Clearance of overhead lines

Clearances required for electric field to be below 9 kV/m for 400 kV lines

We discuss on a different page the effect of the clearance of the conductors above ground on the electric field.  The higher the clearance, the lower the field.  Because the electric field exposure limit for the general public in populated areas in the UK is 9 kV/m, we list here the clearances required for the field to be below 9 kV/m.  If the clearance is greater than the stated value, the electric field will be below 9 kV/m.

The field depends on several factors as shown in the table: the tower design, the conductor bundle, and the phasing (transposed, T, or untransposed, U - other phasings lie between these values).

tower conductor bundle phasing
number of conductors conductor type spacing T U
L13 triple Araucaria 500 mm 8.4 m 9.3 m
L12 twin Araucaria 500 mm 7.8 m 8.8 m
Redwood 500 mm 7.9 m 8.9 m
triple (pointing sideways) Araucaria 500 mm 8.3 m 9.7 m
L8 twin Rubus 400 mm 7.5 m 8.5 m
Sorbus 400 mm 7.6 m 8.5 m
Matthew 400 mm 7.5 m 8.5 m
L6(M) quad Zebra 305 mm square 8.5 m 9.2 m
500 mm horizontal
300 mm vertical		 8.7 m 9.5 m
500 mm square (L6(M), BICC and BB) 8.9 m 9.8 m
500 mm square (BK and JLE) 8.8 m 9.9 m
triple (pointing sideways) Araucaria 500 mm 8.4 m 9.1 m
triple (pointing down) Araucaria 500 mm 8.3 m 9.1 m
twin Araucaria 500 mm 8.0 m 8.5 m
Redwood 500 mm 8.1 m 8.6 m
L2 twin Zebra 305 mm 7.2 m 8.3 m
Rubus 500 mm 7.5 m 8.8 m
Rubus 400 mm 7.4 m 8.6 m
Sorbus 400 mm 7.4 m 8.6 m
Matthew 400 mm 7.4 m 8.6 m

All calculations for 400 kV, 1 m above ground level, and using the infinite-straight-line approximation in accordance with the Code of Practice on demonstrating compliance.  Clearances are at 0.1 m resolution (the first 0.1 m increment of extra clearance needed to reduce the field below 9 kV/m, not the nearest 0.1 m).