This test doesn't examine insulation or the turns ratio, but rather the copper path itself: are the conductors, joints, and tap-changer contacts sound? A small increase in a winding's resistance signals a hot spot waiting for the moment of breakdown.
What Does It Measure and How?
The instrument passes a known direct current through the winding and measures the voltage drop, from which it calculates the ohmic resistance of the conductors and joints along the path. DC is used to eliminate the effect of inductance, while waiting for the reading to stabilize (windings are inductive and need time to settle).
What Do the Results Reveal?
| Observation | Possible Significance |
|---|---|
| Noticeable difference between the three phases | Poor connection, corroded tap-changer contact, or conductor damage in the anomalous phase |
| General increase over factory values (after temperature correction) | Deteriorated joints or a problem at connection points |
| Unstable reading | Intermittent connection — a real hazard under load |
Key Practical Controls
- Temperature correction: copper resistance changes with temperature; compare values after referring them to a common reference temperature.
- Measure at each tap position if possible — irregular differences between positions point to the tap-changer contacts.
- Compare the results against factory values within the stated tolerance and against the other phases.
- After the test, safely discharge the winding's inductive energy through the instrument before disconnecting the leads.
Sample answer: It usually indicates a localized problem in that phase's path: a poor connection at a joint, corrosion or looseness in a tap-changer contact, or damage to the conductor itself. This point of excess resistance will become a localized hot spot under load, so the cause must be identified and fixed before energization, with the measurement repeated to confirm.
Comparing measurements taken at different temperatures without correction. A rise in temperature naturally increases copper resistance, which may be mistakenly interpreted as a fault — or conversely, may mask a real fault.
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