If you’re a spec engineer, plant manager, or anyone who’s ever had to keep industrial equipment running smoothly, you’ve probably stared at a dry-type transformer and wondered, “What’s the big deal about moisture anyway? It’s just water, right?” Trust me, I get it—when we sell these units, most folks fixate on kVA ratings, voltage levels, and whether they fit in the old switchgear closet. But after 12 years as a dry-type transformer supplier, I can tell you moisture is the silent killer that creeps up on you when you least expect it, and it’s not just a minor hassle. It can tank your transformer’s performance, shorten its lifespan, and even cause that whole plant shutdown you’ve been trying to avoid. Let’s break this down like we’re chatting over coffee, no stuffy textbook jargon—just real-world stuff I’ve seen firsthand. Dry Type Transformer
First, let’s start with how dry-type transformers are actually insulated, because that’s key here. Unlike oil-filled ones (you know, the big, sealed tanks outside that everyone calls “oil cans”), dry-type units use solid insulation, usually epoxy resin-coated copper windings, right? Some older ones use paper insulation, but these days epoxy is the go-to because it’s durable and fire-resistant—perfect for places like food processing plants, hospitals, or downtown office buildings where oil leaks would be a disaster. That epoxy is supposed to be like a waterproof barrier, keeping moisture out of the windings, right? Well, not exactly. Epoxy isn’t 100% impenetrable. It’s porous in tiny, micro ways—like a sponge that doesn’t feel wet until it’s been sitting in a steamy bathroom for a week. And the windings underneath? Even if they’re coated, they’re still made of material that can absorb and hold moisture, same as a wooden table or a towel.
Here’s where it gets tricky: insulation works by stopping electrical current from jumping where it shouldn’t—between windings, from windings to the metal core, or to the grounded frame of the transformer. If that insulation is wet, its electrical resistance drops hard. I’ve had a customer in the Pacific Northwest (you know, the place where it rains for 10 months straight) call me in a panic two years ago. Their 750 kVA dry-type transformer was acting up—fault codes popping up, occasional arcing they could hear through the switchgear. When our techs tested the insulation resistance, it was half what it should’ve been. Turned out, the unit was installed in a partially finished storage area where the HVAC had broken, and the humidity hit 90% for three straight weeks. The windings had soaked up moisture through the epoxy, and that’s what caused the resistance to plummet.
Wait, let’s get specific about that resistance drop because it’s not trivial. IEC standards (I’ll mention real ones later, don’t worry) say a healthy dry-type transformer should have an insulation resistance (IR) of at least 1 megohm per kV of rated voltage, right? So a 480V unit would need around 0.5 megohms? No, wait, let me get that straight—wait, no, for low-voltage dry-type, the rule of thumb is IR = rated voltage (in kV) times 1000, minimum. So a 0.48 kV unit should have at least 480 megohms when dry. When my customer’s transformer was wet, it was clocking in at 120 megohms. That’s a 75% drop. Small, sure, but enough to let leakage current sneak through the insulation. Leakage current isn’t a big deal on its own, but over time, it heats up the insulation from the inside. That heat accelerates the breakdown of the epoxy and the winding insulation—think of it like leaving a plastic cup on a hot stove; it’ll get brittle and crack way faster than normal.
And here’s the thing about heat and moisture working together: it’s a vicious cycle. Once the insulation starts to degrade from the leakage current, it gets even more porous, so more moisture gets in. Then that moisture causes more leakage current, more heat, more degradation. I’ve seen that cycle take a perfectly good transformer that should last 30 years and kill it in 8. Another customer, this one in a metal stamping plant in Ohio, had their transformer installed near a washdown area. Every time they cleaned the floor, water would seep through the gap in the concrete and wick up to the transformer’s base, and the humidity from the spray got inside the cabinet. Within two years, the winding insulation was so degraded that they had to rewind the whole unit—cost them $15k, not to mention the 3 days their line was down. That’s way more than the $2k we would’ve charged them to add a moisture seal and a dehumidifier as an optional add-on when we installed it.
Wait, but not all moisture is the same, right? That’s a point a lot of folks miss. There’s “surface moisture” and “absorbed moisture.” Surface moisture is the obvious stuff—like condensation sitting on the outside of the cabinet, or water droplets dripping into the unit. That’s easier to spot, but it’s also more immediate. Condensation usually happens when the transformer is sitting idle, right? If it’s turned off for a few days (like over a weekend or a holiday) and the surrounding air is warm and humid, then the transformer’s metal windings cool down, and the air inside the cabinet can’t hold all that moisture anymore—so it forms droplets. I had a customer in Canada learn this the hard way last winter. They fired up a 1500 kVA unit after Christmas break, and it tripped the main breaker 10 minutes later. Turned out, the temperature had dropped 20 degrees overnight, condensation had formed on the windings, and when they turned the power on, the leakage current was so high it caused an overcurrent trip.
Absorbed moisture is the sneaky kind, though—you don’t see it, it doesn’t cause an immediate fault, but it’s eating away at the insulation little by little. Like I said earlier, the epoxy coating has tiny pores, and over time, especially in high-humidity environments, moisture gets trapped inside. Our techs test for this with a dielectric absorption ratio (DAR) test, which checks how the insulation resistance changes over time—dry insulation has a high DAR, wet insulation has a low one. Most new transformers pass the DAR test with flying colors, but after 5 or 10 years in a steamy environment, that number starts to drop, and that’s when you need to start monitoring closely.
Another big impact of moisture: partial discharges. If the insulation is weak from moisture, tiny sparks can form inside the windings, between turns, or between the windings and the core. Those sparks—partial discharges—don’t always cause a big, obvious arcing, but over time, they erode the insulation from the inside. Think of it like sandblasting a wall, but way smaller. I’ve seen transformers that passed a basic visual inspection and worked fine for years, but had partial discharges that eventually caused a short circuit. And partial discharges can also generate ozone, which breaks down epoxy even faster—another cycle of destruction.
Wait, but here’s the good news: it’s not all doom and gloom. Moisture is a problem, but it’s one you can manage if you pay attention. As a supplier, I don’t just sell transformers and disappear—we give our customers tips to keep moisture out, because we want our units to last, and we want them to call us when they need more, not when their transformer is fried. For example, we recommend adding NEMA 12 enclosures for transformers in environments with high humidity or washdowns—those enclosures are sealed way better than standard NEMA 1 ones, keeping water and moisture out. We also talk about optional dehumidifiers that mount inside the cabinet—they run on tiny power, and keep the air inside dry even if the outside is steamy. For customers who have transformers that sit idle for long periods, we suggest periodic “hot runs” where you turn the unit on at partial load for a few hours every few weeks—this heats up the windings, evaporates any trapped moisture, and prevents condensation.
I also want to bust a myth here: some folks think because dry-type transformers don’t have oil, they don’t need to worry about moisture. That’s not true at all. Oil actually helps insulate and cool, and it’s pretty good at keeping moisture out of the windings—dry-type’s solid insulation is more exposed, so it’s more vulnerable, not less. I’ve seen oil-filled transformers in swampy areas last 40 years, while dry-type ones in the same area fail in 15 because of moisture. That’s not a knock on dry-type units—they’re amazing for the right applications, but you have to plan for moisture when you install them.
Let’s circle back to that customer I mentioned in the Pacific Northwest. We didn’t just sell them a new transformer—we helped them fix their space: we added a proper HVAC unit to their storage area, sealed the gaps around the transformer cabinet, and recommended the dehumidifier. A year later, they called to say the IR test was still perfect, no faults, no issues. That’s what it’s all about, right? Not just selling a product, but making sure it works for you.
Now, let’s talk about testing, because if you own a dry-type transformer, this is what you should be doing on a regular basis. Most manufacturers, including us, recommend testing insulation resistance at least once a year, and DAR tests every 2 to 3 years, especially if you’re in a humid or extreme temperature environment. If the IR drops below that 1 megohm per kV rule, or the DAR is below 1.5 (that’s the threshold for wet insulation), you need to take action fast—either dry out the transformer, fix the seals, or add a dehumidifier. Ignoring those numbers is how you end up with an unplanned shutdown.
Wait, but what if moisture is already in the transformer? Can you fix it? Yeah, usually, if you catch it early. We offer a drying service where we put the transformer in a temperature-controlled chamber, heat it up to around 100°C, and blow dry air through the windings to evaporate all the trapped moisture. For minor cases, that can bring the IR back up to normal. For more serious cases where the insulation is already degraded, we can rewind the windings or coat them with a fresh layer of epoxy. It’s way cheaper than buying a new unit, obviously, but it’s better to prevent it in the first place.
I’ve been in this game long enough to see trends—more and more customers are choosing dry-type transformers for their safety and environmental benefits, which is great. But more of them are also asking us about moisture management, which tells me they’re learning from the mistakes I mentioned earlier. That’s why we’re always updating our advice, adding better seal options, and working with our techs to spot potential moisture issues before they become problems.
Let’s wrap this up, because I don’t want to bore you with all the details. The key takeaway here is: moisture is a major threat to dry-type transformer insulation, but it’s manageable. It lowers insulation resistance, causes leakage current that heats and degrades the insulation, leads to condensation that can trip the unit, and sparks partial discharges that erode things over time. But with the right installation (sealed enclosures, dehumidifiers), regular testing (IR and DAR), and a little bit of proactive maintenance, you can avoid most of these issues.
If you’re not sure if your current transformer has moisture issues, or you’re looking for a new dry-type unit that’s built to handle humid conditions, reach out. We don’t do pushy sales pitches—we’ll ask about your environment, your load, your maintenance schedule, and help you pick the right unit and accessories to keep things running smoothly. No jargon, no hidden fees, just real advice from someone who’s seen the good, the bad, and the ugly of dry-type transformers.
References
Rectifier Transformer International Electrotechnical Commission. IEC 60076-11: Power transformers – Part 11: Dry-type transformers. International Electrotechnical Commission, 2018.
Institute of Electrical and Electronics Engineers. IEEE C57.12.00: General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers. Institute of Electrical and Electronics Engineers, 2020.
National Electrical Manufacturers Association. NEMA Standards Publication No. 250: Enclosures for Electrical Equipment (1000 Volts Maximum). National Electrical Manufacturers Association, 2021.
Zhejiang Jiangshan Hengli Electrical Co., Ltd.
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