Resistance and continuity
Winding resistance testing measures the direct-current resistance of a winding with a low-resistance instrument. It finds poor connections, broken strands and worn tap contacts in transformers, motors, reactors, regulators and instrument transformers, and results are compared between phases, with factory data after temperature correction, and with earlier tests.
Winding resistance testing measures the direct-current resistance of the electrical windings in a piece of equipment. A test set drives a steady current through the winding and measures the voltage across it, reporting the resistance, which is typically small. The test is applied to power and distribution transformers at every tap position, to the stators of induction and synchronous motors and generators from phase to phase, to the field and exciter windings and discharge resistors of synchronous machines, to shunt and current-limiting reactors, to step voltage and induction regulators and load tap-changers, and to the secondary windings of current and voltage transformers, including relaying cores. It is one of the standard electrical checks because it needs little equipment, takes little time and reveals defects that are invisible from outside, such as broken strands, high-resistance joints and bad contacts inside a tap-changer. The measured resistance reflects the entire current path, including the bushing connections, the internal leads, the joints and every turn of the winding.
A winding is only as sound as its weakest joint. Poor brazed or crimped connections, strands that broke during shipping or from a through-fault, contacts in a tap-changer that have worn or become coked, and loose terminal connections all raise the resistance without necessarily showing up in other tests. Such defects cause local heating under load, which can degrade insulation, produce gas in liquid-filled equipment and eventually open the circuit. In motors and generators, a high-resistance joint or an unbalanced phase leads to unequal currents, extra heating and torque pulsation. On a current transformer, a secondary winding with the wrong resistance affects accuracy and the ability to drive the protection burden. Winding resistance testing is therefore standard at acceptance, to catch manufacturing and shipping problems and set a baseline, and at maintenance, to detect developing problems and after any fault or event that could have damaged the equipment. It is also the companion to dissolved gas analysis, since an unexplained thermal fault may point to a bad connection that this test can confirm.
The resistance of a winding equals the voltage across it divided by the steady direct current through it, and it depends on the length, cross-section, material and temperature of the conductor, plus the resistance of every joint along the path. Because inductance offers no opposition to steady direct current, the reading is purely resistive, but it takes time to reach, because a winding with high inductance needs time for the current to settle before the measurement is valid. Large transformers can take a while, and faster instruments use techniques such as higher source current, demagnetizing or injecting on several windings at once. Resistance rises with temperature, in a known proportion for copper and aluminum, so a reading is corrected to a reference temperature before comparing it with other data. The measurement uses separate current and voltage leads, which keeps lead and clamp resistance out of the reading. When the three phases of a winding are nominally identical, their resistances should be nearly equal, and a difference points to a defect in one of them. When the test is done at each tap position, the resistance should change by a smooth, expected pattern, and a break in the pattern points to a contact or lead problem at that tap.
The equipment is de-energized, isolated, grounded and locked out under the owner's procedures, and external connections that could provide parallel paths are removed from the winding terminals. Because the test charges an inductance, the winding is discharged safely before leads are removed, and the leads are handled with care when the source is on. A low-resistance test set with a current output suited to the size of the winding is connected with separate current and voltage leads at the terminals of the winding being measured. Clean contact surfaces are essential. For a transformer, each winding is measured in turn, and the test is repeated at each tap position, with the tap changer moved through its range and returned to the position noted before the test. For a motor, readings are taken between each pair of stator terminals, and for field and exciter circuits the winding and any discharge resistor are measured separately. For an instrument transformer, the secondary winding is measured across the relevant tap or core. The current is allowed to stabilize, the reading is recorded, and the winding temperature is measured, with care taken to use a representative temperature because liquid-filled equipment can be warmer inside than on the surface. Tests on a transformer are usually sequenced so that direct-current tests that magnetize the core follow any frequency response work. After the test the current is brought down slowly and the winding is discharged before the leads are moved.
The record holds the resistance measured for each winding, phase pair and tap position, together with the winding temperature and the way it was determined, and the current used. It notes the instrument and its calibration status and the as-found condition of the connections. The tap or core to which each reading belongs, and any reading that was retaken, is documented. The reference temperature and the correction applied are noted when results are shown in a corrected form, so that a later reader can reproduce the comparison. Taken together, these details allow a result to be compared with the factory data and with future tests on equal terms.
This page states no numeric limits. The published criteria take several shapes that are used together. The first is comparison between phases or between corresponding windings, where the readings of nominally identical windings should be close and the outlier is investigated. The second is comparison with factory test data or nameplate information, after the temperature correction that brings both to the same reference. The third is the consistency of the tap-to-tap pattern, which should change smoothly. The fourth is trending against earlier results on the same equipment, because a slowly rising value reveals developing wear or a loosening joint. For instrument transformers the secondary winding resistance is judged against manufacturer data and the guidance in IEEE C57.13-2016, and for step voltage and induction regulators against IEEE C57.15-2017, with reactors judged against manufacturer data. The tests are specified in ANSI/NETA ATS-2025 for acceptance and in ANSI/NETA MTS-2023 for maintenance, and further related guidance is found in IEEE C57.13.1-2017. Use your licensed copies of these documents for the governing values and wording.
At acceptance the test is performed carefully on every winding and tap, because it establishes the baseline against which the equipment will be compared throughout its life, and it can reveal damage in transit. In maintenance, a shorter set of readings is often taken, with attention to taps in regular service, and the winding temperature is determined as consistently as possible. Liquid-filled transformers retain heat, so a stabilized temperature matters; dry-type units are easier to measure for temperature. Motors and generators are tested from phase to phase because the neutral may not be accessible, and the stator winding temperature is often derived from embedded detectors. Synchronous machine field and exciter circuits and their discharge resistors are measured separately. Instrument transformers have small windings and small readings, so lead handling and instrument resolution are important. When a load tap-changer is present, the static winding resistance at each tap is paired with dynamic winding resistance, which looks at what happens between taps. The measurement is also one of several used to investigate a thermal fault suggested by dissolved gas analysis.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
The resistance of copper and aluminum rises as temperature rises, so the same winding reads higher when warm than when cool. Correcting each reading to a common reference temperature lets it be compared with factory data and with tests taken on other days or at other temperatures.
It finds series-path defects such as loose or high-resistance joints, broken strands, worn tap-changer contacts and bad terminal connections. Insulation tests look at the insulation between parts, which is a different aspect of condition, so each test finds problems the other can miss.
A large winding has high inductance, so the direct current rises slowly toward its steady value, and the reading is not valid until it has settled. Test sets use higher source current, demagnetizing steps or simultaneous injection to shorten the wait without affecting the result.
Not reliably. A few shorted turns reduce the total resistance only slightly, often less than normal variation, so other tests such as turns ratio, excitation current and frequency response are better suited to finding them. Winding resistance is most effective for connection and conductor faults.
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