MIL-STD-188-124B
Summary of Sections

Grounding, Bonding and Shielding for Long Haul/Tactical Communication Systems Including Ground Base Communications - Electronics Facilities and Equipment

DOD 1. This Military Standard is approved and mandatory for use by all Departments and Agencies of the Department of Defense in accordance with OASD Memo.

Forward 3. This standard is also recommended for applicable use on any ground facility or equipment where grounding, bonding, shield, personnel safety, lightning protection and EMC are required.

Page 1 Scope 1. 3 .....this includes ATC and Navigational Aid Facilities, Radio Sites, Satellite Ground Terminals, Telephone Central Offices, Microwave and Data Communication Systems, as well as C-E Transportables, Aircraft Simulators, Computer Centers, and Weapons Assembly Facilities. Use of this standard for other ground C-E facilities or equipment is also encouraged.

Page 4 Section 2.4 Order of Precedence: In the event of a conflict between the text of this standard and the references cited herin, the text of this standard shall take precedence.

Section 5.1 Grounding: The facility ground system consists of four electrically connected subsystems. These are A. Earth electrode subsystem B. Fault protection subsystem C. Lightning protection subsystem D. Signal reference subsystem

Section 5.1.1.1.3 Minimum Configuration. The basic earth electrode subsystem configuration shall consist of driven ground rods uniformly spaced around the facility and placed 2 feet to 6 feet outside the drip line of structures. The rods shall be interconnected with a 1/0 AWG bare copper cable buried at least 18 inches below grade level. Large size cables as well as greater burial depths shall be specified where earth and atmosphere considerations so dictate. The interconnecting cable shall be brazed or welded to each ground rod and shall close on itself to form a complete loop with the ends brazed or welded together. Where ground wells are employed, acceptable compression type connectors may be utilized to bond the cable to the ground rod. Coverage of the earth electrode subsystem by asphalt, concrete, etc. Shall be discouraged and kept to a minimum in an effort to maintain the effectiveness of the subsystem.

Section 5.1.1.1.3.1 Resistance to Earth. (DO) The resistance to earth of the earth electrode subsystem should not exceed 10 ohms at fixed permanent facilities. Resistance to earth for tactical and transportable systems should not exceed the TO established for the particular system.

Section 5.1.1.1.3.1.2 Additional Considerations: Where 10 ohms are not obtained at fixed permanent facilities or the required resistance established for tactical or transportable systems due to high soil resistivity, rock formations, or other terrain features, alternate methods for reducing the resistance to earth shall be considered. See MIL-HDBK-419

Footnote: MIL-HDBK-419 Section 2.2.2.2: Department of Defense Communications Electronics Requirements - The NEC 25 ohm resistance is not acceptable for C-E facilities when consideration is given to the large investments in personnel and equipment. A compromise of cost versus protection against lightning, power faults, or EMP has led to establishment of a design goal of 10 ohms for the earth electrode subsystem (EES) in Mil-STD-188-124b. The EES designed in MIL-STD-188-124b specifies a ring ground around the periphery of the facility to be protected. With proper design and installation of the EES, the design goal of 10 ohms should be attained at reasonable cost. At locations where 10 ohms has not been attained due to high soil resistivity, rock formations, or other terrain features, alternate methods listed in Paragraph 2.9 shall be considered for reducing the resistance to earth.

Footnote: MIL-HDBK-419 Section 2.2.3 ….. Experience has shown that a grounding resistance of 10 ohms gives a fairly reliable lightning protection to buildings, transformers, transmission lines, towers and other exposed structures. The lower the resistance, the greater the protection, therefore, attempts should be made to reduce the resistance to the lowest practical value.

Footnote: MIL-HDBK-419 Section 2.6.2 and 2.6.2.1 ….Most of the resistance of a single electrode (ground rod) is obtained within a reasonable distance from the electrode. For a vertical rod, better than 90 percent is realized within two rod lengths. If two or more electrodes are closely spaced, however, the total effective resistance of neither is realized. This interaction prevents the resistance of N electrodes connected in parallel from being 1/N times the resistance of one of the electrodes. For this reason, the crowding of multiple vertical rods is not as beneficial in terms of dollar cost per ohm as is achievable with fewer rods properly spaced. If the electrodes in a multiple electrode installation are separated by adequate distances, the interactive influence is minimized. The separation between driven vertical ground rods in a group of rods should not be less than the length or greater than twice the length of an individual rod. Consider two rods driven into the earth with their tops flush with the surface. The two rods are electrically parallel, but the presence of one rod affects the resistance of the other.

Section 5.1.1.1.4 Ground Rods shall be copper-clad steel, a minimum of 10 feet in length, spaced apart not more that twice the rod length, and shall not be less than ¾" in diameter. The thickness of the copper jacket shall not be less than .012 inch.

Section 5.1.1.1.7 Resistance Checks. The resistance measurements of the earth electrode subsystem to earth shall be accomplished every 21 months after the initial 12 month period by the facilities engineering activity.

Section 5.1.1.2.1 General. ....... A ground bus shall be provided in all switch boards and panel boards and a separate connecting (green) 1. Wire shall be carried within the same raceway or cable tray with the AC power conductors. 1. Green Wire: green, green with yellow stripes or bare wire with green tape.

Section 5.1.1.2.5.1 ... The size of the ground conductor shall be specified in Table 1-20 or Mil-HDBK-419 or Table 250-95 of the NEC. In each facility, all power distribution neutrals shall be isolated from the C-E equipment case and the structure elements so that no AC return current flows through the equipment and fault protection subsystem or the signal reference network. Conduit shall not be used in lieu of separate grounding, green, wire.

Section 5.1.1.2.5.2 .... Standby Generators: Motor and generator frames and housings shall be grounded in accordance with Article 250 of the NEC.

Section 5.1.1.2.5.4 ...... Frames of Generators: The frames of motors, generators and other types of electrical rotating machinery shall be grounded to the fault protection subsystem, according to Article 430 of the NEC.

Section 5.1.1.2.5.6 ...... Metallic battery racks should be grounded to the facility ground system at the nearest point.

Section 5.1.1.3.1 …General: Lightning protection shall be provided as required for buildings and structures in accordance with the NFPA No. 780

Section 5.1.1.3.2 …Building and structures: This protection shall be extended to all electrical, electronic, or other elements which are a part of, or are in support of all C-E facilities. Such elements shall include, but shall not be limited to, substations, power poles, towers, antennas, masts, etc.

Section 5.1.1.3.3 .... Down Conductors - 8" radius and bends NOT GREATER THAN 90 DEGREES. Where practicable, a separation of at least 6 feet shall be maintained between open conductors of power and communications systems and lightning down conductors. Down conductors shall be continuous and shall be bonded in accordance with 5.1.1.1.5 and 5.2.3 to an earth electrode subsystem or to a ground rod bonded to this subsystem installed as near as practicable and within 6 feet from the structure.

Section 5.1.1.3.4 .... Bonding - Lightning protection systems, - UL approved high compression clamping devices or welding

Section 5.1.1.3.8.1 .... Towers within 20 feet of the structure - one earth electrode system is provided and bonded by two 1/0 AWG bare copper cables with independent routes.

Section 5.1.1.3.8.4 …Down Conductors shall be bonded to the tower legs at the base. Conductors connecting to the earth electrode subsystem shall be protected against mechanical damage. Connecting cables passing through foundations shall be installed in plastic or non-metallic conduit.

Section 5.1.1.3.8.5 .... Waveguide grounding: All waveguides shall be grounded at three points: near the antenna, at the vertical/horizontal transition near the base of the tower and at the waveguide entry panel/plate. The entry panel shall be connected by the most direct route to the earth electrode subsystem using a 1/0 stranded copper cable.

Section 5.1.1.3.8.6 … Coaxial Cable Grounding: Coaxial cables shall be bonded to the building or facility entrance plate and in turn to the earth electrode subsystem.

Section 5.1.1.3.12 .... Lightning Arrestors: Shall be provided and installed at the point of entrance into the facility in accordance with NEC Article 280.

Section 5.1.1.3.13 .... Security/Perimeter Fences shall be grounded according to MIL-HNBK-419.

Section 5.1.1.4.2 .... Signal Reference Subsystem - High frequency network require equipotential ground plane which is installed under the equipment at new construction and above the equipment for retrofit.

Appendix B 1.0.C. Footnote 2 - All military communication stations are treated as high frequency facilities.

Section 5.1.2.1 ... Higher Frequency Network: To minimize the voltage differential between points in the higher frequency signal reference network, the dc resistance between any two points within a chassis or equipment cabinet serving as a reference for higher frequency signals shall be less than 1 milliohm (0.001ohm). The grounding conductor shall not penetrate equipment cabinets or cases but rather shall be terminated on the ground stud peripherally welded to the metal barrier.

Section 5.1.2.1.1.1 .... There shall be no isolation between equipment chassis and case for higher frequency equipment.

Section 5.1.2.1.1.2 .... Equipment Signal Ground Terminations: All equipment cases and all racks and cabinets shall have a grounding terminal, as long as it permits convenient and secure attachment of the ground strap.

Section 5.1.2.1.1.3 .... Coaxial Cables: Shields of coaxial cables shall be terminated by peripherally grounding the shield to the equipment case.

Section 5.1.2.2.3 ... AC Power Neutral: In all electrical and electronic equipment, the AC power neutral (white wire) shall be insulated from the equipment chassis, case, and facility ground system except for one point at the facility power service entry.

Section 5.2.1 ... Bonding General: Bonding refers to the process by which a low impedance path for the flow of an electric current is established between two metallic objects in order to minimize electric shock hazards, provide lightning protection , establish references for electronic signals, etc.

Section 5.2.3.1 .. Corrosion Protection: Each bonded joint shall be protected against corrosion by assuring that the metals to be bonded are galvanically compatible. Bonds shall be sealed with a silicone or petroleum-based sealant to prevent moisture from reaching the bond area.

Section 5.2.3.2. .... Compression Bonds not exposed SHALL NOT require sealing.

Section 5.2.4 …Bond Resistance: All bonds for ground conductors shall have a maximum dc resistance of 1 milliohm.

Section 5.2.5.3 ... Clamps: Where bonding clamps are not avoidable, they shall conform to AN735 or AN742.

Section 5.2.6 .... Direct Bonds - High pressure contact between mating surfaces with bolts without the use of auxiliary conductors.

Section 5.2.6.3 ... Bonding of Copper to Steel: Either brazing or exothermic welding shall be used for the permanent bonding of copper conductors to steel.

Section 5.2.7 ... Indirect Bonds may be used where the direct joining is impossible.

Section 5.2.8.2 ... Paint, primers, rust, oxides and nonconductive finishes shall be removed so as not to compromise the bond between metal surfaces.

Section 5.2.9 ... Dissimilar Metals: A protective coating is required between non-coupling metals.

Section 5.2.10.2 ... Equipment chassis or case shall be directly bonded to each other by direct bonding fasteners and to adjacent cabinets and racks.

Section 5.3.2.5 ... Wire & Cable Routing: Cables or wires carrying high level signals shall be routed as far as feasible from low level signal lines. Power lines and control lines subject to large transients shall be routed away from sensitive digital or other susceptible circuits.

Footnote: MIL-HDBK-419a Volume 2 Section 1.3.3.5.27 Grounding of Unused Wires: All unused wires/pairs of communication cable runs should be connected to ground at each end. This action will reduce transients on the unused lines which otherwise could be coupled to in-service lines of the cable.

Section 5.3.2.6 ... Telephone cable shields shall be bonded to each other and the earth electrode subsystem through the steel entry plate. This measure eliminates harmful differences of potential between the various telephone cables entering the facility.

Additional information and site survey information can be obtained by contacting

POC: Mike Helms mike@lightningmike.com