IEC 60061 has four parts and they are easy to confuse. Part 1 covers lamp caps, part 2 covers lampholders, part 3 covers gauges, and part 4 covers the general guidelines that explain how the first three fit together. Part 3 is the one that ends up on a bench, because it is the only part that gives a physical object with which an inspector can accept or reject a production lot.
A gauge sheet does not repeat the cap dimensions from part 1. It defines a separate artefact whose critical dimensions are derived from the cap limits so that the gauge accepts every cap inside the tolerance band and rejects caps outside it. That is why a gauge dimension is often not a round number: it is the worst-case combination of the cap limits plus the gauge manufacturing tolerance.
A cap that passes its go gauge and fails its no-go gauge is dimensionally interchangeable with any holder made to part 2 limits. It does not mean the cap is safe, correctly wired, or correctly marked. Safety requirements live in part 1 and are tested by other means, including insulation resistance, dielectric strength, and cap temperature rise.
When a supplier claims IEC 60061 compliance, ask which gauge sheets were used and whether the gauges were calibrated within the last twelve months. A test report that names the cap but not the gauge sheet cannot be reproduced by anyone else, and a gauge that has worn out of tolerance will keep passing parts long after the parts have drifted out of specification.
