Fundamentals of grounding and bonding
Key concepts Conductors should be bonded at the service enclosure and connected to a grounded electrode. Grounded and equipment grounding conductors should never be interchanged.
Conductors should be bonded at the service enclosure and connected to a grounded electrode.
Grounded and equipment grounding conductors should never be interchanged.
Bonding jumpers ensure a solid electrical connection between conductive parts.
The National Electrical Code (NEC) defines grounding as "connecting to earth or to some conducting body that serves in place of the earth." Bonding is defined as "the permanent joining of metallic parts to form an electrically conductive path." Although bonding can apply to parts at any voltage, it is assumed that bonded parts are at ground potential.
When there is a neutral, it is always grounded; for example, the center of a 208 or 440-V wye-connected transformer or the center tap of one leg of a 230-V delta transformer. For systems without a neutral, the ground location could be any point. Although not prevalent, delta-connected three-wire systems do exist with one corner grounded.
There are exceptions to grounding for critical systems such as health-care life support, which must remain energized and are designed to tolerate a single fault without shutting down. In these cases, ungrounded power systems are used, but they contain ground fault detection devices that provide a warning when a fault exists.
For this article, a grounded supply system is assumed (Fig. 1).
Fig. 1. Grounding diagram for a typical electrical system shows NEC-defined terminology.
The grounded service conductor, grounded conductor(s), and equipment grounding conductor(s) are all bonded together at the service equipment enclosure; and are also connected to a grounding electrode. This grounding electrode could be an underground water pipe, structural member, or fabricated ground rod; and in many cases consists of more than one of these bonded together. This location is the only point where a system should be connected to the earth.
It is important to understand the distinction between the grounded conductor and the equipment grounding conductor. The grounded conductor , commonly referred to as the neutral, is the normal current-carrying circuit conductor to the grounded side of the supply, and is always white or natural gray color. The equipment grounding conductor is bonded to all exposed metal and only carries current in the event of a fault, except for normal, small leakage currents. It may be bare, but if insulated, it is always green or green with yellow stripes.
Both of these conductors are bonded to the supply system ground at the service entrance, but they perform radically different functions and should never be interchanged. They should also never connect to each other, or to the earth, anywhere except at the service entrance.
The requirements for grounding within a facility are covered by Article 250 of the NEC . A new appendix E was added in the 1999 Code , which contains a cross-reference to corresponding section numbers in the 1996 version. All references here are to the 1999 Code . Section 250-2 (d) describes fault path performance this way: "The fault current path shall be permanent and electrically continuous, shall be capable of safely carrying the maximum fault likely to be imposed on it, and shall have sufficiently low impedance to facilitate the operation of overcurrent devices under fault conditions. The earth shall not be used as the sole equipment grounding conductor or fault current path."
It would be impossible to even begin to cover all of the detailed Code requirements relating to grounding and bonding. Instead, the basic concepts behind them will be examined, since once these are mastered, it becomes much easier to understand and apply the Code requirements.
Fault current from an ungrounded supply conductor flow through the lowest impedance path back to the grounded supply conductor. Since there is ac current, the term impedance rather than resistance is used, which becomes especially important for long lengths of wiring or higher capacity systems, where the inductive effects can be significant. If multiple paths exist, the current splits, with the most current flowing through the lowest impedance.
Fig. 2 .Fig. 2. Fault voltage depends on fault current and ground impedance.
A low impedance ground path is required in order to limit fault voltage to a safe level. The voltage at the location of a ground fault with respect to the earth can be determined from the fault current and the ground circuit impedance (Fig. 2). For example, with a fault current of 100 amp, a 0.1-ohm ground impedance would produce 10 V at the fault location. If the ground impedance increased to 1 ohm, due to a corroded terminal, for example, the voltage at the fault would increase to 100 V, which would be hazardous.
A low-impedance ground path is necessary to allow sufficient fault current to flow and trip the circuit protector. The maximum fault current that flows is determined from the open circuit voltage and the total loop impedance (Fig. 3). The loop impedance is the sum of all impedances in the faulted circuit, including the supply transformer, the ungrounded conductors, and the grounding circuit. For example, a loop impedance of 1 ohm on a 120-V circuit would allow 120 amp to flow, causing the circuit protector to trip very rapidly. If the impedance were to increase to 5 ohms, the current would be reduced to 24 amp, which could flow indefinitely on a 30-amp circuit.
The ground path must be capable of carrying the maximum available fault current. Obviously, if the ground circuit opens up when a fault occurs, then it is not able to perform its intended function.
The earth is a poor conductor. Its impedance varies considerably with soil type, moisture content, and temperature. It cannot reliably provide the necessary low impedance ground path. Therefore, metallic conductors must be used.
All exposed noncurrent-carrying conductors, including raceways, conduit, fit- tings, and enclosures, must be grounded. Equipment grounding conductors can be wires, metallic conduit, electrical tubing, or listed flexible conduit with approved fittings. All wire terminations must be an approved mechanical type or welded. Grounding conductors are sized initially according to NEC Table 250-122, based on the overcurrent protection rating of the circuit. They need to be increased above these sizes, if the circuit conductors are increased from their standard sizes (to reduce voltage drop, for example), by the same area ratio as the circuit conductors.
A grounding conductor should always run together with the other conductors for the circuit that it protects, since the magnetic interaction between the conductors results in lower impedance. While conduit or metallic tubing can be used as the grounding conductor, it produces a higher impedance circuit and is less reliable than wires, due to poorer conductivity and threaded connections. Copper wires should be used for grounding whenever possible.
Bonding jumpers are used to ensure a solid electrical connection between individual conductive parts, and may consist of wire, hardware, conduit, or fittings. All fittings that are relied upon for bonding must be approved for the application by a nationally recognized agency and marked as such, and must be installed properly.
Some specific bonding situations
- Nonmetallic conduit, fittings, or boxes would interrupt a grounding circuit through the conduit. A bonding jumper must be used around them to ensure that the individual pieces are electrically connected.
- Eccentric or concentric reducing knockouts on most existing boxes cannot carry fault current reliably, since the small tabs that connect them to the box will act as fuses and melt open. Bonding jumpers must be used between the conduit and the box. Some boxes are now available with approved reducing knockouts that do not require jumpers, and are marked to identify them as such.
- Paint and other nonconductive coatings prevent contact between an enclosure and fittings. Good contact must be obtained by scraping away the coating or by using approved fittings or bonding jumpers.
- Flexible conduit and fittings that are not approved for grounding purposes require a bonding jumper.
- Bonding requirements within factory supplied equipment are addressed by the manufacturer. Methods may include jumpers (Fig. 4), clean metal-to-metal contact, star washers to pierce nonconductive coatings, and grounding conductors in power cords. It is essential to replace these properly when maintaining or servicing the equipment.
- Whenever there is uncertainty about the existence of a reliable electrical connection, a bonding jumper should be used.
With the proliferation of sensitive electronic and computerized equipment, as well as harmonic producers such as solid state drives, the issue of electrical "noise" has become of much greater importance. Noise consists of higher frequencies, and therefore sees a higher impedance from the ground circuit. Much greater attention to details, such as grounding conductor size and the integrity of all bonding jumpers, is necessary to provide a low-impedance path for noise. Copper wires are mandatory. A perfectly good power ground might be totally ineffective for purposes of eliminating noise.
The NEC is concerned only with the safety aspects of a ground system, and not the effect on operation of computers or other equipment. While the ideal arrangement for a sensitive computer might be an isolated ground wire to a separate earth electrode, this approach clearly violates the Code since the fault path would be through the earth. One possible solution is an isolated ground wire run directly to the service equipment, with no intermediate connections (Fig. 5). While the conduit is isolated from the load equipment to prevent stray currents, it still must be bonded to the ground system. Other more complex arrangements are sometimes required.
It is essential to use the proper test equipment and understand and interpret the readings correctly when analyzing ground systems. For example, one very popular piece of test equipment is a receptacle tester consisting of three neon lights in a plug-in housing. While this device indicates problems such as reversed wiring or an open wire, it is useless for indicating ground circuit integrity. Since a neon lamp requires only a few milliamps to light, a ground impedance of thousands of ohms gives a "normal" indication. The same problem occurs if a high-impedance voltmeter is used between a line and ground. It reads almost full line voltage, even with extremely high ground impedance.
The typical impedance of a ground circuit should be 0.25 ohm or less, with higher capacity circuits being much lower. A standard multimeter cannot be used to measure this, since it does not give accurate resistance readings below 1 ohm, and is severely affected by the resistance of the test leads. Instead, a low-resistance ohmmeter should be used, which can measure less than 1 milliohm (1/1000 of an ohm), and can accurately indicate the resistance of wires, connections, and other components in the ground path. While it technically measures the dc resistance and not the ac impedance (which is always equal or higher), it still gives a very useful indication of the ground system condition.
Ground impedance testers are available that measure the actual ac impedance of the ground wiring. An ac loop impedance tester, when connected to an energized system, indicates not only the ac impedance of the total loop, but also the magnitude of fault current that will occur at the point of measurement.
To measure the effective resistance of grounding electrodes, a ground resistance tester designed for this purpose must be used. These typically use three or four probes buried in the earth to measure the current and potential difference, and are greatly influenced by the location of the probes relative to the electrode being tested. The manufacturers of these instruments provide information and training on proper application techniques.
For cord-connected equipment, the same low-resistance meter can be used to measure the resistance between the power cord grounding conductor and the equipment housing, which should typically be 0.1 ohm or less. Ideally this measurement should be made with a test current of 25 amp in accordance with IEC procedures. This amount not only ensures that the connection exists, but also that it can reliably carry fault current. Testers are available that perform this 25-amp test with short pulses of current, preventing any inadvertent damage to the equipment under test.
Proper grounding and bonding result in a system that is safe and reliable. Most situations can be addressed by applying the basic principles discussed above. The results are well worth the effort, and could save lives.
-- Edited by Joseph L. Foszcz, Senior Editor, 630-320-7135, firstname.lastname@example.org
Why most ac power systems are grounded at their source
- System voltages are stabilized.
- The potential difference across insulation materials is limited.
- Voltages that occur during a ground fault are minimized.
- A path is provided for fault currents to allow circuit protective devices to operate.
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