Ground clearance in the crossing span is determined from the following formula :
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a = L / 2 + TH / WL
where,
a = Distance of lowest point of wire from higher tower.
L = Actual span
T = Tension at 750 C & N0-wind for equivalent tower.
h = Height difference of two river crossing towers.
W = Weight of conductor / meter (kg / m)
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From a, span 2a is calculated and sag SA for River crossing span this span is determined as per following formula.
SA = [2a / Eq. Span]2 x Sag at 750 N/W for equivalent span.
From above, the max sag is determined by adding 4% error in sag to the above.
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R.L. of Null point is determined as follows :
R.L. of Null point = R.L. of bottom cross arm of higher 'Rx' tower after deducting length of insulator string & length of cleat and D-shackle and maximum Sag. Ground clearance available
= (R.L. of Null point - R.L. of H.F.L.)
