Just as all the water in the world tends to fall to a common level, which is sea level, so does all generated electrical energy tend to fall to a common zero level, which we call ground potential. And just as gravity is the driving force that compels water to flow to the sea, so is the difference in electrical potential between a generated voltage and the ground potential of the earth the driving force that compels the flow of the generated voltage through an electrical circuit. In any electrical circuit, therefore, if there isn’t a connection to ground, there can't be a flow of electricity through that circuit. Indeed, we can say that in any circuit the basic function of a switch or a fuse is to interrupt the flow of electrical energy to ground .
Occasionally, defects in the wiring or insulation of a circuit—defects that are called faults—allow the electrical cur rent to bypass its normal route, to take a short cut to ground, so to speak. This short cut is called a short circuit, and it can occur (1) between two hot wires, (2) between a hot wire and a neutral wire, or (3) between a hot wire and some unintentional ground. What we are going to discuss in this article is the third case, a short circuit (or fault) to ground. Any such short circuit is potentially very dangerous. If the flow of current is great enough, the short circuit can kill a person, or it may so overheat the wires through which the current is flowing that the insulation melts and combustible materials touching the bare wires are set a fire.
NOTE: Residential electrical systems are described in the article ELECTRICAL SYSTEM; particular electrical circuits are described in the article ELECTRICAL WIRING CIRCUITS; it's suggested that these two articles be read first.
Figure 1 shows a typical electrical circuit containing a fuse, a switch, and a motor. Electricity is assumed to flow through the hot wire to the motor windings and thence through the neutral wire to ground. If either the fuse or switch is opened, the circuit will remain hot up to the point of disconnection. The circuit beyond that point will be at ground potential. Once the fuse or switch has been opened, we can disconnect the motor or do any kind of repair or maintenance work we want without receiving a shock.
Note that both the fuse and the switch are on the hot side of the circuit. If the switch, for example, were to be located on the neutral side of the circuit, opening the switch would mean that the circuit would be hot right through the motor, even though current couldn't flow through the circuit to ground, and , consequently, the motor would not operate. But if we were to try to work on the motor we would, the moment we touched a hot wire, complete the circuit to ground and would, therefore, receive a severe shock. For this reason, installing a switch, fuse, or circuit breaker on the neutral side of any electrical circuit can be highly dangerous and is strictly forbidden by all electrical codes.
It should be emphasized that in a 115- or 120-volt residential electrical system it's primarily the rate of current flow through a circuit, that's , the amperage, that makes any short circuit potentially dangerous. And the amount of current it takes to kill you in a 115- or 120-volt system can be quite small. E.g., a milliampere (ma) is one-thousandth of an ampere. If you were to touch a 115- or 120-volt wire through which 5 ma or less of current were flowing, you would feel a slight tingle in your hand. As the current flow increased to about 15 ma, the shock would become increasingly unpleasant, and most people would jerk their hand violently away from the wire. If the current flow were to exceed 15 ma, and if you were actually grasping the wire, “muscle freeze” might occur in which you couldn't let go of the wire at all, If, at the same time, you happened to be standing in a puddle of water and therefore were part of an extremely low resistance path to ground, the electricity would very likely kill you.
Whether or not any given flow of current will prove fatal depends on many things—one’s body size, the amount of voltage driving the current through the circuit, how well one is grounded or isolated from ground—so that a current flow that would kill one person would leave another with nothing more than a tingling sensation. In a 115-or 120-volt system, it's only when a current exceeds 75 ma that it's almost invariably fatal, and even 75 ma is a very small amount of electricity.
The techniques by which an electrical system is safely grounded in a domestic installation are spelled out in very complex detail in the National Electrical Code (NEC). To understand the reasoning behind the requirements of the NEC, one has to make a basic distinction between the main system ground that protects the entire electrical installation against short circuits and an equipment ground in a branch circuit that protects an individual piece of electrical equipment. The main system ground is installed close to the point at which the public utility conductors enter the house, and the equipment ground connections are located in the individual branch circuits.
Electricity is supplied to most dwellings through three large conductors (or wires) that are connected to the public utility’s distribution system. These conductors are known collectively as the service entrance, and it will suffice here to note that two of these conductors carry 115 or 120 volts each (depending on the utility’s practice); the third conductor is the neutral cable. For a detailed description of the service entrance see ELECTRICAL SYSTEM.
The electricity supplied to most dwellings is generated and transmitted by the utility at 2300 or more volts. This voltage is reduced to the 115/230 or 120/240 volts used in most dwellings by nearby transformers.
The neutral conductor of the service entrance is connected to ground at both the transformer and at the dwelling, the latter connection being described below. Thus, the neutral conductor of the service entrance is at ground potential over its entire length, and , in addition to its basic function of maximizing the efficiency with which voltage can push current through the system, the neutral conductor also protects the entire electrical system against such major calamities as lightning striking the service entrance or storm damage that causes the 2300-volt power line to short across the incoming 115- or 120-volt service-entrance conductors. If the neutral conductor weren’t present and properly grounded, either of these events could result in a surge of high-voltage electricity through the wiring system of the house. The entire wiring system might very well burn out. In addition, any appliances that happened to be connected to the system at the time of the short would have their circuits burned out also. But if the neutral conductor is adequately grounded, as it most probably would be, this surge of high-voltage electricity will be conducted immediately to ground and harmlessly dissipated.
Figure 2 shows a typical system ground installation including the connections between the incoming neutral conductor and a ground wire and between the ground wire and the main ground itself; what the NEC refers to as the grounding electrode.
In most dwellings the service entrance (and the utility’s responsibility) ends at a service disconnect switch that's located within a metal enclosure (i.e., a distribution panel often called a panel box) mounted either on the outside or just inside the dwelling. This panel box usually contains the main and branch-circuit fuses or circuit breakers as well.
The incoming neutral conductor, or wire, which is easily recognized since its insulation invariably has a white or light gray color, unless it's completely bare (the hot wires have either black or red insulation), ends at a heavy buss or neutral strap made of copper or a copper alloy to which it's securely fastened by a large bolt. (There may also be terminals on the neutral strap to which are connected the neutral conductors of such major household appliances as an electrical range or freezer, as well as smaller terminals to which are attached the neutral conductors that are part of the house’s branch circuits.) Since the neutral strap is at the same time securely bolted to the panel box housing, it, the panel box, the neutral conductor, and the ground wire are all at ground potential.
The ground wire is usually made of copper because this metal is both an excellent conductor of electricity and non-corrodible. It can be in the form of a solid bar or rod, or stranded or solid wire, and it may or may not be covered with insulation. The main thing is that the ground wire be of a certain minimum size; this size depends on several factors, the main one being the capacity in amperes of the incoming electrical service. The ground wire must certainly be large enough that it does not introduce any electrical resistance of its own into the circuit. If the ground wire is No. 6 American Wire Gauge (AWG) in size or larger, it can be installed as is. If it's No. 8 AWG or smaller, it must be located within a conduit for protection, and the conduit, if it's made of metal, must then be grounded itself to both the panel box and the main ground.
In all communities that have an underground water-supply system in which the water pipes are made of metal, these pipes serve as the main electrical grounds. In each house, the end of the ground wire is connected to the cold-water supply pipe just where the pipe enters the house. The water pipe is used not because water happens to be flowing through it, but because the exterior of the metal pipe is in solid contact with damp soil, which is of course at zero potential.
The ground wire is connected to the water pipe using special clamps that are specifically designed for this purpose. When the water pipe is made of iron or steel, the clamp must be made of iron or steel; when the water pipe is made of copper, the clamp must be made of copper or brass—the similarity of materials being to prevent the electrolytic corrosion of the clamp, which might occur if two dissimilar metals were joined together. If corrosion should occur, it would increase the electrical resistance of the ground circuit and thus decrease its effectiveness. Under no circumstances can any connection be made by soldering the ground wire to the main ground.
If a water meter should be installed in the water-supply line, the ground connection must be made on the supply side of the meter. Then, if the meter should ever be removed for repairs, the electrical system won’t inadvertently be disconnected from its ground. Besides, the pipe joints connecting the meter to the water-supply pipes often contain rubber or cork gaskets that will also prevent the transmission of ground currents; again, the system would in effect be disconnected from its ground.
If there should be a main shutoff valve installed in the water- supply line, the ground connection must also be on the supply side of this valve, in case the valve is ever removed for repairs.
Sometimes the layout of the house makes it difficult to install the ground wire on the supply side of a water meter and /or shutoff valve. In this case jumper lines are connected to the pipe around both meter and valve. In many communities in which the water is hard, a homeowner may install a water- softener unit in the main water-supply line. Very often this equipment is connected to the water-supply lines by rubber hoses. When this is the case, a jumper line must also be connected to the water-supply lines around the water-softening unit.
As for the water pipe itself, it shouldn't be painted, enameled, or coated with an asphalt compound, all of which tend to reduce the ability of the pipe to transfer an electrical current into the soil. Before he connects the ground wire to the pipe, the electrician must also make sure that the pipe actually extends at least 10 ft into the soil without interruption. This may sound like a curious precaution to take, but many new water-supply installations have plastic gaskets installed between pipe sections to secure good seals at these joints. These gaskets will, of course, electrically isolate the sections of the pipe from each other. Plastic pipe is also being used more and more for cold-water supply systems. The electrician, therefore, must also make sure that he isn't connecting the ground wire to a plastic pipe, which of course would result in no ground at all.
If the water-supply line can't be used as the main ground, it may be necessary to connect the ground wire to an incoming gas line. But before a gas line can be used as a system ground, the electrician must obtain the permission of both the local gas utility and the local building inspectors. The electrician must also make certain that nonmetallic gaskets haven’t been installed between the sections of pipe. Lacking this alternative, it will be necessary for the electrician to install special made electrodes into the soil to which he can connect the ground wire.
A made electrode is usually necessary for an isolated house. The electrode consists either of a length of pipe or rod (see Fig. 3), a flat metal plate, or perhaps the metal casing of a well. The preferred materials are either copper or galvanized steel. Whichever of these two metals is used, it shouldn't be painted, varnished, or otherwise covered with a finish that will reduce its ability to conduct a ground current into the soil.
Rods and pipes must be driven at least 8 ft into the soil, leaving the end of the electrode exposed a few inches above the ground to enable the electrician to clamp the ground wire to it. The fact that the connection is above ground also allows it to be inspected from time to time.
Whatever the type of electrode used, the soil into which it's driven or buried must be damp; otherwise it's very likely that ground currents will not be adequately dissipated in the soil. This would most likely be a problem in arid regions. It may be necessary, therefore, to drive the electrode a considerable distance into the earth before permanently damp soil is reached, or, alternatively, several electrodes can be driven into the earth and the ground wire connected to them in parallel. But when more than one electrode is installed, the electrodes should be at least 6 ft apart.
Since 1968, another type of electrode has been approved by the NEC. It is called the Ufer electrode, after its inventor, and it consists of a bare copper wire having the same diameter as the ground wire (but at least No. 4 AWG in diam.) and at least 20 ft long that's buried within the concrete footing of the house (see Fig. 4). The copper wire must not actually touch the soil, but it must be located at the bottom of the footing, within 2 in. of the soil. One end of the Ufer electrode is clamped to the ground wire at the point where it emerges from the concrete. The Ufer electrode works, and it works well, because the soil underneath most foundation walls is damp, because concrete is permeable to water, and because the weight of the house pressing down upon the damp soil assures a solid electrical contact between the wire and the soil below.
An equipment ground has an entirely different purpose from a system ground. The purpose of an equipment ground is to ground the housing of an electrical appliance in case an internal short should occur between the appliance’s electrical system and its housing. The situation is shown in Fig. 5, which s the same circuit as Fig. 1 but with an internal short between the motor windings and the motor housing.
This internal short circuit will not prevent the motor from operating, nor will it result in excess current flow that will cause the branch-circuit fuse to blow. But anyone who touches the motor housing will complete a low-resistance path to ground and receive a shock. The severity of the shock will depend on the extent of the fault and on how well the person is insulated from ground.
For safety’s sake, therefore, the housing must be grounded. This is accomplished by connecting a special grounding wire to the housing. What the other end of this grounding wire is connected to depends on the type of wiring installation in the dwelling, of which there are two basic types.
In one type of wiring installation, the wires making up the electrical system are contained within metal conduits that run from one outlet box to another throughout the house. These conduits are securely fastened to each outlet box by metal bushings and locknuts. The conduits are also securely connected to the main panel box by special bushings to which short jumper wires can be fastened. These jumper wires are then fastened to the neutral strap located within the panel box. Thus, the entire system of conduits is grounded.
In the second basic type of wiring installation, the wiring Consists of what is called nonmetallic sheathed cable, which means that the wires are surrounded only by rubber or plastic insulation. (Armored cable is also used for dwellings, but the principle of installation remains the same.) In older houses, those built before the 1960s, say, electrical cables contained only two insulated wires—the hot wire and the neutral wire. But beginning in the 1960s cable that included a separate grounding wire within the outer cable wrapping has come to be used more and more. This grounding wire is easily identified because it's completely bare of insulation.
In a house that's wired with this type of cable, each outlet box is grounded by attaching the bare grounding wire to the outlet box with a screw or special clamp. In this way, the entire electrical system is grounded. As with a conduit installation, the cables are ultimately connected to the neutral strap located within the main panel box.
Note that neither the grounded conduits nor the grounding wires of the cable system have anything at all to do with the neutral side of the electrical wiring circuits, even though everything eventually ends up at the same neutral strap. For safe and efficient operation of the electrical circuits, it's important that the neutral wires and grounding circuits be kept completely separate from each other.
To return to our discussion of electrical-equipment grounds, in almost all motor-driven appliances manufactured today, the appliance housing is grounded through a wire enclosed in the cord that supplies electricity to the motor. This grounding wire is identified by the green insulation that covers it. One end of this grounding wire is connected to the appliance housing. The other end of the wire is connected to a blade on the connecting plug. There are three such blades (see Fig. 6). Two serve the usual function of connecting the hot and neutral wires to the electrical circuit via the duplex receptacle. The third blade, which may have either a round or U shape, leads to the system’s grounding circuit.
To accommodate a three-bladed plug, the receptacle into which it's inserted must have three mating slots—two for the hot and neutral wires and another for the grounding connection. Such a three-slot receptacle, which is called a grounding receptacle, is shown in Fig.7. The grounding connection from the appliance housing emerges from the side of the receptacle at a green-tinted hex-head screw. A grounding wire from one of the cables entering the outlet box is connected to this screw, or, in a conduit system, a jumper wire connected to the side of the outlet box grounds the appliance housing. In this way the safe operation of the appliance is assured.
Appliances manufactured before the 1960s are connected to receptacle outlets via two-bladed plugs. These plugs can be inserted into the three-slot grounding receptacles shown in Fig.7, of course. The opposite isn't true, however. An appliance having a three-bladed plug can't be connected to a two- slotted receptacle. The preferred solution to this problem is to replace the old receptacle with a new grounded receptacle. But when this is done, it's important that the newly installed receptacle be grounded to the outlet box with a clip—after one has made certain that, in fact, the outlet box is grounded out. If it isn’t, all that one has gained is a false sense of security for which one may pay dearly some day.