About Calibration of Phase Sequence Meter
The calibration of a phase sequence meter involves verifying that the instrument correctly identifies the phase rotation (R-Y-B or clockwise/counter-clockwise) against a known, controlled reference signal generated by master calibration equipment. The "Non-NABL" aspect simply means the calibration is performed by a lab that is not formally accredited by the National Accreditation Board for Testing and Calibration Laboratories (NABL), though they may still follow established national or international standards. (Thefebricity Calibration And Technical Services)