the transformer starts to saturate during one half of each AC cycle. This causes a multitude of problems for the AC power transmission system.

Telluric Currents

Any electric current flowing through the earth can be called a telluric current. As the early telegraph pioneers quickly discovered, there are many natural sources of telluric currents. For example, thunderstorms cause the earth beneath them to develop a significant electrical charge, causing current to flow through the earth’s surface. Lighting strikes can cause very high, if brief, currents through the earth’s surface. These currents create significant step and touch potential hazards. More lighting deaths are reported from people experiencing hazardous step potentials near a lighting strike, than from people who are directly hit by the lighting bolt itself.

Telegraph operators have long known that local weather conductions can interfere with the operation of their communication system. The first tarns-atlantic telegraph cable used an earth/sea water return path, as did many other submarine telegraph cables. Operators had tremendous difficulty the day they first tried to use the cable, because there was a storm near the North-American end of the line. The next day, when the weather was clear, it was far easier to operate. Logs show the usefulness of the line routinely varied from day-to-day and hour-to-hour as the naturally occurring telluric currents varied.

Thunderstorms are only one of the possible sources of telluric currents. Telegraph operators in the far northern latitudes, found their systems were harder (or sometimes easier) to use during times of intense Aurora Borealis activity (northern light displays).

Image
5/24/2010, Southern Aurora from ISS

Image
Aurora Borealis (from ground)

Eventually people determined the aurora, and the telluric currents that accompanied them, were being caused by storms - on the sun.

The 1859 Solar Storm (The Carrington Event)

People have known since at least 800 BCE that the sun had dark areas, commonly called sunspots.

Image

With the invention of the telescope in the 1600’s, it was possible to accurately measure the size and number of spots. And there was considerable speculation as to what caused them, and what effects they may have.

About noon local time, on September 1, 1859 British astronomers Richard Carrington and Richard Hodson were independently observing the sun. Both noticed an intensely bright spot of light appear near one of the spots, slowly drift across the sun’s surface, and go out after about five minutes. This was the fist documented case of what we now call a solar flare.

Image
Solar Flare

Solar flares are reasonably common. Their frequency varies, along with the number of sunspots, on a roughly 11 year cycle (we don’t know why). During times of peak solar activity, there are many flares per day, while during solar minimums they occur about once a week.

Carrington and Hodson did not immediately realize it, but the flare they observed was associated with another event, called a coronal mass ejection, or CME. Flares and CME are often associated with each other, however it is possible to have one without the other.

A CME is when the sun ejects a mountain’s worth of matter into space, sort of a solar burp. The matter is ejected as a gas or plasma, so it is spread out over a large region of space, compared to something like the Earth.

Image
Coronal Mass Ejection

While the light from the sun travels at the speed of light, the CME is conventional matter, which moves a bit slower. Material from a typical CME takes about 3 to 4 days to reach Earth. But the CME associated with the solar flare mentioned above was an over-performer. It made the sun-to-Earth trip in 17 hours. That means its particles were moving at about 1% of the speed of light, or two thousand kilometers per second. By a meteor entering the atmosphere is typically moving slower than 100 kilometers per second.

The sun is constantly ejecting particles into space, creating a stream of matter known as the solar wind. The CME is simply an unusually large blast of such particles (sort of like a gust of wind, solar wind). Nearly all of the particles in the wind/CME are electrically charged - individual electrons and nuclei of atoms without their surrounding electron shells. This means the particles will experience a force when they encounter a magnetic field.

The orientation of the Earth’s magnetic field naturally deflects the particles away from the planet.

Image
Earth’s Magnetosheath

This is critically important for life on the planet. Without the magnetic field, the high velocity solar wind would slowly strip away our atmosphere. There is a theory that Mars once had a magnetic field, and a denser atmosphere. But then it lost its magnetic field, and after that much of its atmosphere.

The magnetic field allows some particles to enter the atmosphere near the magnetic north and south poles. The high speed particles hit the atmosphere, causing the auroras.

The auroras of September 2nd, 1859 were like none people of that day had seen before, and have not been seen since. There was so much matter in the CME, and it was moving so rapidly, that the Earth’s magnetic field was unable to deflect all of it. A large amount entered the atmosphere, even far away from the magnetic poles. Auroras were visible as far south as the island of Cuba, at 20 degrees North Latitude. Many people reported the sky was so bright, that they mistook it for a coming sunrise. In some places birds started chirping, as they would before sunrise. They produced enough light that one could read a newspaper.

There have probably been other times in human history where CME of this magnitude have hit the Earth. But this one had a new twist. For the first time in human history, there were long-distance electrical conductors in use - telegraph lines.

The massive amount of high velocity charges in the CME induced unusually large telluric currents. Many telegraph systems stopped operating, as the telluric currents overwhelmed the currents produced by their batteries. High voltages were induced on many telegraph lines, causing them to arc across their insulators. In one case, telegraph operators noticed that if they simply removed their batteries from their system, they could communicate using the CME induced currents.

While it is tragic that some telegraph operators were injured, aside from that the CME was little more than a curiosity. The telegraph system was not an indispensable part of the economy, so it was not a major inconvenience if some lines were inoperative for a few days. It would have also been reasonably easy to quick to replace the damaged equipment. The same might not be true today.

Today we have not only communication wires, but power transmission lines and pipelines that cover very long distances. The large currents a CME could produce in these systems could damage equipment. Power systems transformers are particularly sensitive to CME induced currents, and are difficult and expensive to replace. There is serious debate about how much damage a Carrington level CME might do today to global infrastructure, how long it would take to repair, and how well the world’s political and social systems would deal with the disruption.

The 2012 Near-Miss

We almost had a chance to test modern electrical systems. In July 2012, a CME even larger than the 1859 Carrington-event was released by the sun. Fortunately, it occurred on the far side of the sun from the Earth. NASA considered it a near-miss. The sun rotates on it’s axis every 25 days. If this CME had occurred 9 days earlier, it would have directly impacted Earth.

Here is a link to some videos of that eruption: https://svs.gsfc.nasa.gov/4177. We know a lot about that particular CME, because of two spacecraft called STEREO-A and STEREO-B which were observing the sun from approximately the orbit of the Earth, but ahead and behind the Earth by roughly 120 degrees at the time of the CME

Image

Here is a simulation, based on the observed data, of how large that CME was. The planets are not drawn to scale, but I believe the CME is to scale when compared to the orbits of the planets.

Image

It is inevitable that, sooner or later, a large CME will strike the Earth. Hopefully we will discover that power, communication, pipeline, and satellite systems are all up to the challenge.

The Common Return Problem

Natural phenomenon like solar storms are not the only problem with using the earth as an electrical conductor. When two human-produced electrical systems share a segment of earth as a common conductor, there can be undesirable interactions between the systems. This is known as the common return problem.

Common return problems show up not just in earth-return systems, but in any systems that share a common conductor. For example, a printed circuit board often has a path - or even an entire layer of copper - dedicated to allowing currents to return to their source. This is known as the circuit ground, a name taken from the practice of using the earth for this purpose.

To a first approximation, circuit grounds are considered ideal conductors. But upon closer inspection, we quickly find the conductor is not ideal, and if not used properly, can lead to annoying interaction between circuits. There are technical solutions to this problem, such as differential signalling.

One of the first places the common ground problem came up, was when telephone systems tried to share the earth return path with the electrical power distribution systems being deployed at the same time.