I often read or hear smart people say that electrical current wants to go to the ground (or “dirt”), and that this is the reason for the grounding electrode and “grounding.” This is just not true. It takes a heck of a lot of voltage to push current through the soil and this amount of voltage is generally not available in a residence.
The grounding electrode (also known as ground rod or Ufer ground) is primarily installed to provide a path to ground (the dirt or “earth”) should lightning strike near or at the building and to dissipate static charges instead of letting them build up in a building’s metal systems. Secondarily, the grounding electrode at the house reintroduces the earth reference to maintain the voltage potential of 120 volts to ground (or the grounded conductor, also known as “neutral”).
A Review of Basic Electricity Terminology
A term of measurement to indicate the pressure behind the electrons to get them where we want the work done. This is analogous to water pressure: The higher the pressure (voltage), the greater potential for a leak and the greater the push behind electrons to induce current into a material.
A term of measurement to indicate the volume of electrons (current) being pushed by the voltage (pressure) through a conductor. This is analogous to gallons per minute: The more gallons you want (or current), the larger the pipe or conductor must be to obtain the desired volume without damage or loss from resistance.
Resistance (measured in ohms,
also known as impedance)
A term of measurement that indicates the amount of resistance a material has to current flow. Plastic has high resistance and most soil has high resistance. Metals have low resistance, but some are better conductors than others. We often use copper as a conductor in cables. We also use aluminum; however, aluminum has more resistance than copper, so we must upsize aluminum at least one wire size to get the same safe current capacity as copper. When we try to shove too much current (amperes) through an inadequate conductor, the result is heat, which could mean melting insulation or metal failure.
Most homes have a transformer to step down distribution voltage (generally, this is rather high [2,400-20,000 volts], but it could be higher) to the convention used inside most homes in the United States (120/240 volts).
Generally, the service to the house is three conductors—two ungrounded and one grounded (created at the transformer). Some utilities provide a grounded conductor (or “neutral”) with distribution conductors in the street, some don’t. If they do, the utility neutral is connected to the common neutral connection.
Also known as the “neutral,” this usually white insulated conductor is created at the transformer by connecting a “neutral” conductor to the center tap, along with another wire that is run to a metal rod stuck in the dirt (this grounding electrode conductor can be seen at many electric utility poles, if not stolen) to introduce the earth reference, or 120 volts to ground potential.
The neutral is not needed for a 240-volt circuit. This earth potential is again reintroduced by the grounding electrode(s) at the building. This would be the Ufer, ground rod(s) and any other available electrode such as a metal underground pipe in contact with earth for 10 feet or more. The grounded conductor is the primary return path to the transformer for a 120-volt circuit current. Electricity always wants to return to the transformer once it leaves through the ungrounded conductor(s).
The reason we don’t connect neutrals and equipment grounds in subpanels is to prevent the induction of current into the equipment grounding system from the neutral. Note that some optional systems in a house, such as an emergency generator, are separately derived systems, meaning that the neutral is created at the generator so that current returns to the generator, and thus not going into the non-emergency circuits or the grid.
Also known as “line” or the “hot,” the ungrounded conductor is usually black or red and insulated (most houses have two), created at the transformer and supplying current pushed by voltage to power household appliances.
The hots or lines will most likely have 120-volt potential to ground (neutral) and 240-volt potential between them. In rare cases of residential homes, there could be 208 volts hot to hot (line to line) and 120 volts hot to neutral (or ground). Note that “line” is incoming power, and “load” are downstream devices such as receptacles, where power would be used by devices.
Equipment grounding conductors
Also known as the “ground” or “safety ground,” these equipment grounding conductors are usually bare or green insulated wires and they are the emergency path for current back to the main panel neutral or ground terminal bar should there be a current “leak” (also known as a ground fault).
A leak could be current being induced into the metal frame of an appliance, such as a clothes washer, due to damaged insulation at the flexible power supply cord. The equipment grounding wires are connected to the neutral terminal bar in the main panel to provide a path for current back to the transformer. This allows current flow to “clear the circuit” by tripping a breaker or fuse.
Equipment grounding wires help prevent the potential for a person to be in a “series” path to the transformer, like bonding wires. You always want to keep people in a parallel path with any potential current path, and most of the current will flow in the lower resistive path that would be a metal wire—generally, an equipment ground or bonding wire.
Often confused with “ground wires,” bonding wires are used to connect metal components (not designed as part of the circuit current carrying components) that might become energized with undesired current. These could include metal piping or ductwork systems in the house or metal enclosures. Bonding wires are connected to the neutral terminal bar in the main panel to provide a path for current back to the transformer to allow current flow to “clear the circuit” by tripping a breaker or fuse.
If current does not flow to trip the breaker, it will wait for a potential path, which could be through a person. Bonding wires help prevent the potential for a person to be in a “series” path to the transformer. Again, you always want to keep people in a parallel path with any potential circuit. Most of the current will flow in the lower resistive path, which would be a metal wire. We also use grounding or bonding wires to create equal current potential to ground in all metal should something become energized.
Once current leaves the transformer with 120 or 240 voltage (pressure) behind it, the current’s only function is to return back to the transformer. We direct this current through the breakers or fuses in panels to branch circuits and panel feeders to our devices, such as switches and receptacles (so electricity is readily available to plug in a cord-connected appliance) and to our appliances, such as air-conditioning units and water heaters.
Once we use the voltage (pressure) and current to operate our appliances, current (in amperes) returns to the transformer through the grounded conductor (the neutral), although with less voltage. Think of the hot or line side as the pressure side and the neutral as the drain pipe. I like using plumbing analogies because I can picture them well. What comes out must go to the drain or it’s a “leak.” With 240-volt circuits, each hot takes a turn being the return to the transformer with an alternating current, as each leg is 180 degrees out of phase.
I experienced the difficulty of trying to induce current into the soil in a house I once lived in. A contractor working at the street cut the neutral conductor from the transformer to our house. I was out back working in the yard, and my wife told me the cooktop was not working and the house seemed kind of dark.
Of course, I went in and had to be sure that the cooktop did not work! Also, I did notice that the lights seemed to be dimmer than usual. I went to the main panel and checked voltage with several different multimeters at one of the buses to ground (80 volts), and then I checked between the buses (160 volts).
Next, I walked to the end of the driveway and could see the problem. The contractor had called our local power supplier. The reason for the low voltage at my panel was because the neutral was cut. As a result, current was trying to return to the transformer at the street through the grounding electrode, which was close to and connected by a bond wire to the copper water service pipe to our house, which passed by the transformer grounding electrode at the street that used to create the neutral conductor.
Due to the high resistance of soil, we experienced a significant voltage drop in the house because much of that voltage (pressure) was being used by the neutral current trying to return to the transformer to complete the circuit. Once the neutral was repaired, all voltages returned to normal.
I hope these examples and insights help inspectors better understand electricity and how it works. My experience is that once you understand the theory, you can confidently inspect just about anything.
Mike Casey, ACI, MCI, and ASHI Technical Committee, Claims & Building Consulting, Inspector Higher Education & Technical Support, www.MichaelCasey.com
The opinions expressed in this article are those of the author only and do not necessarily reflect the opinions or views of ASHI. The information contained in the article is general, and readers should always independently verify for accuracy, completeness and reliability.