1. Engineering Background: The Quality You Cannot See

After galvanising, every weld on a metal power pole looks the same. A sound longitudinal seam and one with 30 percent lack of fusion are both smooth, grey and covered in zinc. This is why weld quality is the easiest specification for a low-cost supplier to reduce without the buyer noticing, and why it is the first item an experienced inspector asks about. On a 69kV class pole, the longitudinal seam runs the full height and carries a share of the bending stress; the shaft-to-base flange weld concentrates the highest stress in the structure. Neither can be assessed visually. Verification requires a defined non-destructive testing plan, acceptance criteria written into the purchase order, and records that show what was tested rather than a certificate that says everything passed.

2. Technical Analysis: Which Weld Needs Which Method

Weld families and risk. There are three families. The longitudinal seam runs the full shaft and any lack of fusion directly reduces section capacity. Circumferential butt welds join tube lengths, and misalignment there creates an eccentricity that multiplies local stress. Flange, base plate, arm and stiffener welds sit at the highest stress concentration and are the origin of most fatigue failures.

Ultrasonic testing. UT is the primary method for full-penetration butt welds. It detects lack of fusion at the sidewall, lack of penetration at the root, slag inclusions and porosity clusters, and it reports their length and depth. In pole work with wall thickness between 2.5 and 10 mm, probe selection and calibration on a matched reference block are critical, because thin-wall UT is far more sensitive to technique than thick-section work.

Magnetic particle testing. MT is the correct tool for fillet welds, weld toes, flange connections and repair verification. It detects surface and near-surface cracks, undercut and toe cracking, which is exactly the defect population that drives fatigue. Wet fluorescent MT is preferred for toe inspection on the tension side of the shaft, where the alternating stress range is highest.

Why both. UT will not reliably find a tight surface-breaking toe crack; MT cannot see internal lack of fusion. A plan that uses only one method leaves half the risk unexamined, which is why the standard practice for 69kV class poles combines 100 percent UT on the longitudinal seam with 100 percent MT on the shaft-to-flange and arm welds.

3. Technical Parameters: Test Coverage and Acceptance Levels

Levels below follow common international practice and must be confirmed against the project specification.

Illustrative acceptance practice: a lack of fusion indication longer than 10 mm in a longitudinal seam on a high-moment section is treated as a reject rather than a repair candidate, because grinding out and re-welding a long indication in a thin wall leaves a heat-affected zone with lower toughness than the parent metal. By contrast, isolated porosity under the level B limits may be accepted without repair. Writing these two rules into the specification prevents the argument that usually happens at pre-shipment inspection.

4. Inspection and Design Recommendations

1. Name the acceptance standard and level in the purchase order; "sound weld" is not a specification.
2. Require indication maps showing weld identification, test coverage, defect location and length, not only a pass certificate.
3. Require welder qualification for the actual thickness range and welding position used in production.
4. Confirm that the longitudinal seam is machine or automatic welded; manual seam welding over 15 m is a repeatability risk.
5. Require dimensional verification of the assembled shaft before welding, because misalignment cannot be tested away.
6. Require repair records with location, method and re-test result, and re-test all repairs by MT.
7. Schedule the inspection before galvanising for weld and dimensional hold points, and after galvanising for coating checks.

5. Technical FAQ

Q1. Is a third-party inspection certificate enough on its own?
No. It proves an inspection happened, not what was found. Ask for the indication maps and the coverage percentage alongside the certificate.

Q2. Why specify 100 percent UT on the longitudinal seam?
Because that seam is the primary load path for bending and because sampling misses the defect that matters. On 69kV class poles the cost of full UT is small relative to the consequence of a seam failure.

Q3. Can galvanising hide weld defects?
It conceals surface appearance but does not close internal defects, and it can make small cracks harder to find later. This is why weld testing must be completed before galvanising.

Q4. How should a repair be accepted?
By re-testing the repair zone to the same level as the original weld, plus a documented record of grinding depth. Repairs without re-test are treated as unresolved defects.

6. Conclusion: Buy the Records, Not the Certificate

Weld integrity is invisible at delivery and decisive in service. The buyer who specifies the method, the coverage and the acceptance level, and who asks for the indication maps rather than a compliance certificate, receives a pole whose rated capacity is real. Our Q355 and Q460 poles for 10-35kV and 69kV lines are delivered with the complete inspection dossier: procedure and welder qualifications, UT and MT reports with defect maps, repair records and dimensional data. That dossier costs a fraction of one re-stringing outage and it is the only evidence that survives galvanising.