Electrical Safety: Grounding and Bonding of Electrical Systems and Equipment

Introduction

Electrical grounding and bonding seems to be one of the most misunderstood subjects and requirements of the National Electrical Code (NEC). The Occupational Safety and Health Administration (OSHA) documented more than 20,000 violations of grounding in the time period of one single year. Most of these violations came from loose, damaged, or missing external grounds on electrical equipment and structures. Just imagine the increased numbers of violations if OSHA inspectors opened electrical equipment to check equipment grounding and bonding connections or if they used test equipment to verify proper grounding! The real problem is that most electrical equipment will operate without a ground connection. For example, an electric drill will not work if the hot or neutral wire is open; however, if the ground wire is open, the drill will run properly. Even though the drill works properly without a ground connection, it is not safe to use.

This section addresses several issues dealing with general grounding and bonding, system grounding, and equipment grounding. Proper grounding is an issue that must be seriously considered for all electrical installations and equipment. Strict adherence to federal and local regulatory standards, such as OSHA 29 CFR 1910.304(g) and NEC article 250, as well as the numerous consensus standards and reference books, reduces the risk of electrical shock from contact with inadvertently energized equipment, enclosures, and structures.

The information in this section is not intended to be a substitute for the NEC or OSHA requirements. Nor is it intended to replace or be a substitute for any other standard quoted herein. For proper grounding and bonding of electrical systems and equipment, always com ply with the requirements contained in the current editions of standards and regulations.

Electric Shock Hazard

Energized conductors and circuit components, installed within or on electrical equipment, are insulated from the equipment's metal enclosure to protect personnel who operate or otherwise make contact with the equipment from being exposed to dangerous voltages.

When aging or malfunction causes the insulation to break down, the energized conductors within the equipment can make direct contact with the metal enclosure, thereby energizing it. Anyone making contact with an energized equipment enclosure could be injured or killed.

Equipment grounding and bonding are ways of mitigating this shock hazard. An equipment grounding conductor forms a very low-impedance path back to the overcurrent protective device to cause it to trip in the event of a ground fault. Bonding jumpers are installed, where required, to join together all non-current-carrying metal parts, enclosures, and components of electrical equipment to prevent a difference in potential between two or more different surfaces.

Simply stated, proper bonding and grounding of electrical equipment will substantially reduce the risk of electrical shock by effectively eliminating a difference in potential.

General requirements for Grounding and Bonding

Grounding of Electrical Systems

Grounded electrical systems are required to be connected to earth in such a way as to limit any voltages imposed by lightning, line surges, or unintentional contact with higher voltage conductors. Electrical systems are also grounded to stabilize the voltage to earth during normal operation. If, for example, the neutral of a 120/240 v, wye-connected secondary of a transformer were not grounded, instead of being 120 v to ground, the voltage could reach several hundred volts to ground. A wye-connected electrical system becomes very unstable if it is not properly grounded.

The following requirements are taken from OSHA 29 CFR 1910.304(g) and can also be found in the National Electrical Code (NEC) Section 250.20.

(g) Grounding. Paragraphs (g)(1) through (g)(9) of this section contain grounding requirements for systems, circuits, and equipment.

(1) Systems to be grounded. Systems that supply premises wiring shall be grounded as follows:

(i) All 3-wire DC systems shall have their neutral conductor grounded;

(ii) Two-wire DC systems operating at over 50 volts through 300 volts between conductors shall be grounded unless:

(A) They supply only industrial equipment in limited areas and are equipped with a ground detector; (B) They are rectifier-derived from an AC system complying with paragraphs (g)(1)(iii), (g)(1)(iv), and (g)(1)(v) of this section; or (C) They are fire-alarm circuits having a maximum current of 0.030 amperes;

(iii) AC circuits of less than 50 volts shall be grounded if they are installed as overhead conductors outside of buildings or if they are supplied by transformers and the trans former primary supply system is ungrounded or exceeds 150 volts to ground;

(iv) AC systems of 50 volts to 1000 volts shall be grounded under any of the following conditions, unless exempted by paragraph (g)(1)(v) of this section:

(A) If the system can be so grounded that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts; (see FIG. 1) (B) If the system is nominally rated three-phase, four-wire wye connected in which the neutral is used as a circuit conductor; (see FIG. 2)

FIG. 1 voltage-to-ground less than 150 volts. (Courtesy AVO Training Institute, Inc.)

FIG. 2 480/277 volt systems. (AVO Training Institute, Inc.)

FIG. 3 Four-wire delta system with neutral. (AVO Training Institute, Inc.)

(C) If the system is nominally rated three-phase, four-wire delta connected in which the midpoint of one phase is used as a circuit conductor; (see FIG. 3); or (D) If a service conductor is uninsulated;

(v) AC systems of 50 volts to 1000 volts are not required to be grounded under any of the following conditions:

(A) If the system is used exclusively to supply industrial electric furnaces for melting, refining, tempering, and the like;

(B) If the system is separately derived and is used exclusively for rectifiers supplying only adjustable speed industrial drives;

(C) If the system is separately derived and is supplied by a transformer that has a primary voltage rating less than 1000 volts, provided all of the following conditions are met:

(1) The system is used exclusively for control circuits;

(2) The conditions of maintenance and supervision assure that only qualified persons will service the installation;

(3) Continuity of control power is required; and (4) Ground detectors are installed on the control system;

(D) If the system is an isolated power system that supplies circuits in health care facilities; or

(E) If the system is a high-impedance grounded neutral system in which a grounding impedance, usually a resistor, limits the ground-fault current to a low value for 3-phase ac systems of 480 volts to 1000 volts provided all of the following conditions are met:

(1) The conditions of maintenance and supervision ensure that only qualified persons will service the installation;

(2) Continuity of power is required;

(3) Ground detectors are installed on the system; and (4) line-to-neutral loads are not served.

(2) Conductor to be grounded. The conductor to be grounded for ac premises wiring systems required to be grounded by paragraph (g)(1) of this section shall be as follows:

(i) One conductor of a single-phase, two-wire system shall be grounded;

(ii) The neutral conductor of a single-phase, three-wire system shall be grounded;

(iii) The common conductor of a multiphase system having one wire common to all phases shall be grounded;

(iv) One phase conductor of a multiphase system where one phase is grounded shall be grounded; and (v) The neutral conductor of a multiphase system in which one phase is used as a neutral conductor shall be grounded.

(3) Portable and vehicle-mounted generators. (i) The frame of a portable generator need not be grounded and may serve as the grounding electrode for a system supplied by the generator under the following conditions:

(A) The generator supplies only equipment mounted on the generator or cord- and plug connected equipment through receptacles mounted on the generator, or both; and (B) The noncurrent-carrying metal parts of equipment and the equipment grounding conductor terminals of the receptacles are bonded to the generator frame.

(ii) The frame of a vehicle need not be grounded and may serve as the grounding electrode for a system supplied by a generator located on the vehicle under the following conditions:

(A) The frame of the generator is bonded to the vehicle frame;

(B) The generator supplies only equipment located on the vehicle and cord- and plug connected equipment through receptacles mounted on the vehicle;

(C) The noncurrent-carrying metal parts of equipment and the equipment grounding conductor terminals of the receptacles are bonded to the generator frame; and (D) The system complies with all other provisions of paragraph (g) of this section.

(iii) A system conductor that is required to be grounded by the provisions of paragraph (g)(2) of this section shall be bonded to the generator frame where the generator is a component of a separately derived system.

(4) Grounding connections. (i) For a grounded system, a grounding electrode conductor shall be used to connect both the equipment grounding conductor and the grounded circuit conductor to the grounding electrode. Both the equipment grounding conductor and the grounding electrode conductor shall be connected to the grounded circuit conductor on the supply side of the service disconnecting means or on the supply side of the system disconnecting means or overcurrent devices if the system is separately derived.

(ii) For an ungrounded service supplied system, the equipment grounding conductor shall be connected to the grounding electrode conductor at the service equipment. For an ungrounded separately derived system, the equipment grounding conductor shall be connected to the grounding electrode conductor at, or ahead of, the system disconnecting means or overcurrent devices.

(iii) On extensions of existing branch circuits that do not have an equipment grounding conductor, grounding-type receptacles may be grounded to a grounded cold water pipe near the equipment if the extension was installed before August 13, 2007. When any element of this branch circuit is replaced, the entire branch circuit shall use an equipment grounding conductor that complies with all other provisions of paragraph (g) of this section.

5.6 (5) Grounding path. The path to ground from circuits, equipment, and enclosures shall be permanent, continuous, and effective.

(6) Supports, enclosures, and equipment to be grounded-(i) Metal cable trays, metal raceways, and metal enclosures for conductors shall be grounded, except that:

(A) Metal enclosures such as sleeves that are used to protect cable assemblies from physical damage need not be grounded; and (B) Metal enclosures for conductors added to existing installations of open wire, knob and-tube wiring, and nonmetallic-sheathed cable need not be grounded if all of the following conditions are met:

(1) Runs are less than 7.62 m (25.0 ft);

(2) Enclosures are free from probable contact with ground, grounded metal, metal lathes, or other conductive materials; and (3) Enclosures are guarded against employee contact.

(ii) Metal enclosures for service equipment shall be grounded.

(iii) Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and metal outlet or junction boxes that are part of the circuit for these appliances shall be grounded.

(iv) Exposed non-current-carrying metal parts of fixed equipment that may become energized shall be grounded under any of the following conditions:

(A) If within 2.44 m (8 ft) vertically or 1.52 m (5 ft) horizontally of ground or grounded metal objects and subject to employee contact;

(B) If located in a wet or damp location and not isolated;

(C) If in electrical contact with metal;

(D) If in a hazardous (classified) location;

(E) If supplied by a metal-clad, metal-sheathed, or grounded metal raceway wiring method; or (F) If equipment operates with any terminal at over 150 volts to ground.

(v) Notwithstanding the provisions of paragraph (g)(6)(iv) of this section, exposed non current-carrying metal parts of the following types of fixed equipment need not be grounded:

(A) Enclosures for switches or circuit breakers used for other than service equipment and accessible to qualified persons only;

(B) Electrically heated appliances that are permanently and effectively insulated from ground;

(C) Distribution apparatus, such as transformer and capacitor cases, mounted on wooden poles, at a height exceeding 2.44 m (8.0 ft) above ground or grade level; and (D) listed equipment protected by a system of double insulation, or its equivalent, and distinctively marked as such.

(vi) Exposed non-current-carrying metal parts of cord- and plug-connected equipment that may become energized shall be grounded under any of the following conditions:

(A) If in hazardous (classified) locations (see § 1910.307);

(B) If operated at over 150 volts to ground, except for guarded motors and metal frames of electrically heated appliances if the appliance frames are permanently and effectively insulated from ground;

(C) If the equipment is of the following types:

(1) Refrigerators, freezers, and air conditioners;

(2) Clothes-washing, clothes-drying, and dishwashing machines, sump pumps, and electric aquarium equipment;

(3) Hand-held motor-operated tools, stationary and fixed motor-operated tools, and light industrial motor-operated tools;

(4) Motor-operated appliances of the following types: hedge clippers, lawn mowers, snow blowers, and wet scrubbers;

(5) Cord- and plug-connected appliances used in damp or wet locations, or by employees standing on the ground or on metal floors or working inside of metal tanks or boilers;

(6) Portable and mobile X-ray and associated equipment;

(7) Tools likely to be used in wet and conductive locations; and (8) Portable hand lamps.

(vii) Notwithstanding the provisions of paragraph (g)(6)(vi) of this section, the following equipment need not be grounded:

(A) Tools likely to be used in wet and conductive locations if supplied through an isolating transformer with an ungrounded secondary of not over 50 volts; and (B) listed or labeled portable tools and appliances if protected by an approved system of double insulation, or its equivalent, and distinctively marked.

(7) Non electrical equipment. The metal parts of the following nonelectrical equipment shall be grounded: frames and tracks of electrically operated cranes and hoists; frames of non-electrically driven elevator cars to which electric conductors are attached; hand operated metal shifting ropes or cables of electric elevators; and metal partitions, grill work, and similar metal enclosures around equipment of over 750 volts between conductors.

(8) Methods of grounding fixed equipment. (i) Non-current-carrying metal parts of fixed equipment, if required to be grounded by this subpart, shall be grounded by an equipment grounding conductor that is contained within the same raceway, cable, or cord, or runs with or encloses the circuit conductors. For dc circuits only, the equipment grounding conductor may be run separately from the circuit conductors.

(ii) Electric equipment is considered to be effectively grounded if it is secured to, and in electrical contact with, a metal rack or structure that is provided for its support and the metal rack or structure is grounded by the method specified for the non-current-carrying metal parts of fixed equipment in paragraph (g)(8)(i) of this section. Metal car frames supported by metal hoisting cables attached to or running over metal sheaves or drums of grounded elevator machines are also considered to be effectively grounded.

(iii) For installations made before April 16, 1981, electric equipment is also considered to be effectively grounded if it is secured to, and in metallic contact with, the grounded structural metal frame of a building. When any element of this branch circuit is replaced, the entire branch circuit shall use an equipment grounding conductor that complies with all other provisions of paragraph (g) of this section.

(9) Grounding of systems and circuits of 1000 volts and over (high voltage). If high-voltage systems are grounded, they shall comply with all applicable provisions of paragraphs (g)(1) through (g)(8) of this section as supplemented and modified by the following requirements:

(i) Systems supplying portable or mobile high-voltage equipment, other than substations installed on a temporary basis, shall comply with the following:

(A) The system shall have its neutral grounded through an impedance. If a delta-connected high-voltage system is used to supply the equipment, a system neutral shall be derived.

(B) Exposed non-current-carrying metal parts of portable and mobile equipment shall be connected by an equipment grounding conductor to the point at which the system neutral impedance is grounded.

(C) Ground-fault detection and relaying shall be provided to automatically de-energize any high-voltage system component that has developed a ground fault. The continuity of the equipment grounding conductor shall be continuously monitored so as to de energize automatically the high-voltage feeder to the portable equipment upon loss of continuity of the equipment grounding conductor.

(D) The grounding electrode to which the portable or mobile equipment system neutral impedance is connected shall be isolated from and separated in the ground by at least 6.1 m (20.0 ft) from any other system or equipment grounding electrode, and there shall be no direct connection between the grounding electrodes, such as buried pipe, fence, and so forth.

Grounding of Electrical Equipment

OSHA 1910.304(g)(9)(ii) All non-current-carrying metal parts of portable equipment and fixed equipment including their associated fences, housings, enclosures, and supporting structures shall be grounded. However, equipment that is guarded by location and isolated from ground need not be grounded. Additionally, pole-mounted distribution apparatus at a height exceeding 8 ft above ground or grade level need not be grounded.

In 29 CFR 1910.303, "General," OSHA states under "(b) Examination, installation, and use of equipment (1) Examination" that "Electrical equipment shall be free from recognized hazards that are likely to cause death or serious physical harm to employees." This section continues in paragraph (viii) with "other factors which contribute to the practical safeguarding of persons using or likely to come in contact with the equipment." One of these "other factors" is proper grounding and bonding of the electrical equipment. If the non-current-carrying metal parts of electric equipment are not properly grounded and bonded, and these parts become energized, then any person "using or likely to come in contact with the equipment" is at risk of an electrical shock that may or may not be fatal.

This is a risk that must not be taken. Proper grounding and bonding can effectively eliminate this shock hazard by providing a permanent and continuous low-impedance path for ground-fault current to follow in order to clear the circuit protective device(s).

Bonding of Electrically conductive materials and other Equipment

Bonding is the permanent joining of metallic parts of materials and equipment. When different metal parts are not bonded together, a difference in potential could exist between the metal parts. This creates an electrically hazardous condition between the parts. Anyone simultaneously coming into contact with the metal parts would be subject to electrical shock, burns, or electrocution. When all conductive materials and parts of equipment are permanently bonded together, there is only one piece of metal and no potential difference exists between the parts. The metal parts must also be grounded to earth in order to be at earth potential. This minimizes the risk of touch-potential and step-potential hazards when working on or around metal enclosures that could become energized.

Other metallic equipment that is in contact with or adjacent to the electrical equipment should also be grounded and bonded to prevent a difference in potential in the event that a ground fault occurs in the electrical equipment. This could include other piping as well as ducts in ventilation systems as shown in FIG. 4.

As discussed in the previous section, all non-current-carrying components of electrical equipment must be grounded and bonded. Equipment grounding conductors are installed and connected to the required terminal in the equipment to provide the low-impedance path for fault current to clear the circuit. All other metallic components of the equipment must be bonded to the grounded portion of the equipment to prevent a difference in potential between the components. An example of this would be service enclosures. FIG. 5 illustrates bonding of service enclosures.

FIG. 4 Bonding of other piping and duct systems. (Courtesy AVO Training Institute, Inc.)

FIG. 5 Bonding service enclosures. (Courtesy AVO Training Institute, Inc.) Performance of fault path.

OSHA makes a very simple statement on the performance of the fault path as found in 1910.304(g)(4), which states: "Grounding path--The path to ground from circuits, equipment, and enclosures shall be permanent and continuous." NEC Section 250.4(A)(5) also requires the fault current path to be able to safely carry the maximum ground-fault current, provide a low-impedance path in order to clear the overcurrent devices, and perform in a way that the earth is not used as the sole equipment grounding conductor for fault current.

The fault current path must meet these criteria in order for the overcurrent protection to clear the circuit in the event a ground fault occurs. A low-impedance conductor path must be used because the earth provides an extremely high-impedance path and would not allow a sufficient amount of fault current to flow to clear the overcurrent device. Another factor is that a low-impedance conductor path for fault current to flow through minimizes the possibility of step- and touch-potential hazards by limiting the voltage to ground.

Arrangement to prevent objectionable current

Grounding of electrical equipment and systems must be accomplished in a manner that would prevent objectionable current. The main point here is to install an effective grounding system without creating an objectionable current situation. Circulating current is one form of objectionable current that can occur when multiple grounds are utilized.

Alterations to Stop objectionable current

Objectionable current flow can occur when using multiple grounds in electrical systems and equipment. If this occurs, there are several different options to prevent or at least minimize this current flow:

1. One or more of the grounds may be discontinued. Never discontinue all of the grounds.

2. The grounding connection may be changed to another location.

3. The conductive path for the grounding connections may be interrupted.

4. The authority having jurisdiction may grant other remedial action.

Temporary currents not classified as objectionable current

One thing to keep in mind when dealing with this subject is that there are times when cur rent will flow in the grounding system or through the non-current-carrying parts of electrical equipment. Ground faults in equipment do occur and that is when the grounding system performs its function. These currents are not classified as objectionable current for the purpose of this discussion.

Connection of Grounding and Bonding Equipment

Conductors for grounding and bonding of electrical equipment must be connected using an approved method such as exothermic welding, listed irreversible compression-type connections, listed pressure connections, listed clamps, and other approved means. Solder must not be used as the sole connection. Solder is too soft and has a very low melting point and, therefore, becomes a fuse in the grounding connection. Also, never use sheet metal screws to make connections between the grounding conductor and the enclosure. The use of sheet metal screws would provide limited metal-to-metal contact between the screw and the enclosure and, therefore, may not provide the required low-impedance connection.

Protection of Ground Clamps and Fittings

All grounding connections must be protected from physical damage either by location or by means of an enclosure made of wood, metal, or equivalent. Damaged grounding conductor connections can result in loss of continuity in the ground path, which will create a potential shock hazard.

Clean Surfaces

If the grounding connection point is contaminated with paint or other such coatings, good continuity may not be accomplished. All surfaces must be cleaned as needed to remove any such coatings or other contaminants that could interfere with the continuity of the grounding connection. As was stated earlier, the grounding system must create a sufficiently low impedance path for circuit protective devices to clear the circuit in the event of a ground fault.

cont. to part 2 >>