Identify the real source first
Measurements and simulation regularly show that the low-voltage side, where currents are highest, produces the dominant external field. Shielding the transformer alone then delivers disappointing results.
Geometry before material
Reducing the spacing between phase conductors reduces the far field roughly with the square of the distance between them. Transposition, tighter bundling and optimised phase order routinely deliver 50 to 80% reduction at no material cost.
When to shield
Shielding is the right answer when geometry is fixed and distance is unavailable — typically ceilings under apartments and walls facing occupied rooms. A layered aluminium/steel shield covering only the critical surfaces is usually sufficient.