In our main page on the phasing of a double-circuit power line, we explain how an arrangement called transposed phasing produces lower fields to the sides of the power line than the alternative untransposed phasing.  This is because the magnetic field produced by each circuit is, roughly, a dipole, and with transposed phasing the two dipoles are in opposite directions, leading to greater cancellation of the fields.  (See some drawings of the actual field lines)

This effect varies with the ground clearance of the conductors (see also how the ground clearance affects the field directly).  Well to the sides of the power line, it is always true that transposed phasing gives lower fields, whatever the clearance.  But between the two circuits, a different effect comes into play.

Each circuit on its own produces a dipole field, shown in simplified form here:

As you pass from one side to the other of the circuit in question, the field lines change direction - the vertical component of the field reverses direction.

Now consider the fields from both circuits simultaneously:

As you go from outside one circuit to inside that circuit, the vertical component of the field from that circuit has reversed direction - but the field from the other circuit has not.  So if the two fields were cancelling each other outside the circuits - the situation with transposed phasing - now, inside the circuits, they are not cancelling so well - the horizontal components may still partially cancel, but the vertical components are adding.

This is more significant at lower clearances because the vertical component is more significant.  So at the lowest clearances, transposed phasing actually gives higher fields than untransposed at ground level between the two circuits.

Similarly, with untransposed phasing, to the sides of the line the two  dipoles add, but between the conductors, they partially cancel.  This leads to a "dip" in the field on the centreline, half way between the two conductors.

(Actually, whatever the phasing, if the clearance is low enough this dip will appear.  That is because if you are close enough to the actual conductors of one circuit, the field you experience basically comes just from that circuit.)

These effects are illustrated in the following graph.

• At a relatively high clearance, 20 m, the transposed phasing is lower than the untransposed everywhere
• At a relatively low clearance, 8 m, the untransposed phasing dips between the two circuits and the transposed phasing gives the higher field in this region
• At intermediate clearances, 12 m, typical of actual power lines, the two are about the same between the circuits
• To the sides of the line, transposed phasing is always lower.

Note: these graphs are calculated for L12 lines with 500 A in each circuit.  In real power lines, the currents are not always equal both between circuits and within a circuit, and these reduce the effectiveness of the transposed phasing.  See more detail on exactly how fast fields fall with distance under these different conditions.