Codes and Standards

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Chapter 10. Marking and Lighting of Catenary and Catenary Support Structures - Medium Intensity Flashing White Lights

100. PURPOSE.

Lighting catenary support structures with a medium intensity (1.-866) omnidirectional flashing white lighting system provides conspicuity both day and night. In addition, the unique sequential/simultaneous flashing light system alerts pilots of the associated catenary wires. In those instances where normally marking and red lighting of the structures would be deemed adequate for conspicuity, the medium intensity flashing white lighting system would be the preferred system. The use of spherical markers shall be considered and is a separate issue involving additional factors.

101. STANDARDS.

a. Levels. A system of three levels of sequentially flashing light units should be installed on each supporting structure or adjacent terrain. Install one level at the top of the structure, one at the height of the lowest point in the catenary, and one level approximately midway between the other two light levels. The middle level should normally be at least 50 feet (15m) from the other two levels. The middle light unit may be deleted when the distance between the top and the bottom light levels is less than 100 feet (3Om). If the installation presents a potential danger to maintenance personnel, or when necessary for lightning protection, the top level of lights may be mounted as low as 20 feet (6m) below the highest point of the structure.

1. Top Levels. One or more lights should be installed at the top of the structure to provide 360--degree coverage ensuring an unobstructed view. If the installation presents a potential danger to maintenance personnel, or when necessary for lightning protection, the top level of lights may be mounted as low as 20 feet (6m) below the highest point of the structure.

2. Horizontal Coverage. The light units at the middle level and bottom level should he installed so as to provide a minimum of 180--degree coverage centered perpendicular to the flyway. Where a catenary crossing is situated near a bend in a river, canyon, etc., or is not perpendicular to the flyway, the horizontal beam should be directed to provide the most effective light coverage to warn pilots approaching from either direction of the catenary wires.

3. Variation. The vertical and horizontal arrangements of the lights may be subject to the structural limits of the towers and/or adjacent terrain. A tolerance of 20 percent from uniform spacing of the bottom and middle light is allowed. If the base of the supporting structure(s) is higher than the lowest point in the catenary, such as a canyon crossing, one or more lights should be installed on the adjacent terrain at the level of the lowest point in the span.

These lights should be installed on the structure or terrain at the height of the lowest point in the catenary.

b. Structures 500 Feet (153m) AGL or Less. When white lights are operated 24 hours a day, painting can be ommitted. This system with its unique flash rate and sequence precludes a pilot from mistaking the support structures for stand-alone towers. When white lights are used during nighttime and twilight only, painting should be used for daytime marking. Spherical markers may also be needed.

c. Structures Exceeding 500 Feet (153m) AGL. The white lights should be used during nighttime and twilight and are recommended for use 24 hours a day. Painting should always be used for daytime marking.

d. Flash Sequence. The flash sequence should be middle level, top level, and bottom level with all lights on the same level flashing simultaneously. The time delay between flashes of levels is designed to present a unique system display.

e. Synchronization. Although desirable, the corresponding light levels on associated supporting towers of a catenary crossing need not flash simultaneously.

102. CONTROL DEVICE.

The light intensity is conlxolled by a device that changes the intensity when the ambient light changes. The system should automatically change intensity steps when the northern sky illumination in the Northern Hemisphere on a vertical surface is as follows:

a. Twilight-to-Night. This should not occur before the illumination drops below five footcandles (53.8 lux) but should occur before it drops below two footcandles (21.5 lux).

b. Night-to--Day. The intensity changes listed in a. above should be reversed when changing from the night to day mode.

103. AREA SURROUNDING CATENARY SUPPORT STRUCTURES.

The area in the immediate vicinity of the supporting structure's base should be clear of all items and/or objects of natural growth that could interfere with the line-of-sight between a pilot and the structure's lights.

104. THREE OR MORE CATENARY SUPPORT STRUCTU RES.

Where a catenary wire crossing requires three or more supporting structures, the inner structures should be equipped with enough light units per level to provide a full coverage.

105. CATENARY LIGHTING.

Lighted markers are available for increased night conspicuity of high-voltage (69KV or higher) transmission line catenary wires. Lighted markers provide conspicuity both day and night; installation, size, color, and pattern should follow the guidelines of paragraphs 34 and 36. These markers should be used for increased night conspicuity of high-voltage (69KV or higher) transmission line catenary wires near airports, heliports, across rivers, canyons, lakes, etc. The lighted markers should be manufacturer certified as recognizable from a minimum distance of 4,000 feet (1219m) under nighttime, minimum VFR conditions or having a minimum intensity of at least 32.5 candela. The lighting unit should emit a steady burning red light. They should be used on the highest energized line. If the lighted markers are installed on a line other than the highest catenary, then markers specified in paragraph 34 should be used in addition to the lighted markers. (The maximum distance between the line energizing the lighted markers and the highest catenary above the lighted marker should be no more than 20 feet.) Markers should be distinctively shaped, i.e., spherical, cylindrical, so they are not mistaken for items that are used to convey other information. They should be visible in all directions from which aircraft are likely to approach.

a. Size and Color. The diameter of the markers used on extensive catenary wires across canyons, lakes, rivers, etc., should be not less than 36 inches (91cm). Smaller 20-inch (51em) markers are permitted on less extensive power lines or on power lines below 50 feet (15m) above the ground and within 1,500 feet (458m) of an airport runway end. Each marker should be a solid color such as aviation orange, white, or yellow. (See paragraph 131 for color standards.)

b. Installn~ons.

1. Spacing. Lighted niarkers should be spaced equally along the wire at intervals of approximately 200 feet (61m) or fraction thereof. Intervals between markers should be less in critical areas near runway ends (i.e., 30 to 50 feet). If the markers are installed on a line other than the highest catenary, then markers markers specified in paragraph 34 should be used in addition to the lighted markers. The maximum distance between the line energizing the lighted markers and the highest catenary above the markers can be no more than 20 feet. The lighted markers may be installed alternately along each wire if the distance between adjacent markers meets the spacing standard, This method allows the weight and wind loading factors to be distributed.

2. Pattern. An alternating color scheme provides the most conspicuity against all backgrounds. Mark overhead wires by alternating solid colored markers of aviation orange, white, and yellow. Normally, an orange marker is placed at each end of a line and the spacing is adjusted (not to exceed 200 feeO to accommodate the rest of the markers. When less than four markers are used, they should all be aviation orange.

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