IEC 62271-100 is the part of the IEC 62271 series covering alternating-current circuit breakers for voltages above 1 kV. It sets their ratings, design requirements and, above all, the type and routine tests they must pass.
For field work what matters is that it standardises the operating sequences used in timing tests — O, CO, O-0.3s-CO and similar — the pole scatter criteria and the tolerances on opening and closing times. It is the document a breaker analyzer report refers back to.
It applies alongside IEC 62271-1, which collects the clauses common to all switchgear, and its North American counterpart is IEEE C37.09, with which it shares its philosophy although the procedural detail differs.