11/6/2020: Initial Post
11/16/2020: Text and Graphics added
2/9/2021: Graphics added; minor edits
This is an introductory article, written to provide a basic understanding of a complex aspect of AC electric systems to an audience with little or no prior background in electricity. This subject is fundamental to AC system wiring in buildings and on boats, and is a prominent underlying part of the discussion in many other articles about AC Shore Power found on this website.
The concepts around “earthing” and “grounding” are at the very core of making electrical systems as safe as possible to people, pets, farm animals and wildlife. But, “earthing” and “grounding” may or may not mean the same thing when used in conversations and when used without context. These subtle concepts and the terminology they involve can be new and confusing to people without prior electrical backgrounds, and are among the most important to electrical safety. “Grounds” and “grounding” are topics that embrace multiple related ideas. “Earthing” and “Grounding” have different implications in residential single-family house settings than they do on boats, and residential electricians often are not aware of issues that apply to electrical safety on boats. Context is very important to understanding these issues, and as always in electricity, there are many “language shortcuts” that occur in group discussions on docks. Boaters will benefit from an understanding of these topics.
In nature, there is a form of electricity called “static electricity.” A major characteristic of static electricity is that it “flows” outside of wired electrical circuits, through the air. Its flow is intermittent and spontaneous. Static electricity is caused by the friction of two surfaces moving across one another. Static electricity results from the accumulation of electrons on one object (“negative charge”) and a deficit of electrons on another object (“positive charge”). It occurs where friction between surfaces creates a negative charge on the surface with excess electrons and a positive charge on the surface from which electrons were taken.
Residents of low-humidity, cold climates are familiar with static electricity. Little static shocks result from walking across a carpeted room in wool socks, petting the cat or dog, putting on a sweater or overcoat, and then contacting a doorknob, another person, or a car door (or any number of similar life activities).
“St. Elmo’s Fire” is a visible ionized corona; a static electricity “charge” that occurs when conditions are still and humid. In St. Elmo’s Fire, a sphere of blue or purple ionized plasma forms at the sharp points of outdoor structures, such as electric utility towers, church spires, chimney’s, masts, spars, flag poles, weather vanes, etc. In the far upper atmosphere, a similar phenomena is responsible for the “Northern Lights.”
In clouds, warm, rising water droplets collide with cold, descending ice crystals, causing static charge to accumulate and eventually result in lightening. The “discharge” of static electricity is a visible flash – an “electric arc” – composed of electrons flowing through ionized air. Electrons “flowing” between two points is the definition of an “electric current.” With static electricity, a voltage difference (electric charge) between the two poles of the static system becomes instantaneously great enough that the insulating characteristic of the normally nonconductive air gap breaks down and conducts. In household situations, the arc is mainly a nuisance, although it can damage modern semiconductor electronics and the “shock,” together with an occasionally audible “snap,” can scare/surprise its animal and human victims.
Lightening is by far the most impressive static electricity discharge phenomena with which we are all familiar. Lightening is static electricity with a massive visible arc composed of many, many thousands of amps. That arc current creates many thousands of degrees of instantaneous temperature rise in the surrounding air, resulting in thunder. Lightening releases massive amounts of energy (mega-joules) and often results in severe damage at its earth contact point. Lightening is more than capable of killing animals and people.
The arc of a static discharge “neutralizes” the accumulated positive and negative atomic charge of the oppositely charged poles of the static “system.” Protecting building electrical systems from being damaged by the discharge arc of lightening involves creating a means to get the arc current to flow AROUND, rather than THROUGH, the electrical system of the building, or its structural components. To protect a building, metallic “air terminals” are placed high, on roofs. A network of heavy electrical conductors connect the air terminals to rods driven into the earth. Large communications towers, bridges and high rise buildings often utilize their own metallic structure as a safe path for guiding discharge currents into the earth. Farm structures (barns, grain elevators, windmill pumps, etc), industrial sites (refineries, chemical plants, chimneys, etc, etc), and hospitals are protected with air terminals and metallic paths to earth ground. These protective devices are apparently considered unsightly and undesirable in suburbia, because they are rarely found on single-family residential buildings. When we lived in Indiana, our neighbor across the street had the chimney blown off his house by a lightening strike to that unprotected structure.
Lightening protection for boats is a separate and complex study; inexact, expensive to install, and impossible to properly retrofit if not built into the initial design at the construction phase of the boat’s life. Boats struck by lightening almost always experience severe electrical system damage and extensive damage to electronic equipment aboard. Lightening can literally blow a hole in a boat’s hull on its way to earth ground.
See my article on “Faraday Cages” for ways to protect sensitive electronic gadgets from lightening; for example, hand-held VHF radios, hand-held GPS, computers, back-up hard drives and cellular telephones.
Residential Electric Circuit “Wire” Naming and Identification
All operational electric circuits require two conductors (wires); one outbound from the source to the load, and one returning from the load to the source. The pair of conductors that lead current to and from the source of power are both called “Current Carrying Conductors.”
In DC circuits on boats, the conductor carrying the positive charge is called “B+,” and can also called the “plus” or “positive” conductor. By conventional agreement, the positive DC conductor is red in color. The conductor that returns current from the load to the source is called the “B-,” or “negative” conductor. By conventional agreement, the negative conductor (in 2020) is yellow in color. Until recent years, DC negative conductors on boats had black insulation, and many such systems are still in service today. In boats with both DC and AC systems installed, the black DC negative wire was easily confused with the black AC energized wire, so the DC color code was changed to “yellow” to eliminate the safety implications of confusing those two wires. In DC situations, the “negative,” or “B-” conductor is sometimes referred to as a “ground,” although that is usually (almost always) not technically correct, since “ground” wires are not intended to carry current in normally operating systems (reasons explained later).
In AC circuits in buildings, the power on the conductors is alternately positive and negative, so the DC nomenclature “B+” and “B-” doesn’t work. In single phase 120V circuits in North America, the two conductors are named for their role in the circuit. The conductor that is considered to be the energized (power suppling) conductor is called the “Ungrounded Conductor,” or “Line 1,” or the “hot” conductor. By code and convention in North America, “L1” is black in color. The other conductor in a 120V circuit is considered to be the return conductor. It is called the “Grounded Conductor” (for reasons explained later), or the “Neutral Conductor,” or simple the “neutral,” and it is white in color.
In single phase 120V/208V and 120V/240V circuits in North America, there are two “Ungrounded Conductors.” They are commonly called “Line 1” and “Line 2.” “L1” is black, and “L2” is red. In these circuits, there is also a “Grounded Conductor,” always referred to as the “neutral,” and white in color.
In electrical engineering, “earth” is the single reference point in an electrical system from which voltages are measured and which provides a direct physical connection to the earth. Since the 1950s, the National Electric Code for AC distribution circuits in buildings has required “Equipment Bonding Conductors” and an “Equipment Grounding Conductor.” In the NEC, Article 250 is the standard for “grounding and bonding.” Each individual conductor that is an individual component that comprises the network of conductors that make up the “ground system” has its own specific name. For the purpose of understanding concepts, the term used here will be the “ground conductor,” or “safety ground.”
The NEC, Article 100, defines an “Effective Ground-Fault Path” as an intentionally constructed, low-resistance, conductive path designed to carry fault current from the origination point of a ground-fault in a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault sensors. The purpose of the safety ground is to create an “effective ground-fault path.” That low resistance path is intended to function as a fault-clearing path; for that single “emergency use” only. “Fault-clearing” means that the circuit breaker feeding that faulting circuit will trip to remove power from the circuit. Under normal conditions in a properly wired electrical system, the safety ground conductors (including the bonding system network on a boat) DO NOT/MUST NOT carry current in normal, routine operation. The safety ground conductors are intended to ONLY carry current when there is a “fault” in the system. In buildings on land, the ground conductor is typically bare copper wire. On boats and in appliances, the ground wire is insulated, and solid green in color, or green with a yellow stripe.
Subtle take-away: the DC “negative” conductor has the same role in a DC circuit that the AC “neutral” conductor has in a residential/boat AC circuit. That is, the DC “Negative” conductor returns current from the load to the power source (battery). In a “grounded DC electrical system” (which is uncommon), there is a third conductor that is part of the DC circuit, just as there is a safety ground in a 120V AC system. The B- conductor is a “Current-Carrying, Grounded Conductor,” and is entirely separate from the actual ground conductor.
NEVER, NEVER use wires of the wrong color for the wrong purpose in a circuit. In new and repair work, always install the correct color of primary wire. Personnel safety and equipment safety depends on colors being correct! It is code-legal to “change” the color of a conductor in cases where that cannot be avoided. Changing the color of a wire is accomplished by wrapping electrical tape of the proper functional color for a distance of several inches at BOTH ENDS of the wire having its color changed. If ever that is found in existing work, DO NOT DISTURB that wrap of tape. The NEC does not allow green safety ground wiring to be changed. Safety ground wiring must be green, and green must not be used for any other purpose.
Note: on some but not all boats built overseas, AC wire colors may be different than the North American standard (NEC and ABYC) colors cited above. On some boats, like some Grand Banks trawlers, one 120V “hot” conductor (L1) is black, but the other (L2) is brown, not red; and the AC neutral conductors are blue, not white. This difference is also common on boats built overseas, because they follow the European color standards. If “strange colors” are found aboard a boat, BE PARTICULARLY CAREFUL to determine how that wiring is used to ensure equipment, fire and personnel safety.
However tedious this discussion seems, an understanding of wiring terminology and color conventions is important to understanding electrical installation instructions for many different types of electrical equipment on boats, and to understanding the host electrical systems, themselves.
Core concept: opposite to the situation with static electricity, in man-made electrical circuits, the electricity originates at a point that is know to be its “source.” This can be a battery, a solar cell, a fuel cell, a generator, or a point-of-connection to the electrical grid. An electric “circuit” is said to exist when an electric “current” has a path that enables electrons to flow out of the source on a conductor, travel through a load to do useful work, and then return to its source on another conductor. The “source” can be DC or AC. Whether DC or AC, a “voltage” can appear at the output terminals of a source (like a battery or generator), but a “circuit” does not exist unless electrons can flow out of the source, through a load, and back into the source. A “circuit” consists of is a round trip of continuous conductive wiring for current flow out of a source and back into the source. A “switch” is any electrical device that “opens” a circuit to prevent electron flow as a matter of convenience and/or function; a relay is a device that interrupts current flow in the circuit that it controls; a “fuse” or “circuit breaker” is a device that “opens” a circuit to protect conductor insulation or remove power as a matter of fire prevention and/or personnel safety; and, a “severed” (“broken”) wire is a “malfunction” (“fault”) that “opens” a circuit so that there is, in effect, no round-trip circuit for electrons.
Fundamental Physics of Electric Circuits
Rule 1: Electric currents MUST RETURN TO THEIR OWN SOURCE.
Rule 2: Electric current will return to its source on ALL AVAILABLE PATHS. Corollary: if there are parallel paths back to the source, current will divide and some portion of the total will take each available path.
Rule 3: An electric “circuit” does not exist UNLESS current has a continuous conductive path on which to flow from source back to source.
Readers will come back to these fundamental rules of electrical behavior over-and-over again when dealing with electrical systems and the concept of electrical faults. The more complex the electrical system, the more numerous and complex the issues, but electrical safety always comes back to the physics that underlies the behavior of electric currents.
The National Electric Code (NEC) and the American Boat and Yacht Council (ABYC) electrical standard, E-11, provide design and installation requirements that define system controls that manage how voltages will be safely removed and currents will be safely stopped (disconnected) in response to faults of various kinds that may occur in an electrical system. It is actually quite easy “to get something to work.” It is much more complicated and much more important to control electricity when something isn’t right. Disconnecting power, and disconnecting power safely, is the only way to prevent fires and electric shock risks to personnel.
Electric Code Grounding Categories
Finally, we get to “earthing” and “grounding.” There are two contexts for electrical “grounding” as required by the NEC.
- System Grounding
- Equipment Grounding (Bonding)
Residential System Grounding
“Ground” is the standard reference point for measurement of voltages. The NEC, Article 100, defines the crust of our beloved home planet as “Ground.” Ergo, Sir Knight, the electrical potential (natural voltage) of the earth’s “soil” is defined to be “zero volts.” All voltages are measured from an earth ground reference point.
The crust of the earth is electrically conductive. The earth’s crust contains many minerals and mineral salts which provide “free electrons.” In response to an impressed voltage, electrons will flow from point-to-point around and within the earth’s crust. An important corollary is that currents flowing in the crust of the earth follow the fundamental rules of electro-physics, including “Ohm’s Law” and “Kirchhoff’s Law.” In order to create a residential electrical system connection to “earth ground,” one or more interconnected metallic rods (often copper) are driven into the earth.
In the North American residential AC system model, three conductors arise from the utility power transformer at the street. All three are “Current Carrying Conductors.” Two of those conductors are considered, by conventional agreement, to be “energized” (“L1” and “L2”) and one is the neutral line (“N”). This is known as a “Single Phase, Center Tapped, Three-Pole,” system. The “Neutral is the transformer’s center-tap connection. As these three lines emerge from the utility transformer in the street, 240V are present between “L1” and “L2,” and 120V is present between “L1” and “N” and between “L2” and “N.” Note, however, that at the street, these voltages “float” with respect to their external environmental surroundings. They are not connected to anything. This situation is referred to as a “floating neutral system,” and in a “floating neutral system,” the voltage between the neutral and earth ground is unlikely to be “zero.”
If these three lines were connected to a distribution panel in a residence, all electrical appliances would work correctly. All of the necessary operating voltages inside the building would be correct. But, measured against a ground reference, it’s entirely likely the neutral would be at some perhaps large voltage difference with respect to the metal sink where food is prepared, or the metal bathtub when the baby gets bathed, or the metal faucets in the family shower. Clearly, a shock hazard would exist. To eliminate that hazard, the “Neutral” is electrically “tied” (connected) to an earth-ground reference point.
To create a system referenced to a known. zero-volt earth ground, copper rods are driven into the earth at the building’s service entrance location. Within the main service panel of the building, the utility-provided neutral conductor is connected (“bonded”) to this network of copper ground rods. This connection results in an earth-ground, “grounded neutral” system, throughout the premises. In a grounded neutral system, the voltage between the neutral conductor and the safety ground conductor is “zero,” or should be very close to “zero.”
While it’s true that the earth is electrically conductive, the earth is not a good conductor. Even at its best, “dirt” is not as good at conducting electricity as aluminum and copper wire (and also not as good as salt water). But rest assured, Ohm’s Law is a fixed “law” of physics, and it does apply to currents flowing in the earth. So while “dirt” may not be a great conductor, it is a very large-diameter conductor, with an infinite number of parallel paths, and with virtually unlimited ampacity. Just how well any local parcel of “dirt” conducts electricity depends on many things, including mineral and moisture content. The NEC requires that ground rods have a minimum contact resistance of 25Ω to earth. Sometimes, that can be achieved with a single 10′ rod driven into the soil; sometimes it requires a long rod driven 40′ – 50′ into the ground; and, sometimes it requires an entire network of long ground rods, all driven deep, and all connected together in parallel.
The essential point here is that “earth ground” is a universal reference point for all terrestrial power distribution systems. It represents the presence of “zero” electrical potential, or stated in the negative, the absence of any voltage. This works well because in a properly functioning, properly wired system, no current flows on the grounding system. Since no current flows, the voltage at the contact point with the copper grounding rods stays reliably at zero volts (as predicted by Ohm’s Law). Electrical faults (discussed later) create vastly different, sometime dangerous conditions.
Important to realize in this discussion, the earth ground alone DOES NOT protect against electric shock. It is merely a reference point against which system voltages are stabilized at “zero.” Earth ground IS NOT a reference for protective devices (fuses, circuit breakers) to trip to remove power when an electrical fault condition occurs. The Earth IS NOT the “source” for any DC or AC electrical energy. Remember Rule 1: “All electric currents MUST RETURN TO THEIR OWN SOURCE.” Electrical currents in residential and boat electrical systems DO NOT originate in the earth, and so, do not return to the earth. However, under some kinds of fault conditions, current can and does return to its source by traveling through the “dirt;” or, through the water in which a boat is floating!
Well then, why do we have the “Earthing” connection? Well, “Earthing/Grounding” in this context is a single-point-of-connection (one point and ONLY ONE POINT) to the earth for the purposes of mitigating:
- Static build-up (wind induced),
- System voltage instability, including:
▪ Unintentional physical contact with a higher voltage system (automobile accident or severe weather incident involving “hot” utility services),
▪ Repetitive intermittent short circuits (dispatched to first responders as “trees on wires, burning!”), and
▪ Utility switchyard and distribution system switching surges (spikes).
- Nearby vicinity lightning splash, and
- Transient interference (from static discharge and local RF emissions).
Not all of these exceptional conditions apply equally to all residential premises systems, but because some do apply in all areas, the National Electric Code treats all alike.
Consider a building’s main electrical service panel as the “source” of AC power (volts and amps) for the building and all of its branch circuits. In a household AC electrical system, current from that source emerges from a wall outlet on one appliance conductor and returns to the wall outlet on the other appliance conductor. Refer to Rule 1: “Electric currents MUST RETURN TO THEIR OWN SOURCE.”
Now consider a hot water heater, washing machine, trash compactor, dish washer, garbage disposal, microwave or toaster oven, each constructed with a metal exterior cabinet. The appliance is an electrical “load.” Electricity is provided to it from the wall and returns from it to the wall. With just two conductors (supply and return), the appliance can work normally. But, what happens if there is a frayed or cut wire inside the cabinet, and in physical contact with the metal cabinet of the appliance? In that event, the cabinet will have a non-zero “touch potential” (voltage) on it’s metal enclosure, and that voltage could easily be a shock hazard to residents. This is exactly how houses were wired before the 1950s, and many people reading this will remember the “two prong” duplex outlets of that time. In those days, people did get shocks from household appliances, fans and table lamps. Sometimes even from the iron. (Did Granny’s iron have fraying cotton insulation at the plug end? Does anyone actually iron anymore?) And sometimes, the shocks were serious. These shocks were the results of “faults” in the circuit.
A “fault” is said to exist when:
1) an electric current does not flow when it should, or
2) an electric current flows in an unintended path to get back to its source.
Clearly, an electric shock – which is a path through a person’s body – is an unintended path. To avoid shocking experiences like this, a third electrical conductor (safety ground) was added to electrical systems in homes, garages, barns, workshops, supermarkets, retail stores, office buildings, malls, commercial offices, workplaces, etc. That is, anywhere people might come into contact with electricity.
This brings us to the next major category of “grounds” and “grounding.” Not to the earth itself, although it is connected to the earth, but rather to a common point in the building’s main electric panel. In this context, the word “ground” is a useful – but misleading – concept, because the ground conductor does not live in the ground and it does not send fault current into the ground. The ground conductor is connected to the ground rods at the service entrance, so it is REFERENCED to ground. That way, that ground conductor is held at “zero” volts with respect to all other components in the electrical system.
The “equipment grounding conductor” (a/k/a the “safety ground”) in residential and boat electrical systems is designed and intended to cause fuses or circuit breakers to trip in order to DISCONNECT POWER in case of a fault. It is, in the words of the NEC, an “Effective Ground-Fault Current Path;” that is, an intentionally constructed, low-resistance, conductive path designed to carry fault current from the origin point of a ground fault in a wiring system to the electrical supply’s source and that facilitates the operation of the overcurrent protective device or ground-fault sensors.
Disconnecting power is the ONLY WAY to protect against fire and personal injury caused by ground faults in an electrical system. Equipment grounding is the intentional (in fact, NEC Article 250.2 mandatory) act of providing a network of conductors that interconnects the metallic cases of all electrical equipment attached to an electrical distribution panel. The bare copper or green-insulated “grounding conductor” discussed earlier is connected to the metallic cabinets of all modern appliances, and to the round ground pin of North American 15A and 20A household electrical utility outlets. The wires that make up the network of grounding conductors in a home have several names, but “safety ground” is representative for this discussion. On a boat, this network of green grounds is called the “bonding system,” of which the AC Safety Ground is a key part.
Residential dwelling units in North America range from tiny houses to single family homes to compounds with outbuildings to multi-family buildings of all kinds. A “ground buss” is always located in the main service panel of a dwelling unit, and in any sub-panels that may be supplied from that main service panel. Ground conductors from all branch circuits in the panel are connected together at the panel’s “ground buss.” Sub-panel grounds are in turn brought back to the ground buss in the main service panel. Boats are wired as sub-panels, not as main service panels.
At ONE PLACE in the main electrical service panel of the building, the “Grounding Conductor” is electrically connected (bonded) to the “Neutral” “Current Carrying Conductor.” By code, there is ONLY ONE “Neutral-to-Ground” bond in a residential electrical system, and it is placed at the Main Service Panel – never in sub-panels. A boat is wired as a sub-panel, so there should NEVER be a neutral-to-ground bond aboard a boat connected to, and operating on, shore power. This mistake in wiring on a boat is a very common cause of boats tripping shore power ground fault sensors on docks.
Now consider the fault case where an internal fault of some amount tries to put a touch potential voltage on the metal cabinet of an appliance. Rule 2 applies; “electricity will return to the source on all available paths.” Since the grounding conductor is attached to that metal cabinet, the Grounding Conductor does two things. First, it holds the voltage of the appliance cabinet at zero volts (because it’s “grounded” at the main service panel to the network of ground rods), which protects people and pets from shock. Second, it provides a very low-resistance path back to the service panel, via the neutral-to-ground connection, which instantaneously draws a very large spike of current through the circuit breaker (or fuse). That instantaneous large overload trips the circuit breaker to REMOVE POWER from the faulting circuit. Removing power is how the system protects buildings against fire and protects people from electric shock.
The earth’s crust is electrically conductive, so that creates two electrical system design and code issues.
Rule 1 again: Electric currents MUST RETURN TO THEIR OWN SOURCE; and
Rule 2 again: Electric current will return to its source on ALL AVAILABLE PATHS;
Enter, our Corollary to Rule 2:: if there are parallel paths back to the source, current will divide and some portion of the total will take each available path. This law of physics is called Kirchhoff’s Law, which states that when there are multiple parallel paths back to the source, current will divide and some portion of the total will take each available path back to its source.
In both home appliances and boat appliances, the two most common causes of “ground faults” are aging water heater elements and aging motor/transformer windings. In a water heater, power can leak through the water in the heater between the energized heating element and the metallic case of the water heater. In a motor, over time, dust and other airborne contaminants build up in motor windings, and at the same time, heating and cooling cycles cause the winding’s insulation to break down and develop micro-pores. In these cases, the fault current isn’t enough to trip a circuit breaker, but small amounts of power can leak to the Grounding Conductor, and then back to their source at the main service entrance panel. This is a ground fault by definition, because ANY current flowing on the safety ground is flowing on an unintended path. In this case, the fault current flows back to the source on the Safety ground’s conductor. More in a couple of paragraphs, but first, some illustrations.
Here’s a homeowner scenario… Dad’s gonna trim up the lawn, trim some plants, and wash the car (he’s young and energetic, unlike myself). He runs a 100′ extension cord in order to power an electric hedge trimmer, grass trimmer, circular saw, reciprocating saw, radio, charcoal fire starter, polisher/buffer, whatever. The extension cord has a ground wire, but the “tools” attached to it by multi-outlet adapter either have only two wires, or the ground pin has been cut off as a “matter of portability convenience.” Tools that aren’t actively in use are lying on the ground, where they and their cords are in contact with the ground. Now there is a path for power to get back to its source through the soil, to the ground rod(s) serving the main electric panel, and back to the neutral in the main service panel. That is a ” ground fault” because it is clearly an unintended and unwanted electrical path through the soil (ground). And at some point in this scenario, Dad will pick up his tools and possibly have a shocking experience. Possibly even, a lethal shocking experience. Without a continuous “effective fault-clearing path,” there is no way to shut off the power to save Dad from a shocking experience
OK, here’s another scenario with which my daughter and I have direct, personal experience. One Halloween “Hell Night,” Kate came home in need of a shower to remove 17 cans of different brands of shaving creme and lord-only knows what else she had encountered while “out with friends.” She went off to the shower, whereupon Peg and I laughed at her state of dishevelment! Note here, one of our sons had just finished his shower from his night “out with friends.” After just a couple of minutes, there arouse a righteous and shrill scream from the upper reaches:
“Daddy! Turn the water back on!”
In my total, complete and absolute innocence, I grunted at Peg: “Huh?”
The house water pressure had disappeared to a dribble while Kate was all lathered up. Mid-shower! Springing into action, Mom was “off to the rescue,” and Dad was “off to the basement.” In the basement, all seemed OK, but alas, there was no house water pressure.
Plumbing leaks? No water on the floor!
Pressure in the well tank? No! Gauge reading “zero.”
Pump Circuit Breaker “on?” Yes; and not tripped.
Pump relay OK? Yes, relay “picked.”
“Uh oh!” “Darn it!” (or words to that effect)! “Must be the well pump!”
Our homestead in the Catskill Mountains – and all of our neighbors – had a private deep-well that supplied our drinking water. Our well was 100′ deep, and the pump lived at the 90′ level (not very deep). As the pump started and stopped over many years, it twisted (torquing) on the end of 90′ of semi-flexible PVC hose. The wires running to the pump abraded against the earth and rock walls of the well, and eventually the wire’s insulation wore through. This created a ground fault connection from the exposed bare wire directly to the earth about 70′ down.
Deep well pumps are usually two-wire, 240V circuits. One conductor of ours was in direct contact with the wall of the well. If the point-of-contact had been within the cast iron portion of well casing, it’s likely the circuit breaker would have tripped, because that metal casing did have an equipment grounding conductor. But in our case the point-of-contact was with sediment or rock, the 240V circuit breaker indeed did not trip. That did, however, create a significant ground fault. The pump was trying to start, but didn’t get enough voltage to overcome the weight of a 90′ column of water. Power was flowing into the earth, but not enough to overload and trip the pump’s circuit breaker. Power divided where the bare wire touched the well’s wall. Some of the power going down that hole got to the pump and returned on the other current carrying conductor, but some of the power going down that hole flowed back to the panel through the earth, to our home’s ground rods, and back to the service panel’s neutral.
In these situations, a newly-installed (since 2002 or so) residential service panel would have been fit with “Ground Fault Circuit Interrupter” (GFCI) to remove power and terminate the ground fault condition. In the case of yard tools creating a shock hazard at the end of an extension cord, GFCI could literally save Dad’s life. In the case of the deep well fault, GFCI could have saved equipment from damage. Our deep-well pump got burned out by the prolonged stall created by the low supply voltage. Relate this to boats on docks with pedestals fit with 30mA “Equipment Protective Devices.” This is a case where a 30mA EPD on the well supply would have saved the well pump from damage, and would have provided a clear hint to the location and nature of the fault.
GFCIs and EPDs work by monitoring the outgoing and returning current on the two Current Carrying Conductors. The currents should balance equally between the two conductors. If not, there is a ground fault and the GFCI device trips power off. What happens if there is no GFCI, as was our case at that time? Well then, the ground fault condition continues, because power flows out from the source, but has multiple parallel return paths, one through the returning current carrying conductor and the other through the earth to the ground rods at the main service panel at the same time.
See my article on causes of ground faults on boats for information specific to that topic.
See my article on GFCIs for more detail on how these devices work.
Ground faults on boats behave in the same manner, but are very dangerous, because instead of flowing through dirt, which is largely inaccessible to people, pets and wildlife, ground faults on boats can and do flow through the water. People – especially children – pets and wildlife are sometimes found in the water.
See my article on “Electric Shock Drowning” to read about ground faults in the water.
Ground faults on land can be quite dangerous in another, subtly different way. Suppose a 240V mercury arc exterior driveway light has a ground fault at the pole base that is not large enough to trip an over-current circuit breaker. We all now know from my well scenario, above, that 240V in direct contact with the earth will probably not trip a circuit breaker. But in that condition, the soil surrounding the point-of-contact between the energized conductor and the soil itself is electrically “hot.” This condition sets up a “voltage gradient” on the surface soil surrounding the point-of-contact. Using 240V in this example, at the point-of-contact with the voltage, the voltage in the soil is the same as the supply voltage, so there is no DIFFERENCE in the pole voltage and the soil voltage. But Ohm’s Law applies here, and however much current is flowing into the ground and back to the service entrance panel is creating a voltage drop along the surface of the soil (or driveway). So, the resistance of the local soil matters. One electrical standard1 assumes that 25% of the total voltage drop due to path resistance will be found in the first foot of distance away from the point-of-contact. One foot away from the point-of-contact, the soil is at 163V of shock “step potential.” Three feet from the point-of-contact, the soil is at 202V. Five feet from the point-of-contact, the soil is at 206V. As you can see, straddling the voltage gradient of the surface soil can create dangerous “step potentials” in the soil. Imagine the potential for what could happen when Rover comes over to “mark his spot” at that light pole.
The same sort of voltage gradient forms in the water around the prop and rudder or a boat if there is an AC ground fault on the boat. That gradient is quite enough to get a diver’s undivided attention. If the fault itself is in a heat pump, and the diver is working on the boat when the heat pump cycles “on,” … Well, that diver would quickly know how Rover felt…
See my article on “Electric Shock Drowning” to read about ground fault voltage gradients in the water.
Ground faults can be very dangerous!
Do not defeat safety devices.
Install GFCI and ELCI on boats.
- ANSI/IEEE 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems (Green Book) [4.1.1]