If you work in the power distribution or transmission space, you already know that the contact resistance of a vacuum circuit breaker (VCB) is one of the most critical performance metrics there is. I’ve been in the vacuum circuit breaker supply business for over a decade, and I can’t tell you how many times I’ve seen a project delayed, or worse, an entire site go down, because someone skipped proper contact resistance testing on a VCB. Early on in my career, I learned that this isn’t just a box to check during commissioning—it’s a non-negotiable step to ensure reliability, safety, and compliance. Today, I want to walk you through exactly how we test contact resistance on our VCBs, the standards we follow, the common pitfalls we see, and why this step matters more than most people realize. Vacuum Circuit Breaker
First, let’s get clear on what contact resistance actually is, because too many people mix it up with other types of resistance in a circuit breaker. The moving and fixed contacts inside a VCB are the parts that make and break current flow, right? When they close, they form a physical connection, but no two surfaces are perfectly smooth. There’s tiny surface roughness, micro-oxidation, and even small amounts of debris trapped between them—all of which create a resistive barrier. Contact resistance is the total resistance of that barrier plus the resistance of the contact material itself and the tiny current paths that form when the two surfaces press together. If that resistance is too high, two bad things happen: first, the contacts overheat under load, which can soften the contact material, cause pitting, and eventually lead to welding of the contacts mid-operation. Second, excessive heat can degrade the insulation around the breaker, leading to partial discharge or full breakdown. That’s why every VCB we ship from our facility goes through contact resistance testing—we don’t leave that step to the field.
Now, before we get into the testing process, it’s important to know the standards that govern this. The two main ones I lean on every single day are IEC 62271-100 and ANSI C37.06. IEC 62271-100 is the international standard for high-voltage switchgear and controlgear, and it specifies that for VCBs rated up to 52 kV, the contact resistance must be measured at the rated current of the breaker—wait, that’s a key point: testing at low current just doesn’t give accurate results. When you test at low current, you’re only measuring the tiny, scattered current paths between the contacts, not the full-area contact that forms when the breaker is carrying its rated load. That’s why so many field tests get misleading data. ANSI C37.06, the North American standard, echoes this, and even adds that resistance must be measured immediately after a close operation, because contact seating changes right after making. For our VCBs, we calibrate every test device to meet IEC and ANSI specs, because we won’t ship a unit that doesn’t comply with both, no matter the region it’s going to.
Now, let’s walk through the actual testing process, step by step. First, you have to prepare the test setup correctly. A lot of technicians skip this part and wonder why their numbers are off. The first thing is ensuring the VCB is in perfect working order: it’s been cleaned, the contacts are free of debris, and the mechanism is lubricated per our maintenance manual. Then, you need a suitable test instrument. For rated current testing, you can’t use a multimeter—those only put out milliamps, which is too low. You need a micro-ohmmeter, specifically one that can supply at least 10% of the breaker’s rated current for low-voltage units, or full rated current for higher voltage VCBs. Wait, let me clarify: for medium-voltage VCBs (up to 38 kV), we typically use a micro-ohmmeter that supplies 100 A of current, which is enough to replicate the current paths under load. For higher voltage units, we use test sets that can push up to 1000 A, but even then, we don’t need the full rated current if we follow IEC guidelines—10% is sufficient for most medium-voltage applications.
Next, connecting the test leads is critical. This is where half the errors happen. You have to use the four-wire (Kelvin) connection method, not the two-wire method that multimeter users default to. Why? Because two-wire measurements include the resistance of the test leads themselves, which can be as high as a few milliohms—enough to throw off a contact resistance reading that’s supposed to be in the range of 50 to 200 microhms for a typical 12 kV VCB. With four-wire Kelvin, two leads supply the test current to the contacts, and two separate leads measure the voltage drop across the contact area only, so lead resistance is eliminated. I always tell our technicians to connect the current leads as close to the contact terminals as possible, and the voltage leads even closer, right at the contact faces if possible. That small extra step cuts out almost all measurement error.
Once the leads are connected correctly, you need to perform the test in the right sequence. First, you should close the VCB three times consecutively, waiting at least 30 seconds between each close. Why? This seats the contacts properly. The first close often has the contacts not fully mated, with small gaps, but after two or three make-and-break operations, the contacts settle into their ideal position, and the contact resistance stabilizes. We see this every time in our in-house testing: the first reading might be 150 microhms, the second 120, and the third comes in at 90, which is the true value. If you only take one reading, you’ll get a number that’s higher than actual, and might incorrectly reject a good unit. After the three closes, you immediately take three consecutive resistance measurements, waiting 10 seconds between each to let the test settle. We then average those three numbers to get the final contact resistance value for the unit.
Now, what’s an acceptable contact resistance, anyway? That’s not a one-size-fits-all answer. It depends on the voltage class, rated current, and contact material. Our VCBs use copper-chromium contacts, which are standard for vacuum breakers because they have excellent arc resistance and conductivity. For a 12 kV, 1250 A VCB, our target contact resistance is between 70 and 90 microhms. For a 24 kV, 2000 A unit, it’s between 50 and 70 microhms, because higher current ratings require larger contact surfaces to handle the load. If a unit comes in at more than 100 microhms for the 12 kV, 1250 A class, we mark it for rework—either we clean the contacts again, or we check the mechanism to make sure the contact pressure is correct. Contact pressure is a huge factor that people overlook: if the breaker’s spring mechanism isn’t adjusted to the right force, the contacts won’t press hard enough, leading to higher resistance. We test contact pressure separately as part of the same process, because even a perfectly matched pair of contacts will have high resistance if they’re not pressed together with the right force.
Now, let’s talk about in-field testing, because a lot of our customers ask if the same process applies when they’re doing maintenance on a VCB that’s already installed. The short answer is: mostly, but you have to account for site conditions. When testing a VCB in the field, you don’t have access to full rated test current, so you use a lower current micro-ohmmeter, but you have to account for that by correcting the reading. IEC 62271-100 has a correction formula for current: if you test at current I_test, the corrected resistance is R_measured * (I_rated / I_test)^0.5, because contact resistance is inversely proportional to the square root of the test current. That’s a simple step that keeps field test results consistent with factory test results, so you can compare apples to apples. Another field pitfall is loose terminal connections on the breaker or the test leads themselves. We always tell our customers to check all connections before testing, because a loose lead can add 10 microhms of resistance, leading them to think the VCB has a bad contact when it’s just a bad connection.
I’ve seen so many avoidable issues come from skipping proper contact resistance testing. A few years back, we had a customer in a remote mining site that installed a VCB from a competitor, didn’t test contact resistance, and six months later, the breaker contacts overheated so badly that they melted, taking the entire mining operation offline for three days. When they brought the old breaker to us for analysis, we measured its contact resistance at 300 microhms—way above the acceptable range for its class. That’s the kind of failure that costs thousands of dollars in downtime, and it’s completely preventable with proper testing. On the flip side, all of our VCBs come with a full factory test report that includes contact resistance data, so our customers have that baseline for future maintenance.
Now, let’s get into the common mistakes I see people make when testing contact resistance. The first is using the two-wire method, like I mentioned earlier. It’s a cheap mistake, but it leads to inaccurate readings. The second is testing only once, right after the first close. As we saw, contacts need time to seat, so the first reading is almost always higher. The third is testing at too low a current. If you use a 1 A micro-ohmmeter on a 2000 A VCB, you’re going to get a reading that’s artificially high, because the tiny test current only makes contact at the highest points of the surfaces, not the full area under load. The fourth mistake is not accounting for contact pressure. A lot of technicians assume the breaker mechanism is set correctly, but over time, springs can weaken, so checking contact pressure is part of the contact resistance test. We supply pressure gauges with all our VCBs to make this easy for our customers. The fifth mistake is not doing the test at the right time. Contact resistance can be affected by temperature, so we always test at ambient room temperature in the factory, and we tell our customers to test when the breaker is at the same temperature as the site, or adjust readings for temperature per IEC guidelines.
Another thing I want to emphasize: contact resistance testing isn’t just for new VCBs. It’s a key part of predictive maintenance too. Every three to five years, depending on the operating environment, you should re-test contact resistance on installed VCBs. If you see a 20% increase in resistance from the baseline factory reading, that’s a warning sign that something is wrong—maybe debris got into the contact chamber, or the contact pressure dropped, or oxidation started building up on the contact surfaces. Catching that early can mean a simple cleaning or adjustment, instead of a full replacement or a catastrophic failure.
At our company, we stand behind every VCB we sell, and that starts with rigorous contact resistance testing. We don’t cut corners—our in-house testing lab has 10 micro-ohmmeters calibrated annually to IEC standards, and every VCB undergoes three contact resistance tests before it’s packed and shipped. We also provide our customers with a free training session on contact resistance testing when they take delivery of a new VCB, because we want them to do it right in the field too. If you’re a project manager, electrical engineer, or maintenance technician working with vacuum circuit breakers, you know that reliability is non-negotiable, and contact resistance is the foundation of that reliability.
If you have questions about testing contact resistance on your existing VCBs, or if you’re in the market for new VCBs with factory-tested contact resistance, we’re here to help. We can provide detailed test reports, on-site testing support, and training to make sure you’re doing contact resistance testing correctly every time. Don’t take chances with your power system—proper contact resistance testing is the best way to prevent downtime and keep your operations running smoothly. Reach out to us to discuss your VCB needs and get the support you need to ensure your equipment performs as expected.
Capacitor References
IEC 62271-100: High-voltage switchgear and controlgear – Part 100: Alternating current circuit breakers
ANSI C37.06: AC high-voltage circuit breakers rated on a symmetrical current basis
IEEE Std C37.09: Standard Test Procedure for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis
Jingdezhen Wanping Electric Co., Ltd.
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