Field Testing, Commissioning, and Documentation
Acceptance versus maintenance testing
Acceptance testing proves a new installation is sound before energization; maintenance testing establishes condition over time. NETA ATS covers acceptance, NETA MTS covers maintenance. A frequent trap offers a maintenance interval as a field acceptance criterion, or treats a factory test report as satisfying a field acceptance requirement - it does not.
Core field tests
Insulation resistance (megger) at a specified DC voltage for one minute, with temperature correction and a polarization index; low-resistance ohmmeter (ductor) measurements of bus and breaker contacts, typically expected in the tens of micro-ohms; transformer turns ratio (TTR) within about 0.5% of nameplate; power-factor/dissipation-factor testing of bushings and windings; breaker timing and travel; CT ratio, polarity, and excitation; relay secondary injection for settings verification and primary injection where ratio or wiring is in question.
Worked example
A transformer nameplate reads 13,800/480 V. Expected turns ratio = 28.75:1. A field TTR result of 28.6:1 deviates by about 0.5% - at the tolerance limit and worth investigating, but a result of 27.9:1 (3%) indicates a tapped winding, a shorted turn, or a misconnected tap, and the unit should not be energized.
Commissioning sequence
Pre-energization checks: visual inspection, torque verification, protective device settings confirmed against the study, clearance and barrier checks, rotation verification, and functional testing of interlocks and trip circuits. Then energize and perform load checks, phase-angle and phasing verification, and trending.
Documentation
As-builts, O&M manuals, test reports, relay setting files, and punch lists form the deliverable. Traps: accepting an as-designed drawing set as as-built, and failing to record instrument serial numbers and ambient conditions on test forms.