Grounding for Safety
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ELECTRICITY ALWAYS FOLLOWS the shortest possible path to the earth, that is, to the ground. In discussions concerning electrical wiring you will regularly meet the terms ground, grounded, and grounding. They all refer to deliberately connecting parts of a wiring installation to the earth. Actually, the connection is made to something that serves as the earth—a metal rod driven deeply into the ground and metal piping that is part of an underground water system. The purpose of grounding is safety. A wiring installation that is properly grounded allows excess electrical current—such as from lightning strikes—to travel into the earth without causing serious injury to people or damage to the wiring system. Grounding facilitates the proper functioning of fuses and circuit breakers, limiting the risk of shock from defective equipment. An installation that is not properly grounded can present an extreme danger of shock, fire, and damage to appliances and motors. For example, suppose a 2,400-volt line accidentally falls across your 120/240-volt service during a storm. If the system is not grounded, you can be subject to deadly 2,400-volt shocks, and wiring and appliances will be ruined. If the system is properly grounded, the highest voltage of a shock will be much more than 240 volts, but very much less than 2,400 volts. Lightning striking on or even near a high voltage line can cause great damage to your wiring and your appliances, and it can cause fire and injuries. Proper grounding throughout the system greatly reduces the danger. Because grounding is so important it is discussed repeatedly throughout this guide. Be certain that you understand it thoroughly. The National Electrical Code (NEC) rules for grounding are extensive and sometimes seem ambiguous. But for installations in homes and farm buildings—the only installations covered in this guide—the rules are relatively simple. This section discusses the basic principles of grounding. The practical details, including installing ground rods and ground clamps and selecting the correct ground wire size, are covered in the next section on installing the service entrance. HOW DANGEROUS ARE SHOCKS? Most people think it is a high voltage that causes fatal shocks. This is not necessarily so. The amount of current flowing through the body determines the effect of a shock. A milliampere is one-thousandth of an ampere (0.001 amp). A current of 1 milliampere through the body is just barely perceptible. Currents from 1 to 8 milliamperes cause mild to strong surprise. Currents from 8 to 15 milliamperes are unpleasant, but usually the victim is able to let go and get free of the object that is causing the shock. Currents over 15 milliamperes are likely to lead to “muscular freeze:’ which prevents the victim from letting go and often leads to death. Currents over 75 milliamperes are almost always fatal; much depends on the individual involved. The higher the voltage, the higher the number of milliamperes that would flow through the body under any given set of circumstances. A shock from a relatively high voltage while the victim is standing on a completely dry surface will result in fewer milliamperes than a shock from a much lower voltage to someone standing in water. Deaths have been caused by shock from circuits considerably below 120 volts, while someone standing on a dry surface could survive shock from a circuit of 600 volts and more. How to help the victim of a shock is discussed on earlier. GROUNDING BASICS This section defines grounding terminology and describes the functions of the wires and equipment in your installation. Italics are used to indicate words for which definitions are provided and to help differentiate among similar-looking terms. Ground: For the wiring installations covered in this guide, the term “ground” means underground metal water piping connected to a driven metal rod forming a continuous conductive path that allows excess current to travel into the earth. The rod driven into the earth is called a ground rod. The water pipe is part of the building’s underground water system. The NEC calls the rod and water pipe “grounding electrodes?’ Properly joined together, they form a “grounding electrode system?” This arrangement is illustrated at the bottom of FIG. 8-6. “When it is said that something is “grounded”: it means that it is connected to ground. Grounding falls into two categories. System grounding is for the current-carrying wires in an installation. Equipment grounding is for parts that do not carry current in the installation—they are listed under “Grounding (green) wire”. Bonding (or bonded) : Bonding means connecting normally non-current- carrying metal parts (conduit, boxes, etc.) to each other and finally to ground, usually using bare uninsulated wire, resulting in a conductive path that will safely carry accidental voltage into the earth. Ground wire : This is the wire that runs from the service equipment to the grounding electrode (which is grounded because it is buried in the earth). The NEC calls it the “grounding electrode conductor?” It is usually bare, but can be insulated and of any color but green. The ground wire is bonded within the service equipment enclosure to the neutral conductor, to the service raceway, to the service enclosure, and to the equipment grounding bus if any. Grounded (white) wire : In a circuit, this is the wire (usually white) that normally carries current and is connected to the ground at the service equipment. The NEC calls it the “identified conductor.” The grounded wire must never be fused or protected by a circuit breaker or interrupted by a switch. Grounding (green) wire: This is a wire that does not carry current at all during normal operation. The NEC calls it the “equipment grounding conductor?” It is bonded to components of the installation that normally do not carry current but do carry current in case of damage to or defect in the wiring system or the appliances connected to it. These components include the equipment grounding bus in the service equipment cabinet (which holds the main fused service switch or the circuit breaker panel-board, whichever is installed), frames of motors, frames of appliances such as electric range and clothes washer, the outlet boxes in which switches or receptacles are installed, and the metal conduit or the armor of armored cable. The grounding wire runs with the current-carrying wires. It must be green, green with one or more yellow stripes, or bare. In this guide it will be called simply a green wire. It must never be used for any purpose except as the grounding wire in a circuit. In the case of wiring with metal conduit, or cable with a metal armor, a grounding wire as such is not installed separately because the conduit or the armor of the cable serve as the grounding conductor where it is bonded to the cabinet. GROUNDING OVERVIEW
Proper grounding involves every part of the installation, including the incoming service wires that run from the power supplier’s transformer to the building to be served. The service equipment cabinet—the heart of your electrical installation—is where the main grounding connections are made. Wires from transformer to building: Figure 7-1 shows the power supplier’s three wires—labeled A, B, and N—that run from the transformer to the building. The grounded wire, labeled N, is grounded both at the transformer and at the building’s service equipment. This is a neutral wire (meaning it is non-current-carrying when the load is balanced on the hot wires). Wires A and B are “hot” wires. (Hot wires carry voltage above zero, or ground. One is usually black and the other red, or both black, but never white or green.) The voltage between A and N, or between B and N, is 120 volts; between A and B it is 240 volts. Between the transformer and the service equipment, the grounded wire N is a neutral wire. In wiring a building, any wire connected to the point where the neutral wire ends in the service equipment is simply a grounded wire. Many people call it a neutral but it is not a neutral; there cannot be a neutral in a two-wire circuit. (Three-wire circuits, which have a grounded neutral wire, are discussed in a later section.) Grounding of 120-volt and 240-volt loads: The grounded wire must run without interruption to all equipment operating at 120 volts, but not to anything operating only at 240 volts. Only hot wires run to 240-volt loads. A separate grounding wire runs to 240-volt loads (unless the conduit or the armor of armored cable serves as the grounding conductor). Note: Anything that is connected to a circuit and consumes power constitutes a “load” on the circuit. The load might be a motor, a toaster, a lamp—anything consuming power. Switches do not consume power and therefore are not loads. A receptacle is not a load, but anything plugged into the receptacle is a load. Grounding at the service equipment At the service equipment all the grounding wires are connected to an equipment grounding bus-bar, which is bonded to the enclosure and connected to the neutral bus-bar (for all the white wires), which is in turn connected (grounded) by means of the ground wire to the water pipe and ground rod. The power supplier also grounds the neutral of the incoming service wires at the transformer serving the building. If the neutral wire is properly grounded both at the transformer and at the building, it follows that if you touch an exposed grounded wire at a terminal or splice, no harm follows—no shock—any more than if you touch a water pipe or a faucet, because the grounded wire and the piping are connected to each other. Any time you touch a pipe, you are in effect also touching the grounded wire. Short circuits and ground faults: If two hot wires touch each other at a point where both are bare, or if a hot wire touches a bare point in a grounded circuit wire, a short circuit occurs at the point of connection. This rarely happens in the actual wires of a properly installed system, but often happens in cords to lamps or appliances, especially if the cords are badly worn or abused. A ground fault results when a bare point in a hot wire, such as where the wire is connected to a receptacle or switch, touches a grounded component such as conduit, the armor of armored cable, or a grounding wire. For both short circuit and ground fault the effect is the same: a fuse will blow or a circuit breaker will trip. HOW GROUNDING PROMOTES SAFETY The diagrams in this section illustrate correct installations as well as faulty conditions in which shock can occur. The motors represented could be either free standing or part of an appliance. The coiled portion of the line on the right side of each diagram represents the motor’s winding. Your risk of shock and danger from a faulty installation will depend on the surface on which you are standing, your general physical condition, and the condition of your skin at the contact point. If you are on an absolutely dry surface you will note little shock. If you are on a damp surface, (as in a basement) you will experience a severe shock. If you are standing in water you will undergo extreme shock or death. As a safety precaution, always stand on dry boards when you must work in a damp or wet location. Grounding and fuse/breaker placement for 120-volt circuit : Figure 7-2 shows a 120-volt motor with the grounded wire connected to the grounded neutral of the service equipment and a fuse in the hot wire. (Actually the fuse and the ground connection would be at the service equipment cabinet a considerable distance from the motor, not near the motor as shown, although there might be an additional fuse near the motor.) If the fuse blows, the motor stops. What happens if, in inspecting the motor, you accidentally touch one of the wires at the terminals of the motor? Nothing happens because the circuit is hot only up to the blown fuse. Between the fuse and the motor the wire is now dead just as if the wire had been cut at the fuse location. The other wire to the motor is grounded, so it is harmless. You are protected. But if the fuse is not blown and you touch the hot wire, you will receive a 120-volt shock through your body to the earth, and through the earth back to the neutral wire at the service equipment.
Fuse/breaker placement error : The same circuit is shown in FIG. 7-3 except the fuse is wrongly placed in the grounded wire instead of the hot wire. The motor will operate properly. If the fuse blows, the motor stops. But the circuit is still energized through the motor and up to the blown fuse. If you touch one of the wires at the motor, you complete the circuit through your body, through the earth, to the neutral wire in your service equipment; you are directly connected across 120 volts and as a minimum you will receive a shock, and at worst will be killed. Accidental internal ground—unprotected : Now see FIG. 7-4, which again shows the same 120-volt motor as in FIG. 7-2. But suppose the motor is defective so that at the point marked G the winding inside the motor accidentally comes into electrical contact with the frame of the motor. As a result, the winding “grounds” to the frame. That does not prevent the motor from operating. But suppose you choose to inspect the motor, touching just its frame. What happens? Depending on whether the internal ground between winding and frame is at a point nearest the grounded wire or nearest the hot wire, you will receive a shock up to 120 volts as you complete the circuit through your body back to the grounded wire. It is a potentially dangerous situation since shocks of much less than 120 volts can be fatal. In fact, it is not uncommon for breakdowns in the internal insulation of a motor to result in an accidental electrical connection between the winding and the frame of the motor. The entire frame of the motor becomes hot. The same situation arises if the motor is fed by a cord that becomes defective where it enters the junction box of the motor so that one of the bare wires in the cord touches the frame. If there is no cord and the motor is fed by the circuit wires, a sloppy splice between the circuit wires and the wires in the junction box on the motor can lead to the same result: the frame of the motor becomes hot.
Accidental internal ground—protected : Now see FIG. 7-5 which shows the same motor as in FIG. 7-4 with the same accidental ground between winding (or cord) and frame, but protected by a grounding wire that is connected to the frame of the motor and runs back to the ground connection at the service switch. When the internal ground occurs, current will flow over the grounding wire. It will sometimes but not always blow the fuse. Even if the fuse does not blow, the grounding wire will protect you because it reduces the voltage between the frame of the motor to substantially zero as compared with the ground you are standing on. You will not receive a shock provided that a really good job of bonding was done at the service. If the bonding is poor, you will still receive a shock. The grounding wire from the frame of the motor (or from any other normally non current-carrying component) maybe green or in many cases bare uninsulated wire. Green wire may not be used for any purpose other than the grounding wire. Other sections discuss when a separate grounding wire must be installed. If metal conduit or cable with armor is used, the conduit or armor serves as the grounding wire. Grounding and fuse/breaker placement for 240-volt circuit : Now refer to FIG. 7-6, which shows a 240-volt motor installed with each hot wire protected as required with a fuse or circuit breaker. Remember that in such 240-volt installations, the white grounded wire does not run to the motor, but is nevertheless grounded at the service equipment. If you touch both hot wires, you will be completing the circuit from one hot wire to the other, and you will receive a 240-volt shock. But if you touch only one of the wires, you will be completing the circuit through your body, through the earth, back to the grounded neutral in your service equipment, and you will receive a shock of only 120 volts: the same as touching the grounded wire and one of the hot wires of FIG. 7-1. The difference between shocks of 120 volts and 240 volts can be the difference between life and death.
Accidental internal ground—protected : Assume that the motor in FIG. 7-6 becomes defective, that the winding of the motor becomes accidentally grounded to the frame, as shown in FIG. 7-7. This as the same as FIG. 7-4 except that the motor is operating at 240 volts instead of 120 volts. Touching the frame will produce a 120-volt shock, but if the frame has been properly grounded as in FIG. 7-8, one of the fuses will probably blow. Even if a fuse does not blow, the severity of the shock you receive from touching the frame will be limited because the frame is grounded, again assuming that a really good ground was installed at the service. GROUNDING CONSIDERATIONS FOR WIRING SYSTEMS In any of the situations of Figs. 7-2 through 7-8, if there is an accidental contact between any two wires of the circuit, the contact constitutes a short circuit, and a fuse will blow or the circuit breaker will trip regardless of whether the short is between one of the hot wires and the grounded wire, or between the two hot wires. For a short circuit to occur, there must be bare places on two different wires touching each other, which does not happen very often in a carefully installed job.
Advantages of metal conduit or armored cable : Consider a wiring system in which all the wires are installed in a metal raceway: metal pipe called conduit, or cable with a metal armor (commonly referred to as “BX”). The raceway or armor is grounded at the service equipment. It is also connected to the motor itself (assuming that the motor is not connected by a flexible cord and plug). No separate grounding wire is then required. If there is now an accidental ground from winding to frame (or to the raceway or armor), it has the same effect as a short between one of the hot wires and the grounded wire, because the grounded wire and the raceway or armor are connected to each other and to the ground at the service equipment. A fuse will immediately blow whether the motor operates at 120 or 240 volts. A considerable advantage has been gained—accidental grounds that otherwise might remain undiscovered are automatically disclosed. See FIG. 7-9. Separate grounding conductors : Many wiring methods use nonmetallic cable sheaths or raceways. In most such cases you install (or the cable includes) a separate grounding conductor that performs the same equipment grounding function as the steel raceway or cable armor. In addition, some designers insist on a separate equipment grounding conductor even within a steel raceway in order to add additional reliability. The NEC actually requires that arrangement for branch circuits serving patient care areas of hospitals. Although hospital wiring is beyond the scope of this guide, that rule points to one of the many instances where you’re likely to encounter separate grounding conductors. Continuous grounding : An outlet box or switch box is used to protect each electrical connection in an installation. When a grounded-neutral wiring system is used (which is 100 percent of the time for installations of the kind discussed in this guide), and you use metal conduit or cable with a metal armor, you must ground not only the neutral wire but also the conduit or cable armor. The white wire is grounded only at the service equipment (at the point where it is connected to the neutral of the service equipment), but the conduit or armor must be securely connected to every box or cabinet. Lighting fixtures installed on metal outlet boxes are automatically grounded through the conduit or armor. (Nonmetallic boxes are also used). Motors or appliances directly connected to conduit or armor are automatically grounded. If using nonmetallic-sheathed cable, the extra wire—the bare uninsulated grounding wire—must be carried from outlet to outlet, providing a continuous ground. The connections for this wire are explained in another section. Regardless of the wiring method used, a continuous ground all the way back to the service is essential. Good workmanship is crucial for this part of the job, because a small defect in a grounding return path can make the difference between an overcurrent device opening promptly or not, leading to a fire or worse. GROUNDING-TYPE RECEPTACLES INCREASE SAFETY Many old receptacles still in use have only two parallel openings for the plug, as in FIG. 7-10. Plugging something into this receptacle duplicates the condition of FIG. 7-2. If you handle a defective appliance that is plugged into an old-style receptacle, you could receive a shock (see FIG. 7-4). This danger led to the development of the “grounding receptacle,” which has three openings (see FIG. 7-11). Note that the grounding receptacle in FIG. 7-11 has the usual two parallel slots for two blades of a plug, plus a third round or U-shaped opening for a third prong on the corresponding plug. In use, the third prong of the plug is connected to a third or grounding (green) wire in the cord, running to and connected to the frame of the motor or other appliance.
Making the ground connection : On the receptacle, the round or U-shaped opening leads to a green terminal screw that in turn is connected to the metal mounting yoke of the receptacle. The details of how to connect the green terminal to the equipment grounding conductor, or to the grounded metal box, are discussed in a later section. In this way, the frame of the motor or appliance is effectively grounded, leading to extra safety as shown and discussed in connection with FIG. 7-5. Where grounding receptacles are required : If an appliance is connected by cord and plug, NEC 250. t 14(3) requires a three-wire cord and three-prong plug on every refrigerator, freezer, air conditioner, clothes washer, clothes dryer, dishwasher, sump pump, on aquarium equipment, and on every hand-held, motor-driven tool such as a drill, saw, sander, hedge trimmer, and similar items. The three-wire cord with three-prong plug is not required on ordinary household appliances such as toasters, irons, radios, TVs, razors, lamps, and similar items. Only one kind of receptacle needs to be installed, because grounding receptacles are designed for use with both two-prong and three-prong plugs. The NEC requires that in all new construction only grounding receptacles are to be installed. GROUND-FAULT CIRCUIT INTERRUPTER (GFCI) The GFCI is a supplementary protection that senses leakage currents too small to operate ordinary branch circuit fuses or circuit breakers. The use of a grounding wire in a three-wire cord with a three-prong plug and a grounding receptacle reduces the danger of a shock in some circumstances, such as when using a portable tool, but it does not eliminate the danger completely. Cords can be defective or wrongly connected. Millions of tools with two-wire cords are still in use. Some people foolishly cut off the grounding prong on a three-prong plug because they have only two-wire receptacles (see Figs. 7-10 and 7-11). Under normal conditions the current in the hot wire and the current in the grounded wire are absolutely identical. But if the wiring or a tool or appliance is defective and allows some current to leak to ground, then a ground-fault circuit interrupter will sense the difference in current in the two wires. If the fault current exceeds the trip level of the GFCI, which is between 4 and 6 milliamperes, the GFCI will disconnect the circuit in as little as one-fortieth of a second. The fault current, which is much too low to trip a normal breaker or blow a fuse, could possibly flow through a person in contact with the faulty equipment and a grounded surface. The use of a GFCI is a highly recommended safety precaution, especially when using electrical equipment outdoors where standing on the ground (especially if it is wet) greatly increases the likelihood and especially the severity of a shock. The GFCI you install must be rated in amperes and volts to match the rating of the outlet or circuit it is to protect. The GFCI should be considered additional insurance against dangerous shocks. It is not to be considered a substitute for grounding. The GFCI will not prevent a person who is part of a ground-fault circuit from receiving a shock, but it will open the circuit so quickly that the shock will be below levels that inhibit breathing or heart action or the ability to let go of the circuit. Where GFCIs are required: NEC 210.8 requires GFCI protection for 15-amp and 20-amp receptacles in these locations in dwellings: at kitchen counters; for counter use within 6 feet of a wet bar sink; outdoors at balconies, porches, patios, decks, and roofs; in grade-level non-habitable rooms of detached accessory buildings; in garages (except where not accessible such as a garage door opener or for appliances in dedicated space such as a freezer); in wired detached garages; in unfinished basements; in crawl spaces; and in all boathouses. GFCI protection is also required for bathroom receptacles in all occupancies, and for permanently installed receptacles used temporarily during construction. Some swimming pool, spa, and hot tub installations (not covered in this guide) also require GFCI protection. Three types of GFCI: Ground-fault circuit interrupters are available as separately enclosed types, or in combination with either a breaker or a receptacle. The separately enclosed type is available for 120-volt, two-wire and 120/240-volt, three-wire circuits up to 60 amps. It is most often used in swimming pool wiring, installed at any convenient point in the circuit. Related to this is the “master trip” GFCI device, which looks like a GFCI receptacle without any slots; it is used for downstream GFCI protection in cases where protection must be provided, but an outlet is not desired or permitted at its location. The breaker type combines a 15-amp, 20-amp, and up to 60-amp circuit breaker and a GFCI in the same plastic case. It is installed in place of an ordinary breaker in your panelboard, and is available in 120-volt, two-wire or 120/240-volt, three- wire types (which will protect a 120/240-volt, three-wire circuit or a 240-volt, two-wire circuit). It provides protection against ground faults and overloads for all the outlets on the circuit. You can at any time replace an ordinary breaker in your panelboard with one of these combination breakers. Each GFCI circuit breaker has a white pigtail that you must connect to the grounded (neutral) busbar of your panelboard. You must connect the white (grounded) wire for the circuit to a terminal provided for it on the breaker. Do not use a single-pole GFCI circuit breaker on a multiwire circuit; it will nuisance trip immediately. Use two-pole GFCI circuit breakers on such circuits. The receptacle type combines a receptacle and a GFCI in the same housing . It provides only ground-fault protection to the equipment plugged into that receptacle or, if it is the “feed through” type, to equipment plugged in to other ordinary receptacles installed “downstream” on the same circuit. This type is a convenient choice when replacing existing receptacles where GFCI protection is desired or required. Be very careful to observe “line” and “load” markings on these receptacles. If you wire them backwards, anything plugged in will still work, and the test button will cause the reset button to trip as usual. However, only downstream loads will be protected; anything plugged into the mis-wired device will have no GFCI protection at all. GFCI testing and potential problems : Regardless of the type or brand of GFCI you install, it is essential that you carefully follow the installation and periodic testing instructions that come with it. Every GFCI has a test button for easy verification of its functional operation. The GFCI is designed to trip if the cords or tools plugged into the protected receptacle outlet are in poor repair and provide a path for current to leak to ground. Even where wiring, tools, and appliances are in perfect condition and there is no ground fault, be on the lookout for these installation problems that will cause tripping of a GFCI: • A two-wire GFCI receptacle (other than an end-of-run type) is connected in a three-wire circuit. Two-pole GFCI circuit breakers will protect these circuits. • The white circuit conductor is grounded on the load side of the GFCI. • The protected portion of the circuit is excessively long (250 feet maximum is the rule of thumb—longer circuits may develop a capacitive leakage to ground). Remember that the GFCI will not prevent shock, but it will render shocks relatively harmless. Also, it will not protect a person against contact with both conductors of the circuit at the same time unless there is also a current path to ground. The GFCI may be pictured as a tiny computer that constantly monitors the current out to a load and back again. The GFCI acts to quickly disconnect the circuit only when the current out to the load and the current returning differ by 0.005 amp (5 milliamperes) or more. LIGHTNING ARRESTERS If you live in an area subject to lightning activity, consider installing a lightning arrester at your service location. Lightning damage to building wiring may not be evident at the time of the strike but may show up later. SURGE SUPPRESSORS FOR COMPUTERS AND OTHER ELECTRONIC EQUIPMENT With the increasing home use of personal computers and other sensitive electronic equipment on general-use branch circuits, there is a need for suppression of voltage surges. These surges are typically of very short duration. Among the possible causes are the switching off or on of fluorescent lights or motors such as air conditioners, or the switching of major loads by the utility, or distant electrical storms. Such surges can cause a personal computer to lose data, or cause other troubles for modems, data terminals, word processors, fax machines, electronic cash registers, computerized sewing machines, and other electronic equipment. Even with a surge suppressor in place, it is a good idea to disconnect (unplug) sensitive electronic equipment when there is an electric storm nearby. Several types of surge suppressors are available including: • plug-in unit resembling a cord adapter • group of receptacles on a strip with a supply cord • permanently installed receptacles that include surge suppression in their design • unit shaped like a circuit breaker that provides only surge suppression—it is plugged into a two-pole circuit breaker space • circuit breakers which, in addition to the usual overcurrent protection features, incorporate a surge suppressor to protect the entire branch circuit Some of these may include a replaceable module, or a light or buzzer to indicate when the suppressor has failed, or both. The less expensive models degrade after doing their duty a few times and may not offer any protection. There is no way to test them. Most designs shunt the excess energy into the grounding circuit, which may damage other equipment on the same circuit. The best designs incorporate capacitors to store the excess energy instead of shunting it to the ground. Manufacturers can request additional testing by UL, in addition to safety testing, to establish Endurance Grades A, B, and C; Performance Classes 1, 2, and 3; and Modes 1 and 2. The best surge suppressors carry the Class 1, Grade A, Mode 1 classification. You can spend less than $10 to more than $200, and in general you get what you pay for. Be sure the unit you purchase is listed. PREV: Circuit Diagrams Home • • |
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