The American Society for Testing and Materials (ASTM) Committee F-18, "Electrical Protective Equipment for Workers," has developed and published a consensus standard for protective grounds, ASTM Designation: F-855. In accordance with Office of Management and Budget (OMB) Circular No. A-119. October 27, 1982, this voluntary consensus standard may be used by all Federal agencies for procurement purposes. Therefore, grounding cables, clamps, and ferrules purchased should meet all the requirements of ASTM Designation: F-855. Aluminum cables shall not be used for personal grounds.

The grounding of high-voltage lines and equipment will provide workers with additional protection from electric shock if grounds are sized, selected, and installed properly. For further information on the construction of personnel protective grounds, see ASTM F855-90.

7.5.3.1 AVAILABILITY

Grounding cables shall be available for use when work is being done on deenergized lines or equipment.

7.5.3.2 APPROVED CAPACITY

Grounding cables shall accommodate the maximum fault current to which the cable or equipment might be subjected.

7.5.3.3 GROUNDING CABLES AND HARDWARE

Personal protective grounding cables consist of appropriate lengths of suitable copper grounding cable, with electrically and mechanically compatible ferrules and clamps at each end. In addition, appropriate hotsticks are required for installing and removing the conductor-end clamps to the conductors. Hotsticks are required for attaching ground-end clamps if the grounded system and the worker are at different potentials. Cluster bars provide a low-resistance means of connecting the ground-end clamps. Each of these components will be discussed in the following subsections.

7.5.3.4 GROUNDING CABLES

Most of the grounding cables in use today (and available for purchase) are actually manufactured for another purpose-principally as welding cable. These extra-flexible copper cables with jackets are manufactured according to appropriate ASTM standards for both cables and jackets, and can be expected to perform satisfactorily as grounding cables.

7.5.3.4.1 STRANDING

There are several classes of flexible cable with various stranding in the sizes normally used for grounding cables.

7.5.3.4.2 JACKETS

Welding cables are nominally insulated at 600 volts. When used as grounding cable, the insulation or jacket serves primarily for mechanical protection of the conductor. The flexible elastomer or thermoplastic jackets are manufactured, applied, and tested according to ASTM standards. Black, red, and yellow jackets are usually neoprene rubber compounds, while clear jackets are ultravioletinhibited polyvinyl chloride (PVC). All jackets should have the American Wire Gage (AWG) size stamped or printed repeatedly along the length of the cable. The clear jacket allows easy visual inspection of the conductor for strand breakage, but becomes stiff and hard to handle at low temperatures. The clear jacket will split or shatter at very low temperatures.

7.5.3.4.3 FERRULES

Ferrules should be threaded-stud copper base compression type. Ferrules should have the filler compound vent hole at the bottom of the cable so that employees can visually check that the cable is fully inserted into the ferrule. Compound should be used with crimped ferrules. The ferrules should be crimped with the ferrule manufacturer's recommended die. The press shall have enough pressure to completely close the die. The area covering the inserted cable jacket should not be compressed. Heat shrink or springs should be installed over a portion of the ferrule to minimize strand breakage caused by bending. In all cases, the manufacturer's recommendations should be followed.

7.5.3.4.4 HANDLING OF GROUNDING CABLE

Personal protective grounds are usually handled and lifted by the cable. However, continuous flexing eventually breaks the conductor strands beneath the jacket. Therefore, employees should minimize the use of sharp bends in the cable.

7.5.3.4.5 SIZE OF GROUNDING CABLE

The size of the grounding cable must be selected to handle the maximum calculated fault current of the power system or specific portion thereof. The minimum size that shall be used for grounding cables is #2 (AWG) flexible copper. In larger substations, the maximum available fault current may require larger cables. If larger cables are not available, parallel cables (with the appropriate derating factor) may be used.

Most manufacturers and suppliers of grounding cables publish tables to assist the user in selecting the proper cable size for a given fault current. These tables show the maximum fault current capability for several sizes of copper grounding cables.

7.5.3.4.6 GROUNDING CABLE LENGTH

Excessive cable lengths should be avoided. Therefore, slack in the installed cables should be minimal to reduce possible injury to workers. Resistance in the cable increases with cable length, and excessive length could exceed the tolerable voltage drop across the body. Longer than necessary cables also tend to twist or coil, which reduces the effectiveness of the cable.

7.5.3.4.7 GROUNDING CLAMPS

Grounding clamps are normally made of copper or aluminum alloys; sized to meet or exceed the current-carrying capacity of the cable; and designed to provide a strong mechanical connection to the conductor, metal structure, or ground wire/rod.

7.5.3.4.7.1 CLAMP TYPES

Clamps are furnished in, but not limited to, three types according to their function and methods of installation:

1. Type I clamps, for installation on deenergized conductors, are equipped with eyes for installation with removable hotsticks.

2. Type III clamps, for installation on permanently grounded conductor or metal structures, have T-handles, eyes, and/or square-or hexagon-head screws.

3. Other types of special clamps are designed for specific applications, such as cluster grounds, underground equipment grounding, and so on.

7.5.3.4.7.2 CLAMP JAWS

Bus clamps should be furnished with smooth jaws for installation on copper, aluminum, or silverplated bus work without marring the surface. Conductor or metal structure clamps should be furnished with serrations or cross-hatching designed to abrade or bite through corrosion products on surfaces of the conductor or the metal structure being clamped. Several styles of conductor and ground-end clamps have jaws that can be replaced when the serrations have worn. Self-cleaning jaws are recommended for conductor-end clamps used on aluminum or aluminum conductor steel reinforced (ACSR) conductors. Several styles of ground-end clamps are designed with a cup-point set screw which should be tightened with a wrench (after the serrated jaws have been securely tightened) to break through paint, rust, galvanized coating, or corrosion on the surface that is to be clamped.

A typical grounding cable for transmission line work used by line crews consists of a 2/0 (AWG) copper cable with an insulating jacket, terminated with an all-angle, self-cleaning aluminum conductor clamp at one end, and a flat-faced clamp with a set screw at the other end for connecting to a tower leg or ground wire/rod.

7.5.3.4.8 GROUNDING CLUSTER BARS

When climbing wood-pole structures, workers may use a grounding cluster bar to connect the phase cables to the pole ground wire, if the ground wire has sufficient capacity to carry the fault current. Cluster bars must have an attached bonding lead. If there is no pole ground wire, the cluster bar for each pole is connected to a common driven or screw-in ground rod with a grounding cable (or cables). In substation grounding, a copper bar is sometimes used to connect the three-phase cables and a fourth cable to a riser from the station ground mat. When installing personal grounds on wood structures from a bucket, the ground cables may be connected between the overhead ground wire (OGW), and the phases without the use of cluster bars provided that an electrical bond of sufficient current carrying capacity exists between the OGW and the structure ground.

7.5.3.4.9 TEMPORARY GROUND RODS

Some typical examples of temporary ground rods used for grounding ungrounded structures or mobile equipment, or during conductor splicing operations, are either:

1. A minimum 5/8-inch diameter bronze, copper, or copper-weld rod at least 6 feet long, driven to a depth of at least 5 feet; or

2. A 6-foot, screw-type ground rod, consisting of a minimum 5/8-inch diameter copper-weld shaft with a bronze auger bit and bronze T-handle, screwed to a depth of at least 5 feet (preferred). The T-handle must be tightly connected to the rod.

If a temporary rod cannot be driven or screwed to a depth of 5 feet, additional rod(s) should be driven or screwed so that a total of at least 5 feet of rod is buried. These rods shall be bonded together with grounding cables prior to installing phase grounds. The rods should be placed 6 to 8 feet apart; however, the 10-foot clearance from the rods should be maintained. OGWs may be used at any time to bond the conductors provided that these wires are electrically bonded to the structure ground, either permanently or by personal grounds.

Groundsmen should stay clear (at least 10 feet where feasible) of items such as down guys, ground rods, maintenance vehicles, and structure legs or ground wires while they are bonded to protective grounds which are in place. When it is absolutely necessary to work on or near these features, employees should use bonded conductive or insulated platforms, or approved insulated shoes to minimize the hazard from step and touch potentials.

7.5.4 TESTING BEFORE INSTALLING GROUNDS

Before grounds are installed, the deenergized line or equipment shall be tested for voltage. Appropriate testers for the nominal voltage involved (audio or visual) should be used. They shall be tested immediately before and after use to verify that they are in good working condition. [See 29 CFR 1910.269(n)(5)].