Death By grounding

All's not what it seems

One of the most often received questions is: Am I grounded?

The second most often received is: Am I grounded well?

All trees are “grounded,” and most of us wouldn’t think twice about seeking shelter under a tree when it starts raining. But because it’s grounded, and because its branches are above ground, as if to reach out and touch something, it’s the most likely spot for lightning to strike, bar none. So being grounded means connected with the soil, electrically speaking, but that in no way infers that that is a “safe” place or condition to be in, and you may want to think twice about seeking shelter under that tree.

So why “grounding?” And why can it be bad?

At the very slim chance that your place would be struck by lightning, you’d want to reduce the electrical stroke’s capability to do damage. So you’d want to “quench” the energy as quickly as possible. Having a good connection with the soil is one way of accomplishing this, and that is achieved in most cases by a metal rod stuck in the soil. Poor soil? No problem! Use a chemical rod, which is a hollow metal pipe with salts within it, that coupled with water create a conductive “root” structure into the soil, and get an excellent connection with the soil / grounding. But at a premium price. A common workaround is a metallic neighborhood water pipe, but that brings on other problems.

A subset of grounding is “bonding” whereby all metallic structures in a dwelling are interconnected with the electrical ground, so that if something energized touches any unintended metal surface, there will be a path back to the breaker / fuse panel, that will produce an immediate corrective action. This bonding provides the added protection that if lightning strikes, the instantaneous flows of current do not develop large enough voltages to do damage as, say, between the sound system and the fridge, etc.

An essential to understanding electrical phenomena is that everything has electrical resistance, or opposition to current flow, that which you demand when you turn something on. Even the common wires in your floors, walls, and ceilings have some resistance. A means of grasping this is to connect a hair drier at an outlet furthest from the breaker panel. When you turn it on, and measure voltage at the outlet, you’d see the typical 120V drop to maybe 116V. Connect it to an outlet by the breaker panel, and you’d measure about 118, when the drier is turn on. Less voltage “loss” due to less electrical resistance, because of less wire between the source and the load, than if you’d plugged it in at the furthest outlet. Small difference? Yes, but most relevant.

Since the electrical resistance occurs on both the supply and return wiring, the voltage “loss” happens on both wires. With the 120V that’s easy to visualize, it drops to 119, or so. But what about the return wire? It’s supposed to be at zero volts, so if it “loses” voltage it would go negative, a condition that would bring us to some unknown parallel universe. So what it does it “gains” voltage, and instead of being at zero volts, it’s then at maybe one volt (1V) or so when the hair drier is turned on. That return wire is no longer at zero volts, so even though it’s “grounded” by the breaker panel, it now has a voltage that can be felt, and possibly cause unintended consequences. So although the voltage loss is easy to follow, the voltage gain is not, and the two occur for the same reason, a differential voltage (between the two ends of the wire) is produced by the current flow. No current, no differential.

In this residential case, all wiring has a common grounding point, by the breaker panel. So you might be tempted to think of that as the “source.”

What about the more realistic source voltage, most typically a local transformer nearby?

Well, in this case the return wire at the transformer and at the home are both grounded. Although good soil electrical conductivity would preclude any voltage differences between them, that is not absolute. Although they are both grounded, the moment current flows on the return wire between the home and the transformer there will be a voltage difference between them, although the “brute force” of the connection with the soil will generally help keep that to a minimum, perhaps less than one volt. Assuming the transformer to be the “perfect source,” in that its grounding point does not change, that voltage difference to the home’s reference (grounding) point will vary dynamically with demand in current. More current, more voltage difference, less current, less voltage difference. Depending on the quality of the soil’s electrical conductivity, that voltage difference will cause some current to flow through the soil. Better soil conductivity, less voltage difference, and more current through the soil. Conversely, worse soil conductivity, more voltage difference, and less current through the soil.

Having established that an electrical ground is not an absolute Zero voltage reference, you may wish to consider what can happen it you touch it, and something else (that is not part of the electric system), simultaneously, and feel a shock.

A shock, of the electrical type, is a reaction to the passage of current through the body. For current to flow, there needs to be a voltage driving force. For voltages less than about 20 volts, our skin offers good electrical resistance, but not if that skin is wet and salty, as in sweaty. For higher voltages the distinction is not so clear cut, but suffice it to say that industrial safeguards are employed when exposed voltages of more than 50 volts are present. However, even a few volts are sufficient to kill, primarily dependent on the extent of the body’s contact surface area.

Consider the simple case above. Path 1 consists of the expected wiring, supply and return, being adjacent to each other. Since there will be a voltage difference between the grounding points at the source and the load, the moment current is demanded, Path 2 comes into play. Due to the ground not being homogeneous, the current path may be tortuous and diffusely scattered over a large area (relative to the size of the wire). Since the Path 1 current is no longer balanced (supply is not equal return), the difference flowing through the soil as Path 2, Path 1 will display a Magnetic field that can be detected yards or tens of yards distant from the wire, while the Path 2 current being widely scattered will produce a much less tangible field.

Life is not so simple.

Consider the above, where several loads are connected to a source. Each of the loads will display a current demand different than the others, and depending on the polarity of the current (N American residential systems have two sources of opposite polarity), may add to, or subtract from, that of the previous load. Note the Path 1 on the wires between the source and each load. Note Path 2 through the soil between each source and the load. Note also the presence of Path 3 between each of the loads. These will change dynamically depending on the amount of current used at each load site. This epitomizes the behavior of the Wye distribution system, where each pole-top transformer serves as a load, as well as behaving as a single transformer (source) feeding several homes, as is usually the case.

Yet, life is even more complicated.

Consider the above (Grounding currents), where homes, or transformers are not located in a straight line, but scattered about. The resulting interaction is more three dimensional, if you can picture what is happening below the soil.

Looking at a typical arrangement above, where the wire is brought to the home underground, the ground rod at the point of use, generally by the meter, will cause a voltage difference between it, and the surrounding soil. This is known as Step Potential, or Ground Potential Rise (GPR). As you move away from the grounding point, the GPR will diffuse out and be less tangible. For animals this may be an issue. Being in physical contact with the soil, they may experience voltages and currents we will not, when with shoes. This can cause behavior modification, especially troublesome in farming environments.

But being near the grounding point should not be much of a concern, as long as the source voltage is small.

Looking at a similar arrangement as the previous one, touching a spigot when near the grounding point, since the spigot is metallically / and electrically connected to the grounding point, may display little voltage difference (low delta V). Being at the other end of the home, the effect will be more pronounced (high delta V). This is known as Touch Potential. Again, this may not be an issue, if you are wearing shoes, but if you go about your backyard on bare feet and go to open a spigot, and get shocked, what’s you first thought? *?R%r+g&r##@*!

GPR can be fatal with high voltages, and low.

In the above, a driver in a rush bypassed a road barrier isolating a downed power line, and when he came to an area where the GPR was high enough, it arced through the tires’ steel treads, ignited the vehicle, and torched him. This was a case of high voltage in contact with the soil.

An interesting twist on this, is where you can be immersed within a GPR in three dimensions.

In this case the pier has convenient electrical outlets, but their ground point is in contact with the water, and the Stray Current can be experienced in three dimensions.

One of these occurrences involved a couple of kids that were walking along a bridge with a dog. For some reason the dog fell in the water. The kids jumped in to rescue the dog, the bridge was metallic and connected to a local electrical ground, as the kids approached the part of the bridge in contact with the water, enough current flowed through the water to kill both kids. This was a case of low voltage in intimate contact with the soil, through the water, and a body’s large surface area contact.

A Wye power system has a ground potential that is derived not just by local usage, but by usage far and wide, as all transformers share a common ground, with voltage differing along the circuit route, due to that elusive wire’s electrical resistance.

A Delta power system is ungrounded on the Primary (high voltage) side, so the ground potential is derived from each local transformer, which may feed one to a few users, and is typically less of an issue.

Solutions involve proper assessment, adequate measurement, and although devices may be “properly wired,” inattention to GPR and how to measure it can be fatal.

Considering that connections begin to fail normally by aging, simply due to build-up of oxides (rust), causing or worsening the conditions so described, should be incentive to consider any electrical system as subject to regular, albeit infrequent, inspections and maintenance, like a car. In stark contrast, however, most individuals I meet up with consider their electric system as forever capable, never needing careful oversight.

© Sal La Duca