Resistance and continuity
Resistor and coil resistance testing measures the direct-current resistance of discrete resistive and wound components, such as neutral grounding resistors, blowout coils, starter and breaker coils and parallel cable runs. It exposes open or damaged elements and loose connections, and results are compared with nameplate data, like components and earlier readings.
A resistor and coil resistance test measures the direct-current resistance of a component whose job depends on having the correct resistance. The tested items include neutral grounding resistors and other impedance devices that limit ground-fault current, the blowout coils that help extinguish the arc in some medium-voltage air circuit breakers and motor starters, the control coils and trip coils that operate contactors and breakers, grid and discharge resistors, and, in a related use, the conductors of a parallel set in a low-voltage cable run, where the resistance of each leg is compared to the others. Depending on the value expected, the measurement is made with a digital low-resistance ohmmeter or a precision resistance bridge. The test is quick, uses small current and does not stress the part. It is called for on breakers, cables, motors and starters, resistor banks and liquid-filled transformers that have a neutral grounding device, and it is a basic way to check that the component in the equipment is the one that was specified and that it is intact.
A resistor or a coil that has drifted from its specified resistance may not do its job. A neutral grounding resistor that has opened, lost part of its element or developed a bad bolted joint fails to limit ground-fault current as designed, and the next ground fault can be far more damaging than the system was built to allow, or the protective relaying can mis-operate. A blowout coil with shorted or open turns weakens the magnetic field that drives the arc into the chutes, so the breaker may clear a fault slowly or not at all. A trip or closing coil with the wrong resistance may fail to operate the mechanism on command or burn out from an excessive current. In a parallel cable set, a leg with a high-resistance termination carries less than its share of current while the others run hot, which can overload and damage the cables. Resistance testing finds these conditions in a de-energized state, long before the part is called on in an emergency.
The direct-current resistance of a resistor or a winding is set by the length, cross-section and resistivity of the material, and, for a coil, by the number of turns of wire. A direct current that has reached steady state sees only that resistance, and the instrument, by comparing the voltage drop with the injected current, reports the resistance. A break in the element gives an open circuit or a very high reading, and a loose or corroded joint adds a smaller series resistance. Shorted turns in a coil lower the resistance slightly, though a small number of shorted turns may be hard to see against normal variation, which is why comparison with similar coils or with prior records is so helpful. Copper and the alloys used in resistor elements change resistance with temperature, copper by a well-known fraction per degree, so readings are taken with the component at a stable and known temperature or are corrected to a reference. The use of separate current and voltage connections lets the instrument remove the resistance of leads and clamps, which matters when the resistance being measured is small. For a parallel conductor set the principle is simply that conductors of the same material, length and size, with good terminations, should show nearly equal resistance.
The equipment is de-energized, isolated, verified dead, grounded and locked out under the owner's procedures, and the component is disconnected from the rest of the circuit if other paths would otherwise run in parallel with it. A neutral grounding resistor is disconnected from the neutral and from ground at the points specified by the manufacturer before it is measured, and a coil is isolated from its control wiring. The instrument is selected for the expected range, and a resistance bridge or a low-resistance ohmmeter with four-wire leads is used for small values. Connections are made to clean bare metal, and tests are applied across each resistor or coil, or across each leg of a parallel set from the same point to the same point, so that the readings are comparable. The reading is allowed to settle, particularly on inductive coils, where the current needs time to build up, and it is recorded. The component temperature is measured or estimated, because resistance depends on it, and this is especially important on a large resistor that has been in the sun or was recently carrying current. Where a coil is on a breaker or starter, the mechanism may be in a specific position as the manufacturer directs. After the test, the connections are restored and tightened to the specified torque.
The record shows the component identified by location and type, the measured resistance of each unit or each leg of a parallel set, and the instrument used and its calibration status. It notes the component temperature at the time of the reading, the nameplate or reference resistance used for comparison, and the as-found condition, such as clean, discolored, corroded or showing signs of overheating. Any connection lifted and restored is also mentioned. When several similar components are measured together, the entries are grouped so that the comparison among them is visible, which helps a later reader see whether a difference is in the component or in the way it was measured.
This page does not give acceptance values. Results are judged by several comparisons together. The first is comparison with the nameplate rating or the manufacturer's published resistance for the component, with the temperature accounted for. The second is comparison among like items, such as the coils of identical breakers, the sections of a resistor bank or the legs of a parallel conductor set, where the readings should be close and an outlier points to a defect. The third is comparison with earlier test results on the same part, taken under comparable conditions, since a trend is more informative than a single reading. For a neutral grounding resistor, the result is checked against the design resistance that the system relies on. Open circuits and values far from the expected are failures regardless of other criteria. The tests are specified in ANSI/NETA ATS-2025 for acceptance and in ANSI/NETA MTS-2023 for maintenance, for the equipment classes named above; use your licensed copies for the governing values and wording.
At acceptance the test confirms that the supplied resistor or coil matches the specification and survived shipping and installation, and it sets the baseline. In maintenance it is repeated at intervals, often alongside other tests on the same equipment, and it is particularly useful for neutral grounding resistors that sit idle for years yet must work on the one day they are needed. On medium-voltage air circuit breakers and motor starters the blowout coil circuit is a distinct item from the trip and close coils, each with its own reference. For cables, the parallel-conductor check applies to low-voltage runs made of several conductors per phase and verifies that terminations and lengths are balanced. On liquid-filled transformers, the measurement applies to the neutral grounding impedance device and is separate from winding resistance, which tests the transformer itself. The expected value changes the instrument: a low-value resistor bank or a heavy winding calls for a low-resistance ohmmeter with four-wire leads or a precision bridge, while a small control coil of higher resistance can be read with an ordinary digital ohmmeter.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
It sits idle until a ground fault occurs and then must limit the fault current to its design level. Corrosion, broken elements or loose joints can develop unnoticed over years, so a resistance check confirms the resistor will behave as designed on the day it is needed.
It shows whether the winding is continuous and whether the resistance is consistent with the nameplate, similar coils and earlier results. An open winding reads very high, and a coil with shorted turns often reads somewhat low, although small shorted sections can hide inside normal variation.
The resistance of copper and of resistor alloys changes with temperature, so a warm component reads higher than a cool one. Recording the temperature lets readings be corrected to a reference or compared fairly with earlier tests taken under different conditions.
The resistance of each conductor in the set is measured between the same two points, with the same instrument and method, and the readings are compared. Conductors of equal size and length with good terminations should read closely alike, and a high leg suggests a poor termination or a damaged conductor.
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