POWER CABLES
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May be used only in combination with diagram 780A.
21.9 Diagram 779 warns of a place where a road passes under an electrified overhead power cable. It must always be accompanied by a plate indicating the safe maximum height vehicle that can pass below the cable. These signs are normally associated with overhead electric railway or tramway cables and should be used at all such crossings. Because of the high voltage of certain cables, it is very important to ensure that adequate warning is given even where cable heights are greater than 16'-6" (5.03 m).
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These plates may be used only in combination with diagram 779. The height may be varied. The height in metric units may be omitted. In diagram 780.1A the arrow may be reversed or omitted, and the distance maybe varied or omitted
21.10 Supplementary plates 780A, 780.1A and 780, 7A or 780.2,4 780.2A indicate this maximum safe height. The dimension may be varied and should show a height which is at least 2'-0" (600 mm) below the height of the overhead conductor for 25kV systems and 1'-6" (460 mm) for systems on lower voltages. At a crossing where the safe height is below 16’-6” (5.03 m), the height shown on the signs should be at least 1'-9" (530 mm) or 1’-3” (380 mm) respectively below the conductor and a load gauge to diagram 781 erected at the safe height. In calculating the safe height, allowance should be made for the effect of the vertical profile of the carriageway on a road vehicle and its load. While the Regulations permit the omission of the height indication in metric units, this is inadvisable. Table 21-1 indicates the heights to be shown on the signs for different cable heights.
LOAD GAUGE
21.11 The load gauge shown in diagram 781 gives beneath cable in an audible warning when the safe height beneath an overhead power cable is exceeded. It should always be used where the safe height is less than 16'-6" (5.03 m); see also para 21.12. A safe height less than 16'-6" is likely to be encountered only at private crossings. At such locations the gauge should be effective as speeds are low and users familiar with the layout. The gauge is intended for use with power cables and not at low bridges over public highways, where speeds are likely to be higher and drivers less able to respond to the sound of the bells in time, or even to hear them at all from inside closed cabs.
21.12 The load gauge must be mounted on two posts coloured black and white in alternate horizontal bands each band being less than 250 mm nor more than 335 mm deep (Direction 43.) Diagram 779 with supplementary plate 780.2A should be used when a load gauge is installed.
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