A NETA-formatted electrical test report runs to dozens of pages of measured values, instrument serial numbers, ambient conditions, and pass/fail flags. Most facility owners flip to the executive summary, see "Pass" on every line item, and file the binder. That is fine until something fails — and then the question becomes: were the numbers actually telling us anything we missed? This article walks through the four most common test types on industrial gear, what their values mean, and the thresholds that should trigger a planned outage even when the report says "Pass".
Insulation resistance (the megger reading)
Insulation resistance is reported in megohms (MΩ) or gigohms (GΩ), measured at 500, 1000, 2500, or 5000 V DC depending on the equipment voltage class. The pass criterion in NETA MTS is typically "greater than 100 MΩ," but that ignores the more useful signal: the trend. A transformer that meggered at 5000 MΩ last year and 2000 MΩ this year is still "Pass" but has lost 60% of its insulation margin in 12 months. That trend, not the absolute number, is what should get attention. Look at the report: does it show prior-year values? If not, ask for them.
Contact resistance (the micro-ohmmeter reading)
Contact resistance through a closed circuit breaker, a disconnect, or a bus joint is measured in micro-ohms (μΩ). NETA acceptance values for vacuum and SF6 breakers typically run 30-100 μΩ depending on size. The number to watch is not the absolute reading but the spread between phases: if A-phase reads 45 μΩ and C-phase reads 90 μΩ, you have a developing issue on C-phase even if both are inside the spec. Phase imbalance greater than 50% is a planned-outage finding, not a "next year" finding.
Power factor / dielectric loss (the Doble reading)
Doble power factor testing is reported as a percentage — typically 0.3% to 1.0% on healthy oil-filled transformers, lower on dry-type. The test catches things meggering misses: insulation aging, moisture, contamination, and bushing deterioration. The trend is even more important than for meggering. Doubling year-over-year (e.g., 0.5% to 1.0%) is a flag. Tripling is a finding that should drive a follow-up DGA and possibly an SFRA. The report should compare to the OEM factory test value if you have it, to last year, and to the NETA recommended limit for the equipment class. If any of those three references are missing from the report, push the testing vendor for a revision.
Partial discharge (the qualitative reading)
PD test results are reported in picocoulombs (pC) for offline tests and as relative magnitude (typically a logarithmic scale) for online surveys. Unlike megger or contact resistance, PD has no clean numerical pass/fail — it is interpretive. A NETA-aligned report includes the PD pulse count, the phase-resolved pattern (a polar or rectangular plot), and the technician's narrative interpretation. The narrative is the most valuable line item; if your report does not include "the patterns are consistent with [void / surface / corona] discharge originating in [bushing / winding / cable termination]," the test was incomplete.
When to push back on a "Pass" report
Push back when: (1) the report shows no trended history; (2) phase-to-phase spread on contact resistance exceeds 50%; (3) Doble power factor doubled year-over-year on any winding or bushing; (4) PD activity is reported but with no narrative interpretation; (5) the technician noted "subject to retest" anywhere in the comments. None of these are pass-fail trips on their own — but each is a signal to schedule a deeper diagnostic before next year's testing window.
Conclusion
NETA test reports are designed to be defensible documentation, not narrative storytelling. The information is there; you have to know where to look. ClarkTE produces test reports that lead with findings ranked by risk, with the trended data on the same page as the threshold for action. We'd rather you reading the executive summary and acting on it than filing the binder. If your incumbent testing vendor produces reports your team finds opaque, that's a switching cost worth considering.
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