12.3 Airfield Lighting Counterpoise for Lightning Protection

Per FAA AC 150/5340-30H, Design and Installation Details for Airport Visual Aids, Paragraph 12.5 or as amended, "The purpose of the counterpoise or lightning protection system is to provide low resistance preferred paths for the energy of lightning discharges to enter the earth and safely dissipate without causing damage to equipment or injury to personnel. The counterpoise is a separate system and must not be confused with equipment safety grounds that provide personnel protection from electrical shock hazards.” Equipment safety grounds shall be provided per Paragraph 12.6 of the above cited Advisory Circular, and as amended later.

To provide a level of protection from lightning strikes to underground power and control cables for airfield electrical equipment owned and maintained by the BWI Marshall and MTN Airports, all projects shall include installation of counterpoise wire above the wiring and conduit intended to be protected. Following are installation requirements for the installation of counterpoise wire:

B. Counterpoise shall be connected to ground rods spaced a maximum of 500 feet apart.

C. Counterpoise shall be bonded to ground rods in accordance with Section 12.3.3.

D. Ground Rods shall be copper or copper-clad steel, 10 feet long by 3/4-inch diameter, minimum.

F. Include provisions in the contract documents that if the resistance to ground of a ground rod exceeds 25 ohms, a supplemental ground rod shall be added.

G. Counterpoise shall be connected to an exterior ground lug on ALL Light Bases that are intended to contain wiring for Airport-Owned and Maintained Lighting, Navigational Aids (Navaids), and other miscellaneous Airfield Electrical Equipment. This includes, but is not limited to, all inpavement and elevated light base cans and sign base cans.

H. A rebar cage shall be installed around inpavement light bases in rigid pavement as well as in concrete encasement for light bases installed in flexible pavement. The counterpoise shall be bonded to the rebar cage.

I. Coat any exposed copper/brass at connections or bonds to base cans with a 6 mil layer of 3M ScotchKote electrical coating or approved equal.

K. Specifications shall require installation to comply with applicable requirements of NFPA 780, Standard for the Installation of Lightning Protection Systems.

L. At the Airfield Lighting Vault or other power source, bond counterpoise to vault or power source equipment grounding system.

Whenever counterpoise wires are bonded to ground rods or other grounding electrodes, bonds shall be exothermic welds. Exothermic welds shall be coated against corrosion where direct buried.

Counterpoise Wire Installation - Provide Counterpoise wire in accordance with the following table:

Counterpoise Wire Requirements
Duct or Conduit Installation Method and Configuration	Counterpoise Wire Installation Requirements
Single conduit, all sizes, not adjacent to pavement edge	4” above line to be protected
Single conduit, all sizes, running beneath pavement (apron area, paved shoulders)	4” above line to be protected
Single conduit, all sizes, running adjacent to edge of pavement, in grass area	8” below grade and located half the distance from edge of pavement to the conduit run centerline.
Multiple conduits and ductbanks, all widths and depths	Height of Counterpoise wire above conduit(s) to be protected shall be within a 45-degree zone of protection below the counterpoise. Provide number of Counterpoise wires as required to meet protection zone criteria. Minimum cover over counterpoise shall be 12”.
Directionally drilled conduit, single, double or in group of three	None – provide ground rod at each end of directionally drilled conduit and bond to counterpoise of circuit being placed in directionally drilled conduits.
Group of 4 or more directionally drilled conduits	#6 AWG, Minimum, in separate directionally drilled sleeve, centrally located. Provide ground rod at each end of directionally drilled conduit and bond to counterpoise wire.
Conduits in steel casing	None – Provide ground rod at each end of casing and bond with #6 counterpoise wire.
Buried FAA cables	Refer to FAA Design Standard 19e or latest edition.

To supplement the above design standard items, include in the contract documents the following specification sections from Advisory Circular 150/5370-10, current edition, ITEM L-108 UNDERGROUND POWER CABLE FOR AIRPORTS.