{"id":3658,"date":"2026-10-27T15:25:25","date_gmt":"2026-10-27T07:25:25","guid":{"rendered":"http:\/\/www.gramstories.com\/blog\/?p=3658"},"modified":"2026-10-27T15:25:25","modified_gmt":"2026-10-27T07:25:25","slug":"what-is-the-accuracy-class-of-a-partial-discharge-tester-48a9-5644be","status":"publish","type":"post","link":"http:\/\/www.gramstories.com\/blog\/2026\/10\/27\/what-is-the-accuracy-class-of-a-partial-discharge-tester-48a9-5644be\/","title":{"rendered":"What is the accuracy class of a Partial Discharge Tester?"},"content":{"rendered":"

As a partial discharge (PD) tester supplier, I\u2019ve spent thousands of hours walking clients through why accuracy class isn\u2019t just a technical box to tick\u2014it\u2019s the single biggest factor that turns a PD test from a quick check into a reliable tool for protecting high-voltage equipment. I\u2019ve seen clients overlook this detail, only to end up with misleading test results that cost them thousands in unplanned maintenance, or worse, a critical asset failure mid-operation. Today, I\u2019m breaking down what accuracy class actually means for PD testers, how it\u2019s measured, and why it should be non-negotiable for anyone buying this equipment. Partial Discharge Tester<\/a><\/p>\n

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First, let\u2019s get the basics straight: Partial discharge testing is all about spotting tiny, invisible flaws in high-voltage gear\u2014think transformers, switchgear, cables, and insulators. Those micro-flaws (like a tiny bubble in insulation or a loose metal connection) may seem harmless, but when voltage runs through them, they spark tiny discharges that can eat away at insulation over years, leading to catastrophic failure. A PD tester measures the magnitude of these discharges, usually in picocoulombs (pC)\u20141 pC is one-trillionth of a coulomb, a unit of electrical charge. That\u2019s so small, even a 1% error in measurement can make a discharge that\u2019s actually dangerous look insignificant, or a harmless spark look like a ticking time bomb.<\/p>\n

Which brings us to accuracy class. In simple terms, accuracy class is a standardized way to define how close a tester\u2019s reading is to the true value of a PD signal. For electrical testing equipment, accuracy class is almost always expressed as a percentage, usually following standards set by organizations like the International Electrotechnical Commission (IEC) or Institute of Electrical and Electronics Engineers (IEEE). For PD testers specifically, the IEC 60270 standard is the gold rule here\u2014it lays out exactly how accuracy class is defined for partial discharge measurement systems, not just the tester itself.<\/p>\n

Wait, let\u2019s clarify that: a lot of people mix up the tester with the entire measurement system. The system includes the PD tester, the couplers (sensors that attach to the high-voltage equipment), and the cables connecting them. Accuracy class applies to the whole system, not just the display unit you hold in your hand. That\u2019s a key mistake I see new clients make when shopping around: they fixate on the price of the handheld unit, only to find the whole system has a poor accuracy rating because of cheap couplers or long, lossy cables that distort the signal before it even reaches the tester.<\/p>\n

So what do those percentage numbers mean, exactly? The most common accuracy classes for PD testers are 1%, 2%, and 5%. Let\u2019s break each down with practical examples, because numbers on a spec sheet don\u2019t mean much until you apply them to real-world use cases.<\/p>\n

A 1% accuracy class system is the most precise, reserved for labs or critical assets where even small errors can have huge consequences. If your test finds a PD reading of 100 pC, a 1% accuracy class means the true value is somewhere between 99 and 101 pC. For utilities testing a 500kV transformer at a power plant, that margin of error matters. A reading of 100 pC is already a cause for monitoring; if your system was only accurate to 5%, you might think the actual value is 95 pC and skip a follow-up test, when in reality it\u2019s 105 pC\u2014still within the \u201cmonitor\u201d range, but if you had a 1000 pC reading, a 1% class gives you a 10 pC error, while a 5% class gives a 50 pC error, which could change whether you schedule a repair.<\/p>\n

Next, the 2% accuracy class. This is the sweet spot for most industrial and mid-voltage applications\u2014commercial substations, medium-voltage transformers, offshore wind turbine cables, and even manufacturing facilities with high-voltage machinery. For 2% class, a 100 pC reading has a 2 pC margin, so true value is 98-102 pC. That\u2019s more than enough precision for routine maintenance testing, which is what 90% of our clients do. We\u2019ve had clients with 5% class systems come to us because they were getting inconsistent results on their 11kV switchgear\u2014sometimes their tester would show 80 pC, then 120 pC on the same test, and they couldn\u2019t tell if the equipment was actually faulting or if the equipment was faulty. A 2% accuracy system eliminates that variability, so their team can trust the readings to make decisions.<\/p>\n

Then there\u2019s the 5% accuracy class. This is the lowest standard you\u2019ll find in most consumer or entry-level PD testers, marketed to small businesses or hobbyists. At 5%, a 100 pC reading has a 5 pC margin, so true value is 95-105 pC. That\u2019s fine for educational use or quick, non-critical checks, but I never recommend it for anyone responsible for high-voltage assets. We had a client last year who bought a cheap 5% class tester to test their factory\u2019s 33kV cables. They got a reading of 150 pC, so they scheduled a shutdown to replace the cable\u2014only to find out after sending the cable to a lab that the true reading was 142 pC, which is well within the acceptable limit for that type of cable. They wasted 3 days of production and $20,000 on unnecessary replacement, all because they skipped checking the accuracy class.<\/p>\n

Now, how is accuracy class actually tested? Because a manufacturer can\u2019t just say \u201cour tester is accurate\u201d without proof. The IEC 60270 standard sets out a formal calibration process where a PD system is tested against a reference signal with a known, traceable value. That reference signal is generated by a calibrated standard pulse generator, which produces PD signals of known magnitudes (10 pC, 100 pC, 1000 pC, etc.). The system is tested across different voltage levels, different signal frequencies, and different environmental conditions (temperature, humidity, noise) to make sure the accuracy holds up in the real world, not just a controlled lab.<\/p>\n

One thing I always emphasize to clients: accuracy class isn\u2019t a one-time test. PD systems can drift over time. The couplers can degrade from exposure to heat or harsh weather, the cables can develop signal loss, and the tester\u2019s internal components can age. That\u2019s why regular recalibration is required\u2014usually every 12 to 24 months, depending on how often the tester is used and the conditions it\u2019s used in. A 1% accuracy class system that hasn\u2019t been calibrated in 3 years might actually be a 3% system, which defeats the purpose of buying the higher precision.<\/p>\n

Wait, there\u2019s also a common myth that higher accuracy class always means a better tester. That\u2019s not true. For example, a 1% accuracy system might be more sensitive to electrical noise than a 2% system, which can be a problem in industrial environments where there\u2019s lots of other electrical equipment running. Noise can distort PD signals, so a higher precision system that\u2019s prone to noise might give you more false positives than a slightly less accurate, more noise-resistant one. It\u2019s a balance, not a race to the highest percentage. That\u2019s why our team works with each client to match the accuracy class to their specific use case, instead of pushing the most expensive option.<\/p>\n

Let me give you a real example. A few months ago, a wind farm operator reached out to us. They had 20 offshore wind turbines, each with 33kV cables, and they were using a 1% accuracy system to test the cables. But they were getting so much electrical noise from the turbines\u2019 converters that the system would flag every test as a fault. We swapped them to a 2% accuracy system with noise filtering designed for turbine environments, and their test results went from 70% false positives to under 5%. They saved thousands in unnecessary repair crews and got accurate data to schedule maintenance. That\u2019s when accuracy class matters most\u2014not in a vacuum, but in context.<\/p>\n

Another point: accuracy class is different from resolution. Resolution is the smallest change a tester can detect, like 0.1 pC. That sounds impressive, but if the accuracy is 5%, a change of 0.1 pC is meaningless because your margin of error is 5 pC. So a tester with high resolution but low accuracy is like a ruler with markings every 0.1 mm, but that\u2019s off by 1 cm\u2014you can measure to the mm, but the whole ruler is wrong. Resolution is a nice spec to have, but accuracy class is what determines if the measurement is actually useful.<\/p>\n

Now, let\u2019s talk about how to verify the accuracy class when you\u2019re shopping for a PD tester. First, don\u2019t just take the manufacturer\u2019s word for it. Ask for a calibration certificate that\u2019s traceable to a national standard, like those from the National Institute of Standards and Technology (NIST) in the US, or the UK\u2019s National Physical Laboratory (NPL). That certificate will show the actual accuracy of the system across different test signals, not just a generic \u201c1% accuracy\u201d label. If a manufacturer can\u2019t provide that, walk away. It means they didn\u2019t have the system tested to the required standard.<\/p>\n

Second, ask about the entire system, not just the tester. A lot of low-cost vendors will label their handheld unit as \u201c1% accuracy\u201d even if the coupler is a cheap, uncalibrated part that adds 5% error to the measurement. I\u2019ve seen this happen so many times\u2014vendors advertising high accuracy on the display, but the whole system is way off. Always confirm that the accuracy class applies to the complete measurement system: tester, couplers, cables, and accessories.<\/p>\n

Third, check for how the accuracy performs under load. PD testing isn\u2019t done at zero voltage\u2014it\u2019s done at operating voltage, or slightly above it to simulate real conditions. A tester might have great accuracy at low test voltages, but when you crank it up to 33kV or 50kV, the accuracy drops off. Look for test data that shows accuracy across the full range of voltages you\u2019ll be testing.<\/p>\n

I mentioned earlier that accuracy class is critical for protecting assets. Let\u2019s circle back to that because it\u2019s the reason this matters more than just a technical specification. A PD fault that starts at 100 pC will grow over time, usually at a rate of 10-20% per year. If you catch it when it\u2019s 100 pC, you can schedule a repair before it gets to 1000 pC, which is the threshold for immediate action. If your system has 5% accuracy, and you get a reading of 100 pC, the true value could be 105 pC\u2014hardly a difference. But if the reading is 900 pC, the true value could be 855 pC or 945 pC, which is the difference between \u201cmonitor quarterly\u201d and \u201cshutdown next week.\u201d That gap is why accuracy class isn\u2019t just a number\u2014it\u2019s a safety net.<\/p>\n

Now, let\u2019s address a common question we get: can you upgrade the accuracy class of an existing PD tester? The short answer is, usually not. The accuracy class is baked into the design of the system\u2014from the components in the tester, to the quality of the couplers, to the calibration methods used during manufacturing. You can recalibrate it to maintain its accuracy class, but you can\u2019t turn a 5% system into a 2% one by adjusting settings. That\u2019s why it\u2019s better to get the accuracy class right from the start, instead of outgrowing it later and having to replace the whole system.<\/p>\n

What about for small operations? If you\u2019re a small commercial facility testing a handful of medium-voltage transformers a few times a year, is a 1% class system necessary? Probably not. A 2% class system will give you all the precision you need, at a price point that makes sense. We often work with small businesses to put together tailored packages with 2% accuracy systems that include basic noise filtering, so they get reliable results without paying for features they don\u2019t need.<\/p>\n

At the end of the day, the goal of any PD test is to make informed decisions. A tester with a clear, verifiable accuracy class is the foundation of that. It\u2019s not the only factor to consider\u2014noise immunity, ease of use, durability in the field, and after-sales support all matter\u2014but accuracy class is non-negotiable. Skip it, and you\u2019re not just wasting money on a tester that doesn\u2019t work; you\u2019re putting your people, your assets, and your business at risk.<\/p>\n

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If you\u2019re shopping for a partial discharge tester, or you have an existing system and want to confirm its accuracy, our team is here to help. We work with everyone from small commercial facilities to large utility companies to match them with the right PD measurement system, explain accuracy classes in plain language, and ensure their equipment is calibrated and ready for reliable testing. Don\u2019t let misleading specs or overlooked accuracy class lead to costly mistakes\u2014reach out to us to discuss your specific needs, and we\u2019ll help you find the right solution.<\/p>\n

Transformer Oil Tester<\/a> References<\/p>\n

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  1. International Electrotechnical Commission (IEC). IEC 60270:2019, High-voltage test techniques \u2013 Partial discharge measurements. Geneva, Switzerland: IEC, 2019.<\/li>\n
  2. Institute of Electrical and Electronics Engineers (IEEE). IEEE Std 400.2-2019, IEEE Guide to Partial Discharge Measurements for Rotating Machinery. Piscataway, NJ: IEEE, 2019.<\/li>\n
  3. National Physical Laboratory (NPL). \u201cCalibration Services for Partial Discharge Measurement Systems.\u201d Teddington, UK: NPL, 2022.<\/li>\n
  4. Chen, L., et al. \u201cAccuracy Evaluation of Portable Partial Discharge Measurement Systems for Industrial Applications.\u201d IEEE Transactions on Dielectrics and Electrical Insulation, vol. 28, no. 3, 2021, pp. 987-994.<\/li>\n<\/ol>\n
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    Refine On (Hebei) Electric Power Technology Co., Ltd.<\/a>
    Refine On (Hebei) Electric Power Technology Co., Ltd. is one of the most reliable partial discharge tester manufacturers and suppliers in China, also supports customized service. Please feel free to wholesale advanced partial discharge tester at competitive price from our factory. Contact us for quotation.
    Address: Building 13, Liandong U Valley, No.64, Jing SAN South Street, Economic Development Zone, Mancheng District, Baoding City, China
    E-mail: victor@transformer-test.com
    WebSite:     https:\/\/www.transformer-test.com\/<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

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