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Sign inProd technique current and spacing
Prods inject current into the surface at two points, so the current spreads through the material between them and sets up a circular field looping around each current path. The field is strongest close to each prod tip and weakest midway between them, which is why the technique is qualified by prod spacing rather than by part size: the spacing fixes both the length of the current path and the depth to which the current spreads.
Because the field falls off away from the current path, a wider spacing needs proportionally more current, and the codes state the requirement as amperes per inch of prod spacing. Thicker sections let the current spread deeper below the surface, diluting the surface field, so they call for a higher figure: 90 to 110 A/in below 19 mm (3/4 in.) of thickness, and 100 to 125 A/in at or above it.
Spacing is bounded at both ends. Below 50 mm (2 in.) the field around each prod is so intense that the background of particles masks real indications, and the prods themselves get in the way. Above 200 mm (8 in.) the current has spread too widely to give a usable field at the midpoint, and the required current becomes impractical. The reliably examined area is the region between the prods, so coverage is built up from overlapping prod positions, repeated at right angles to catch discontinuities in both directions.
Prods pass current through the workpiece at a point contact, and that is the risk. Arc strikes and local burning cause metallurgical damage that is a defect in its own right on pressure equipment. Tips must be clean, contact firm before the current is switched on, and the current switched off before the prods are lifted. Many codes prohibit prods on finished or high-strength surfaces altogether.
Prod spacing S in inches = S / 25.4 Thickness below 19 mm (3/4 in.): 90 to 110 A per inch of prod spacing Thickness 19 mm and above: 100 to 125 A per inch of prod spacing I = (A per inch) x S / 25.4 Prod spacing limits: 50 mm (2 in.) minimum, 200 mm (8 in.) maximum
- The reliably examined area lies between the prods. Build coverage from overlapping prod positions.
- Repeat each area with the prods rotated 90 degrees, because a circular field only reveals discontinuities parallel to the current path.
- Arc strikes and burns from prod contact are metallurgical damage. Keep tips clean, make firm contact before switching current on, and switch off before lifting.
- Many codes prohibit prods on machined, finished or high-strength surfaces. Check the governing specification first.
- The thickness used is that of the section carrying the current, not the weld reinforcement.
- Prove field adequacy with an artificial flaw shim placed on the surface between the prods.
Reference: ASME BPVC Section V (2023 Ed.), Article 7 - prod technique: 90 to 110 A/in of prod spacing for sections under 3/4 in. (19 mm) and 100 to 125 A/in at 3/4 in. and above, with prod spacing between 2 in. (50 mm) and 8 in. (200 mm). ASTM E709-21 gives the same values.
