Electrical Testing Procedures & Equipment Operation Flashcards

During a power factor (dissipation factor) test on a large oil-filled transformer bushing, the measured tip-up between 10 kV and full voltage exceeds 0.5%. According to NETA MTS standards, what does this most likely indicate, and what is the appropriate action?

Answer: Moisture ingress or partial discharge activity within the bushing; the bushing should be removed from service for further evaluation

A tip-up (increase in dissipation factor from 10 kV to rated voltage) greater than 0.5% is a NETA-recognized flag for nonlinear dielectric losses, typically caused by moisture ingress, contaminated oil, or partial discharge within the bushing insulation. The bushing should be taken out of service for oil sampling, drying, or replacement evaluation. A stable dissipation factor that merely rises proportionally with voltage is expected; a non-proportional tip-up signals deterioration.

A technician performs a time-resistance (polarization index) test on a 4,160 V motor. The 1-minute reading is 1,850 MΩ and the 10-minute reading is 2,100 MΩ, yielding a PI of 1.14. The insulation temperature is 22°C. How should this result be interpreted?

Answer: Questionable; a PI below 2.0 indicates contaminated or deteriorating insulation that warrants increased monitoring or drying

IEEE 43 and NETA MTS define a PI of 2.0 or greater as 'good' and 1.0–2.0 as 'questionable' for rotating machine insulation. A PI of 1.14 falls in the questionable range, indicating the insulation resistance does not increase adequately over time — a sign of surface contamination, moisture, or thermal degradation. While the absolute value (1,850 MΩ) seems high, PI evaluates the quality of the dielectric, not just its resistance magnitude. Temperature correction applies to the absolute IR readings but does not change the PI interpretation category.

While performing a transformer turns ratio (TTR) test on a delta-wye transformer (Dyn11), the technician measures a phase-to-phase ratio on the delta primary side and compares it to the calculated wye secondary ratio. The nameplate indicates a 13.8 kV / 480 V ratio. The measured TTR reads 16.63 instead of the expected 16.625. What is the most accurate interpretation?

Answer: The result is within acceptable tolerance; TTR test sets typically have ±0.5% accuracy, and 16.63 deviates only 0.03% from 16.625

A TTR deviation of (16.63 − 16.625) / 16.625 = 0.03% is well within the ±0.5% acceptance criterion per NETA MTS Table 7.1 and IEEE C57.12.90. TTR test sets themselves have inherent measurement uncertainty, and a 0.03% deviation is essentially at the noise floor of the instrument. The result should be recorded as acceptable. A shorted turn would cause a much larger and usually erratic deviation; tap position errors produce systematic differences aligned with discrete tap steps.

A NETA technician is performing a circuit breaker contact resistance test using a micro-ohmmeter at 100 A DC. On a 15 kV vacuum interrupter breaker, one pole reads 48 µΩ while the other two poles read 52 µΩ and 49 µΩ. The manufacturer's maximum is 60 µΩ. What additional concern, specific to vacuum interrupters, should the technician address before closing the test report?

Answer: Contact resistance spread between poles exceeds 10%, requiring a vacuum bottle integrity (hi-pot) test to rule out loss of vacuum

NETA MTS and IEEE C37.09 both specify that for vacuum interrupters, inter-pole contact resistance spread greater than approximately 10% can signal uneven contact erosion or loss of vacuum, which affects both contact integrity and dielectric withstand. The spread here is (52−48)/48 ≈ 8.3%, borderline. More critically, the standard practice for vacuum breakers is to supplement contact resistance testing with a vacuum bottle integrity (hi-pot or power frequency withstand) test across the open contacts because contact resistance alone cannot detect a degraded vacuum — which is the primary failure mode. Low absolute values passing the manufacturer limit do not preclude a vacuum failure.

During a ground grid resistance test using the fall-of-potential (3-point) method at a large substation, the technician cannot achieve a flat region in the resistance-vs.-probe-distance curve even when the current probe is placed at 10× the diagonal of the grounding system. What is the most technically correct corrective action?

Answer: Use the slope method or multi-angle (star) method with extended probe distances to mathematically extrapolate the true ground resistance

For large grounding systems, the fall-of-potential flat region cannot always be achieved within practical probe distances because the spheres of influence of the current and potential electrodes overlap. IEEE Std 81 specifically recommends the slope method (using the 20%, 40%, and 60% distance readings to extrapolate) or the multi-angle (star) method as alternatives when flat-region criteria cannot be met. The clamp-on method measures only loop resistance through a single path and is not appropriate for standalone ground grid resistance. Accepting the minimum reading without correction introduces systematic error. The Wenner method measures soil resistivity, not grid resistance.

A technician is performing partial discharge (PD) testing on a 138 kV XLPE cable system using the off-line damped AC (DAC) method. During the test, PD pulses are detected at 45 pC at a phase angle consistently near the negative peak of the voltage waveform (270°). What is the most likely physical interpretation of this PD source location and type?

Answer: Electrical treeing initiated at a metallic protrusion on the inner conductor shield, because conductor-shield protrusions preferentially discharge on the negative half-cycle

Phase-resolved partial discharge (PRPD) pattern analysis is a key diagnostic skill in advanced cable testing. PD pulses concentrated near the 270° position (negative voltage peak) are characteristic of discharges initiating at a sharp protrusion on the inner (high-voltage) conductor shield. Under AC voltage, field enhancement at a conductor-shield protrusion causes electrons to be injected into the insulation preferentially when the conductor is at its most negative potential (270°), producing asymmetric PRPD patterns. Internal voids produce roughly symmetric discharges at both 90° and 270°. Termination surface discharges typically appear near voltage zero-crossings or 90°. True external corona produces phase-consistent but usually higher-frequency pulse distributions.