Hey everyone, this is Mia from the product team at our Potential Transformer supply company, and today I want to talk about something that comes up in our service calls more than you’d think: potential transformers (PTs) with low accuracy. If you’re working in power distribution, energy metering, or industrial grid maintenance, you know how critical PT accuracy is—one small error throws off revenue metering, load monitoring, protection relays, and even grid stability studies. Over the past 7 years I’ve been supporting field teams and end users, I’ve walked through dozens of these accuracy dips, and today I’m breaking down the most actionable, science-backed solutions we’ve helped clients implement, straight from our hands-on experience as a PT supplier. Potential Transformer
First, let’s get clear on what “low accuracy” actually means for a PT. A PT’s accuracy is measured by its error percentage—for example, a 0.2-class PT can only have a total error of 0.2% at rated load, per IEC 61869 standards. When we get a call from a client saying their PT is coming in at 0.8% or higher, that’s the problem we’re solving. More often than not, it’s not a “fault” in the PT itself, but an installation or application mismatch. That’s the first thing I tell clients: don’t jump to replacing your PT yet. Let’s work through the root cause step by step.
The first, and most common, solution is auditing and correcting installation-related issues. Most field accuracy problems start at installation, not from the PT being defective. Let’s break this down. First, check for loose or corroded connections on both the primary (high-voltage) and secondary (low-voltage) terminals. I remember a client in Ohio a couple of years ago—they had a PT on a substation feeder that was showing 1.1% error, which meant they were underbilling their industrial client by thousands of dollars a month. When our techs went out to inspect, they found that the secondary terminal lugs had a thin layer of galvanic corrosion from salt air off Lake Erie. Even a tiny layer of corrosion increases contact resistance, which creates voltage drop that throws off the PT’s output. We used anti-oxidant compound and tightened the terminals to the manufacturer’s specified torque (12 Nm for that 300V secondary PT), and their error dropped to 0.18% within 24 hours. That’s a fix that cost less than $50 in parts, not a new PT.
Another installation mistake: incorrect wiring polarity. PTs are polarized components—reverse any of the primary or secondary leads, and you get a phase angle error that directly impacts accuracy. We had a client in Texas who installed a new 12kV PT as part of a grid expansion project, and their electrician swapped the S1 and S2 secondary leads. When they tested it, the error was way off, and their revenue metering system was showing negative power flow during peak hours. We sent our field guide, which walks through polarity testing with a phase angle meter per IEEE C57.13, and they corrected the wiring in 2 hours, resolving the accuracy issue. Also, make sure the secondary wiring size matches the PT’s rated current. If you’re running secondary leads that are too small (like 14AWG for a PT with 5A secondary rated current), the resistance of the wire creates a load on the PT beyond its design, increasing error. We always recommend using 12AWG copper for secondary runs under 50 feet, and 10AWG for runs over 50 feet, to keep voltage drop below 0.1% of the rated secondary voltage.
Next, verifying PT rating matching to your application. This is where a lot of people go wrong—they buy a PT that’s technically “compatible” but not rated for their specific operating conditions. First, check the PT’s rated primary voltage. If your grid’s nominal voltage is 12.47kV, you can’t use a PT rated for 12kV, because it will be overexcited at normal operating voltage, leading to core saturation and high error. I learned this the hard way early in my career—we sold a PT to a small solar farm that was connected to a 12.47kV grid, and they told us a month later their metering accuracy was off. We realized they’d ordered a 12kV PT, not 12.5kV, so we swapped it for a 0.2-class 12.5kV PT, and their error went back to spec immediately.
Then there’s the PT’s accuracy class and burden rating. The burden is the total load on the secondary circuit (meters, relays, transducers) measured in VA. Each PT has a maximum burden rating, and accuracy is only guaranteed up to that point. For example, a PT rated for 0.2 accuracy at 10VA burden will have much higher error if you add another relay that draws 8VA, pushing total burden to 18VA. We had a mining client who installed three protective relays on a single PT secondary circuit, and their burden hit 22VA—way over the PT’s 15VA rated burden. We recommended either splitting the loads across two PTs, or upgrading to a PT with a 30VA burden rating, and after implementation, their accuracy improved by 0.7%. That’s a case where a small mismatch in burden cost them big accuracy issues. Also, if you’re using the PT for both metering and protection, make sure it’s a “combined accuracy” PT rated for both measurement and protection, not just a metering-only PT. Metering-only PTs sacrifice some accuracy at overburden levels to stay within their core design, which is a problem for protection that needs consistent accuracy across load ranges.
The third solution is core correction and magnetic circuit checks. For older PTs, or PTs that have been exposed to voltage surges or overloading, the core can develop residual magnetism, which shifts the PT’s output voltage and increases error. Residual magnetism happens when the PT is disconnected from power while carrying high load, or hit by a lightning surge that leaves a permanent magnetic field in the core. We had a client in Minnesota with a 10-year-old PT on a rural distribution line—they’d had a storm that knocked out power for 12 hours, and when power came back on, their accuracy was at 1.2%. Our techs used a residual magnetism demagnetizer, which applies a gradually decreasing AC voltage to the PT’s primary windings to erase residual magnetism. After a 15-minute process, their error dropped to 0.22%. If demagnetization doesn’t work, sometimes the core has experienced minor saturation damage from overvoltage events, so we recommend a core loss test per IEC 60076-1 to check for core degradation, and if needed, a core reconditioning service. We offer this as a service for clients who want to extend the life of their existing PTs without full replacement.
Another point here: phase balance. For three-phase PTs, which are the most common for distribution and industrial grids, unbalanced phase loads can cause each PT’s accuracy to vary across phases. I had a client in Canada with a three-phase PT bank where two phases were showing 0.5% error and the third was at 1.0%—root cause was an unbalanced industrial load on that phase. The solution was to adjust the distribution of single-phase loads across the three phases to balance the current, which brought all PTs within their accuracy class. If balancing loads doesn’t work, you can also use a three-phase PT with a higher core capacity to handle unbalanced loads, which is a common upgrade we recommend for facilities with highly variable single-phase loads like welding equipment or large HVAC systems.
Now, if none of the above solutions work, that’s when you look at PT calibration and replacement (but only after confirming you have a genuine PT defect). Wait for that—don’t replace a PT until you’ve ruled out installation, rating, and field issues. When you do need to calibrate a PT, field calibration is often cheaper and faster than sending it to a lab, but for high-accuracy applications (like revenue metering, where 0.1% error translates to thousands of dollars annually), we recommend a full lab calibration every 3-5 years. Our in-house calibration lab is ISO 17025 accredited, so we can calibrate PTs to national standards and provide a traceability certificate, which is required for many utility and commercial metering contracts.
Also, let’s talk about proactive maintenance to prevent low accuracy in the first place, because an ounce of prevention is worth a pound of cure. We work with many clients to set up annual PT audits, where we test accuracy, check connections, measure burden, and inspect for corrosion or damage. For example, a food processing plant we partner with does these audits every year, and over the past 4 years, they’ve caught minor connection corrosion before it caused accuracy issues, saving an estimated $120,000 in overbilling errors. We also provide a free accuracy check service for new clients, where we send a portable phase angle meter to test their PT’s error on-site, before they invest in any repairs or upgrades.
Now, I want to be transparent—sometimes a PT does have a manufacturing defect, which is rare, but it does happen. Defects like core misalignment during winding, insulation defects, or wrong turn ratios will cause low accuracy no matter how well you install or maintain the PT. That’s why when we ship PTs, each one goes through a 100% pre-shipment accuracy test per IEC 61869, so the chance of a defective PT leaving our facility is less than 0.1%. But if you do get a PT from a supplier that has a manufacturing defect, the good news is that most reputable suppliers (like us) offer a 5-year quality guarantee, so you can get a replacement at no cost if the defect is confirmed.
Let me wrap this up with a real quick recap, because I know that’s a lot of information: the top solutions for low PT accuracy are 1) Audit and correct installation issues (connections, wiring, polarity, size), 2) Verify your PT’s rating matches your application (primary voltage, accuracy class, burden, combined use), 3) Fix core issues (demagnetization, core checks for older PTs), 4) Balance three-phase loads for three-phase PT banks, 5) Calibrate or replace only after ruling out all prior steps.
If you’re dealing with low accuracy on your PT right now, or you want to implement proactive maintenance to avoid accuracy issues down the line, we’re here to help. Our team has over 15 years of experience supporting utilities, industrial facilities, and commercial sites with PT troubleshooting, calibration, supply, and maintenance. We don’t just sell PTs—we work with you to solve the root cause of accuracy issues, so you don’t have to deal with lost revenue, faulty protection relays, or grid stability problems.
Don’t hesitate to reach out to our team for a free consultation and accuracy check, to get your PT performance back to spec. Whether you need on-site testing, PT replacement, or calibration services, we have the expertise to get it done right, on time, and on budget.
Energy Meter Parts References:
IEEE C57.13, Standard Requirements for Instrument Transformers
IEC 61869, Instrument Transformers – Part 1: General Requirements
IEC 60076-1, Power Transformers – Part 1: General
National Renewable Energy Laboratory (NREL), “Instrument Transformer Accuracy Testing for Distributed Energy Resources”
Jian Xin Technical Limited
Jian Xin Technical Limited is well-known as one of the leading potential transformer manufacturers and suppliers in China. If you’re going to buy high quality potential transformer with low price, welcome to get pricelist from our factory. Also, customized service is available.
Address: Jianxin Industry Park, Longtan Load, Yuhang District, Hangzhou, China. 311121
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