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Sign inCentral conductor magnetisation
A central conductor is a copper or aluminium bar threaded through a hollow part and connected across the contact heads. The current flows in the bar, not in the part, so no contact is made with the part and there is no burn or arc-strike risk. The circular field encircling the bar links the part and magnetises both the bore and the outside surface, finding discontinuities that lie longitudinal and radial to the bore axis.
Field strength around a conductor falls as 1/r. A conductor centred in the bore therefore produces a uniform field around the whole circumference and magnetises the part in one shot, but the outer surface always sees a weaker field than the bore, so the current is sized on the full outside diameter of the section being examined - the bore diameter plus twice the wall.
An offset conductor is laid against the bore wall. It gives a much stronger local field, which is what large-diameter or heavy-wall parts need, but only over a limited arc. The accepted working rule is that the effective inspection band extends about four conductor diameters around the circumference, measured from the point of contact, and the current is sized on the conductor diameter plus twice the wall. The part is then rotated through a series of positions with at least 10% overlap until the whole circumference has been covered.
The conductor itself must be non-ferromagnetic and big enough to carry the current without heating. A ferromagnetic bar would take flux into itself and distort the field; an undersized copper bar will simply get hot and sag inside the part.
Wall thickness t = (OD - ID) / 2 Centred conductor: magnetising diameter = ID + 2t (the part OD), full circumference in one shot Offset conductor: magnetising diameter = conductor diameter + 2t I = (A per inch) x magnetising diameter / 25.4 / passes A conductor threaded through the bore n times needs 1/n of the single-pass current Offset inspection band = 4 x conductor diameter, applied with 10% overlap Rotations = circumference / (band x 0.9), rounded up
- The conductor must be non-ferromagnetic - copper or aluminium - and sized to carry the current without overheating.
- A central conductor magnetises the bore surface strongly and the outside surface weakly. Confirm the outer surface field with a shim if OD discontinuities are of interest.
- Offset conductors need the part rotated with at least 10% overlap between shots; mark the start position so no arc is missed.
- Insulate or pad the conductor where it touches the part to prevent local heating at the contact line.
- The field is circular about the bore axis, so it finds longitudinal and radial discontinuities. A separate longitudinal shot is needed for circumferential ones.
- When the conductor passes through the bore more than once, keep the passes bundled tightly together, and for an offset bundle use the bundle diameter as the conductor diameter for the magnetising diameter and the effective band.
Reference: ASME BPVC Section V (2023 Ed.), Article 7 - central conductor technique; ASTM E1444/E1444M-22 and ASTM E709-21. The 300 to 800 A per inch band, the conductor-diameter-plus-twice-wall rule for an offset conductor and the four-conductor-diameter effective band are taken from those standards. ASTM E709-21 also gives the ampere-turn rule for a conductor threaded through the bore several times: the single-pass current requirement divides by the number of passes.
