Contact Resistance and Micro-Ohm Testing in Substations

Contact resistance testing measures the DC resistance of a closed electrical connection, a circuit breaker in the closed position, a bolted bus joint, a disconnect switch jaw and blade, in micro-ohms. The values involved are small, typically in the range of 10–300 µΩ for well-maintained connections, which is why a standard ohmmeter is useless for the purpose. A dedicated low-resistance ohmmeter (DLRO), often called a Ducter after the Megger instrument that made the test common, applies a known high DC current (typically 100A or 200A) through the connection and measures the resulting voltage drop with a precision meter. Ohm's law gives the resistance directly from the ratio.

The test is non-destructive, fast, and required by NETA ATS for virtually every piece of switchgear at acceptance. It is also one of the most diagnostically reliable tests in the substation toolkit, contact resistance correlates directly with heat generation under load and with the let-through resistance during a fault, both of which have direct safety and equipment reliability consequences.

Why contact resistance matters

Every resistance in a current-carrying path dissipates power as heat, P = I²R. At load current, even a modest contact resistance increase produces significant heating. A circuit breaker main contact that tests at 200 µΩ instead of the acceptable 50 µΩ carries four times the resistance, which means four times the heat generation at the same current. At 1200A continuous, that 200 µΩ contact is dissipating 288 watts, enough to bake insulation, degrade lubrication, and drive the contact into a self-reinforcing cycle of oxidation and pitting that accelerates further resistance increase.

The arc flash consequence is less obvious but equally important. A breaker with high contact resistance has higher impedance in the fault current path. Under bolted fault conditions, this reduces the available fault current slightly, but for an arcing fault, where the arc gap itself provides most of the impedance, elevated contact resistance has minimal effect on clearing. What it does affect is the breaker's ability to interrupt: pitted, oxidized main contacts that show elevated resistance may also have degraded surface geometry that increases arc energy during interruption and reduces interrupting capability below nameplate.

For bus joints and cable terminations, the heat issue dominates. An infrared thermography scan will eventually find a hot joint under load, but micro-ohm testing at acceptance finds the problem before the joint is ever energized. Catching a poorly torqued or contaminated bolted connection at commissioning costs a torque wrench and five minutes. Finding it three years later via a thermal event costs a switchgear bay.

The four-wire (Kelvin) measurement method

All accurate low-resistance measurement uses a four-wire connection, also called a Kelvin connection. Two leads carry the test current; two separate leads measure the voltage. Because the voltage leads carry essentially no current, the resistance of the voltage leads themselves does not affect the measurement, only the resistance of the path between the voltage measurement points is captured. This is critical at micro-ohm levels, where lead resistance of a milliohm or two would completely swamp the measurement of interest.

Test sets with four-wire connections have two pairs of clamps or probes, typically color-coded C1/C2 for current and P1/P2 for potential (voltage). The current clamps are placed outside the voltage clamps in all cases. For a circuit breaker contact measurement, the current clamps attach to the line and load terminals of the breaker and the voltage clamps attach just inside them, straddling only the contact path. For a bus joint, the current clamps go on the bus conductors on either side of the joint and the voltage clamps measure across the joint itself.

Test current level and its effect on results

DLRO instruments apply DC test currents ranging from 1A on small instruments to 600A on large units. The most common test currents for substation work are 100A and 200A. The test current level matters for two reasons.

First, a higher test current reduces the signal-to-noise ratio for very low resistances, improving measurement repeatability. A 10 µΩ contact tested at 10A produces a 100 µV signal, easily corrupted by thermoelectric EMF noise in the test circuit. The same contact at 100A produces a 1 mV signal, far more reliable. NETA ATS requires the test be performed at a current level sufficient to produce stable, repeatable readings, 100A is the standard minimum for substation switchgear.

Second, the test current must not saturate a current transformer in the test path. For measurements taken with the CT in the circuit (which should be avoided where possible), test current should not exceed the CT's rated primary current by a large margin. The better practice is to isolate the measurement path from CTs before testing.

Equipment tested and NETA acceptance criteria

Circuit breaker main contacts are the primary application for micro-ohm testing. NETA ATS Table 100.1 (molded case circuit breakers) and Table 100.5 (medium-voltage circuit breakers) specify that contact resistance shall be compared to the manufacturer's published values. In the absence of manufacturer data, NETA's rule of thumb is that contact resistance should not exceed 200% of the average of the three phases, meaning no single phase should read more than twice the average. For most MV circuit breakers, manufacturer specifications call for main contact resistance in the range of 50–150 µΩ depending on breaker rating and design.

Bus connections and cable terminations are tested at acceptance for new installations and after any re-termination. NETA MTS does not set a single absolute threshold for bolted joints, but the accepted practice is that a joint should not measure more than 10% above the resistance of an equivalent length of bare conductor, meaning the joint itself adds no more than 10% to the baseline conductor resistance. In practice, a well-made bolted joint is essentially invisible in the measurement; resistance within a few micro-ohms of the conductor baseline is expected.

Disconnect switch and GOAB switch jaw contacts are tested at acceptance and at major maintenance intervals. Acceptance criteria follow manufacturer specifications, with typical values in the 50–200 µΩ range for distribution-class switches. Phase-to-phase symmetry matters here as much as absolute value, a three-phase switch where one phase reads 50 µΩ and another reads 300 µΩ has a problem on the high phase even if 300 µΩ is below some threshold, because the asymmetry indicates a condition difference that will worsen under load cycling.

Transformer main and LTC contacts are also tested as part of winding resistance measurement programs. Elevated contact resistance in an LTC is identified by comparing tap-to-tap winding resistance across all positions, a high resistance on one or more tap positions relative to adjacent taps indicates a selector or arcing contact problem at those positions. This is covered in more detail in our article on winding resistance and excitation current testing.

Temperature effects and correction

Contact resistance is temperature-dependent. Copper and aluminum conductors increase in resistance with increasing temperature (positive temperature coefficient), so a measurement taken at 35°C will read higher than the same joint at 20°C. For trending purposes, measurements taken at significantly different ambient temperatures should be corrected to a common reference temperature using the same coefficient used for winding resistance correction. For copper, resistance at temperature T is R(T) = R(20°C) × (234.5 + T) / 254.5. For aluminum, the constant is 228.1 instead of 234.5.

In practice, most acceptance testing compares results against manufacturer values at the test temperature rather than correcting to a standard, because the manufacturer values are typically given at ambient rather than a specific reference. The important thing is to record the conductor temperature alongside every micro-ohm result so that future maintenance measurements can be meaningfully compared.

Common failure modes detected

Elevated contact resistance on a circuit breaker almost always traces to one of three causes: oxidation or contamination on the contact surfaces (particularly on silver-plated contacts where the plating has worn through), mechanical misalignment that reduces the contact area or wipe, or spring degradation that reduces contact pressure. All three are correctable at maintenance, oxidized contacts can be cleaned, alignment can be adjusted, and springs can be replaced, but only if the test identifies the problem before the equipment returns to service.

High resistance on a bolted bus joint is typically a torque issue, a contaminated faying surface, or a wrong hardware selection (using standard bolts instead of structural bolts on a high-force joint). Re-torquing with a calibrated torque wrench after cleaning the faying surface usually resolves it. High resistance that persists after re-torquing points to surface damage, galvanic corrosion at an aluminum-copper interface without proper anti-oxidant compound, or a defective bus bar that needs replacement.