Why Milliohms Matter
A high-current connection can look clean and tight while its effective contact area is reduced by wear, oxidation, contamination, poor alignment, or insufficient pressure. Even a small increase in resistance can create additional heat and voltage drop when current is high.
Contact resistance testing applies a controlled current and measures the resulting voltage drop to calculate very low resistance. The method is often used on breaker contacts, disconnects, bus joints, bolted connections, and other current-carrying paths.
Good Measurements Depend on Consistency
Lead placement, surface condition, test current, temperature, parallel paths, and connection geometry affect the reading. Test points should be repeatable, and the equipment should be in a known mechanical position. Comparing phases or identical poles can be useful, but design differences must be understood.
Acceptance criteria should come from the manufacturer, project specification, or applicable test standard. ANSI/NETA ATS provides a structured acceptance-testing framework; maintenance results should also be compared with baseline and prior data when available.
What an Abnormal Result Can Mean
A high or imbalanced reading may point to contaminated contacts, worn contact surfaces, loose or corroded joints, misalignment, inadequate contact pressure, or a measurement setup problem. The test does not identify the cause by itself, so abnormal results should trigger inspection and confirmation.
Breaker condition should also include mechanical operation and protective performance. Currentechnologies’ overview of circuit breaker testing explains why resistance is one piece of a larger decision.
- Use the same test current and connection points for trending.
- Compare like phases, poles, and assemblies.
- Document temperature and equipment position.
- Retest after cleaning, adjustment, repair, or replacement.
Why Trending Beats a Generic Limit
Manufacturer limits and industry guidance are important, but low-resistance measurements are especially powerful when the facility has a clean baseline. Comparing the same pole, phase, joint, and test points over time can reveal movement long before a generic maximum is reached.
Trend comparisons must still account for maintenance. Cleaning contacts, replacing a breaker, tightening a bus joint, or changing the test current can create a step in the data. Record those events with the measurement so a future reviewer can tell the difference between improved condition, changed hardware, and inconsistent technique.
Pair Electrical Data With Thermal Evidence
Infrared inspection under load can show whether a connection is producing abnormal heat in service. Contact resistance testing during an outage can then provide de-energized electrical evidence. Together, infrared inspection and low-resistance measurements give owners a more complete picture.
Currentechnologies can include contact-resistance measurements within acceptance or maintenance testing. To scope breaker, bus, or switchgear work for an upcoming outage, contact the team.
