Resistance and continuity
Contact resistance testing measures the resistance across the closed main contacts of a breaker, switch, contactor, recloser or transfer switch with a low-resistance ohmmeter. Worn, pitted, misaligned or contaminated contacts show up as a reading that is higher than the other poles or than earlier tests, long before they overheat in service.
Contact resistance, also called pole resistance or static contact resistance, is the resistance measured from terminal to terminal across one closed pole of a switching device. The path includes the line and load terminals, the stationary and moving contacts, and any current-carrying parts in between, such as flexible shunts, hinges and blades. A low-resistance ohmmeter, usually called a micro-ohmmeter, drives a steady direct current through the closed pole and reads the voltage drop with separate sense leads, which gives a result in the micro-ohm range. The test is made on medium-voltage and low-voltage circuit breakers, molded-case and insulated-case breakers, air, oil and SF6 breakers, reclosers, sectionalizers, circuit switchers, load interrupter and disconnect switches, switchblade and fuseholder assemblies, medium-voltage motor starters and automatic transfer switches. The result describes how well the closed contacts conduct. It does not describe how the device moves, which is checked by timing and travel tests, nor how it interrupts, which is checked by other means.
The contacts of a switching device carry the full load current whenever the device is closed. Arcing during interruption erodes them, vibration and age loosen the pressure of the springs that hold them together, dust, oil films and oxide contaminate their surfaces, and misalignment reduces the effective area of contact. Each of these raises the resistance of the pole. The heating that results is proportional to the square of the current, so a pole with only modestly elevated resistance can run hot at full load, which accelerates oxidation, raises resistance further, and may end in welded or burned contacts, a failure to carry or interrupt current, or an arc flash event. Because the damage builds quietly, the first visible sign can be a failure. Measuring pole resistance catches the trend early, during an outage, when contacts can be cleaned, adjusted or replaced at planned cost. It is also a quick installation check at acceptance, because it exposes shipping damage, assembly errors and incorrect contact adjustment before the device is energized, and it can be repeated after short-circuit duty or a major repair.
The test is an application of Ohm's law at very small values. A known direct current flows through the closed pole, a voltage drop is measured across it with sense leads separate from the current leads, and the instrument displays the ratio. Four-wire sensing removes the resistance of the leads and their contact with the terminals. The total reading is made up of the resistance of the conductors and the contact resistance at each interface, where current has to squeeze through a few tiny contact spots. The effective contact area depends on the pressure holding the contacts together, their surface condition and their alignment, so degradation of any of these raises the reading. Because the same design and similar current paths repeat in each pole, the three poles of a healthy device read close to one another, and a pole that stands apart suggests a local problem. The direct current used must be high enough to break down thin surface films and give a stable reading, which is why instruments for this work supply a substantial test current. Temperature changes the resistance of metals slightly, so conditions are noted.
The device is de-energized, isolated from all sources, proven dead and grounded under the owner's procedures, and stored energy in springs and capacitors is released. A drawout device is racked to the test position or removed, and the contacts are closed by the device's normal means, mechanically or electrically as the procedure allows. The instrument is chosen for low-resistance work, with a current high enough to give a stable reading, and its calibration and leads are checked. Current leads are clamped on the line and load terminals of one pole, with the sense leads inside them so that the measured path covers the contacts and the parts between the terminals. A steady reading is recorded, and the same is done for each pole. Where the construction allows, the measurement is repeated across individual elements, such as each interrupter on a vacuum starter, each blade on a switch or each fuseholder, to locate a problem more precisely. Terminals and surfaces under the clamps are cleaned so the clamp contact does not contribute. Operating the device a few times before the final measurement is sometimes called for to seat the contacts, as the manufacturer's instructions say. The same instrument, current and lead positions are used on later tests so results can be compared.
The record identifies the device and each pole or element tested and gives the resistance reading for each, the instrument and its test current setting, the calibration status of the instrument and the date. It adds the as-found state of the contacts and terminals where they were visible, such as clean, pitted, discolored or showing signs of heating, the ambient temperature, and whether the device was in the test position or removed. Notes cover any cleaning, adjustment or exercise operations done before or after the reading and any retest. These details let the next technician compare like with like, and they provide the baseline against which later changes are read.
This page states no limit. Pole resistance is judged by comparison. The first comparison is among the poles of the same device, where a pole that deviates from the others by more than the published proportion calls for investigation. The second is with the manufacturer's data for the device, which often gives a maximum for the design. The third is trending against the baseline from acceptance or earlier maintenance, where a steady rise matters even when the pole still resembles the others. The governing criteria and the way they are applied by device type are published in ANSI/NETA ATS-2025 for acceptance testing and ANSI/NETA MTS-2023 for maintenance testing. A pole that fails the comparison is inspected, cleaned or adjusted and measured again, and contact replacement may follow. Results are read together with timing, travel and insulation tests and with visual and infrared findings. For the values, consult your own licensed copies of the standards and the manufacturer's instructions.
At acceptance the test confirms that the contacts arrived and were assembled correctly, and it establishes the baseline for every pole. In maintenance it is mainly a trend tool, so repeating the same test current and lead positions is more important than any one number. The equipment class changes the practice. Low-voltage molded-case breakers are measured pole by pole across the closed breaker, and the readings are small and sensitive to the lead clamping. Medium-voltage vacuum breakers and starters have sealed interrupters, so the reading covers the bottle and its connections. Oil and SF6 breakers carry resistance across the interrupter assembly, and the result is compared with the manufacturer's data. Air and switchblade devices expose their contacts, so the visual condition adds to the reading. Automatic transfer switches are measured on both sources. Where a device has arcing and main contacts in parallel, the measured value reflects the combination. Contact resistance is related to, but different from, dynamic contact resistance, which watches the resistance during operation, and from bolted connection resistance, which covers the fixed joints.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
Contact resistance measures how well the closed contacts conduct, with a very small expected value, while insulation resistance measures how well the insulation blocks current, with a very large expected value. The two tests look for different faults and are both normally part of breaker and switch testing.
A small current can fail to break through thin oxide or oil films on the contacts and give an unstable or falsely high reading. A suitable instrument supplies a substantial direct current and senses the voltage separately, so it reads the real contact resistance and not the leads.
Against the other poles of the same device, the manufacturer's data for that model, and the previous readings on the same pole. A pole that stands apart from the others or has risen over time is flagged, and the published criteria in the standards define the difference that matters.
Sometimes. Light contamination can be removed by cleaning, and low pressure or misalignment can be corrected by adjustment, following the manufacturer's instructions. If the contacts are pitted, eroded or burned, replacement is normally required, and the pole is measured again after the work.
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