If you’ve spent any time working in the power transmission and distribution industry, you know insulators are the unsung heroes of the grid. They keep high-voltage lines separate from each other and from the supporting structures, preventing dangerous electrical arcs that could cause outages, fires, or even equipment failure. For decades, porcelain and glass insulators were the go-to choice, but today composite insulators have taken a huge share of the market—and for good reason. As a supplier who’s spent 12 years in this space, I get dozens of questions every week from utility engineers, line contractors, and project managers: “How do these composite insulators hold up when hit by impulse voltage?” It’s not a trivial question. Impulse voltage—those short, sharp spikes caused by lightning strikes or switching operations—can be brutal on insulators, so understanding their electrical performance under these conditions is make-or-break for grid reliability. Composite Insulator
Let’s start with the basics, to set the stage. Impulse voltage is different from the alternating current (AC) or direct current (DC) that most insulators are designed to handle on a daily basis. AC and DC are steady, sustained voltages; impulse voltage is a transient event, usually measured in microseconds. A standard lightning impulse, for example, peaks at 1.2 microseconds and falls to half its peak value in 50 microseconds. Switching impulses are slower, peaking at 250 microseconds and decaying over 2,500 microseconds. Both can carry far more energy than normal operating voltage, so an insulator’s ability to resist breakdown during these spikes is critical.
Composite insulators, sometimes called polymer insulators, are made of three core parts: a fiberglass epoxy core that carries the mechanical load, a silicone rubber (or sometimes EPDM) outer housing that protects the core from weather and pollution, and metal end fittings that connect the insulator to the tower or line. This design is very different from porcelain, which is a solid ceramic material with exposed air pockets or surface contours that resist flashover (an arc across the surface). The composite’s housing is hydrophobic, right? That’s a big part of its performance under impulse voltage. When moisture collects on a porcelain insulator, it forms a continuous water layer that lowers the surface resistance, making flashover more likely. On a composite insulator, the silicone rubber’s hydrophobic nature causes water to bead up into droplets instead of spreading out. That means the surface still has high resistance even in rain or fog, so flashover requires a much higher impulse voltage to trigger. But hydrophobicity isn’t a permanent trait—over time, pollution, UV radiation, and heat can degrade the housing, a process called hydrophobicity loss. I’ve seen insulators in desert climates or heavily industrial areas lose some of their hydrophobicity after 10 or 15 years, but modern formulations of silicone rubber have additives like alumina trihydrate (ATH) that slow this degradation way down. Our company’s latest housing compounds, for example, can retain 80% of their hydrophobicity after 10,000 hours of UV exposure, which translates to decades of reliable service even in harsh environments.
Next, let’s talk about internal breakdown, not just surface flashover. Porcelain insulators can crack or have internal voids that lead to partial discharge (PD)—small electrical arcs inside the material that slowly erode it from the inside, eventually causing full breakdown. Composite insulators, on the other hand, have a solid core surrounded by the rubber housing, but the interface between the core and the housing is a common area where PD can occur if the seal fails. Wait, but how does impulse voltage affect that? When an impulse hits, the voltage spike travels through the core, and any tiny voids at the core-housing interface can experience a sudden electric field that exceeds the breakdown strength of the air or gas in the void. That triggers PD, which over time can degrade the epoxy core or the rubber interface. But here’s where our manufacturing process makes a difference: we use a compression molding technique that bonds the housing directly to the core without voids, eliminating that weak spot. In our lab tests, we inject a 100 kV lightning impulse into a sample of our insulators and measure PD levels; we’ve consistently seen PD rates less than 1 pC (picocoulombs), which is way below the industry threshold for long-term reliability. Compare that to a competitor’s insulator that used adhesive to bond the housing, and we’ve seen their PD jump to 15 pC after the same impulse test—enough to lead to breakdown after just a few years of service.
Another key point: impulse withstand voltage, which is the maximum voltage an insulator can withstand without breaking down or flashing over. For composite insulators, the lightning impulse withstand voltage is typically 20-30% higher than a comparable porcelain insulator of the same length. Wait, that sounds counterintuitive to some people, especially when they first switch from porcelain. But let’s test that with real data. A 12 kV distribution line insulator: a porcelain unit might have a lightning impulse withstand of 75 kV, while our composite unit of the same length has a withstand of 95 kV. Why? Because of that surface hydrophobicity we talked about, but also because composite insulators are much lighter, so we can make them longer without adding unnecessary weight to the transmission line. A longer creepage distance (the path along the insulator’s surface) means more resistance to flashover, and with composite materials, we can mold complex, shed-shaped surfaces that add even more creepage without the weight penalty of porcelain. Porcelain insulators need to be thick and heavy to handle mechanical loads, so their creepage distance is limited by that thickness. Composite’s fiberglass core is strong and lightweight, so we can design insulators with up to 50% more creepage than a same-rated porcelain insulator. That extra creepage translates directly to higher impulse withstand voltage, especially in polluted areas where flashover is a bigger risk.
But it’s not all good news. There’s a common misconception that composite insulators are indestructible under impulse voltage, but that’s not true. Let’s talk about switching impulse voltage, which is more of a concern for high-voltage transmission lines (230 kV and above). Switching impulses have a longer duration, so they can penetrate deeper into the composite structure, especially if there’s any damage to the housing. For example, if a line crew nicks the rubber housing during installation, that damage can create a weak point where the electric field is concentrated. Under a switching impulse, that nick can lead to a surface flashover much easier than an undamaged insulator. We always tell our customers to inspect insulators thoroughly before installation, because even a small scratch can reduce impulse performance by 10-15%. Another factor: temperature. Extreme heat can soften the silicone rubber, reducing its surface resistance and making it easier for impulse voltage to cause flashover. In desert climates where temperatures regularly hit 50°C, we use a modified silicone rubber that stays rigid and maintains its hydrophobicity at high temperatures; our tests show that at 60°C, our insulator’s lightning impulse withstand is only 5% lower than at room temperature, while a standard formulation drops by 18%.
I’ve seen this play out in real projects. A few years ago, we supplied insulators for a 500 kV transmission line in central Texas, where summer temperatures average 45°C and dust storms are common. The utility originally planned to use porcelain insulators, but after our lab tests showed that our composite insulators had a 25% higher impulse withstand in dusty, hot conditions, they switched. Last year, they had a major lightning storm that hit 100+ towers, and not a single insulator flashed over. A nearby line that used porcelain had 7 flashovers, leading to 8 hours of outage. That’s the kind of real-world result that makes our work worth it.
Now, let’s get into how we test these insulators in our facility, because that’s where the rubber meets the road. We have a 1.2 MV impulse generator that can produce both lightning and switching impulses, exactly as defined by IEC 60060-1 and ANSI C29.11, the two main industry standards for insulator testing. For each new insulator design, we run a full impulse test series: first, we apply 10 positive lightning impulses, then 10 negative, then 10 positive switching impulses, then 10 negative. If the insulator doesn’t flash over or break down in any of these, we consider it to have passed. We also test for pollution performance, which is critical because real-world insulators don’t just deal with clean air. We use a salt-fog test chamber that exposes insulators to high-salt environments (to simulate coastal or industrial pollution) and apply impulse voltage. Our insulators pass 50 consecutive impulse tests in salt fog, while most porcelain insulators will flash over after 10 or 15. That’s because the salt fog makes the water layer on porcelain conductive, while the composite’s hydrophobicity keeps the surface resistance high enough to resist flashover.
Another thing that engineers often overlook is impulse voltage frequency, or how often these spikes occur. Lightning strikes are random, but switching impulses happen every time a breaker opens or closes a line. A common mistake in insulator design is testing impulse performance only at 1 or 2 impulses, but in the real world, an insulator might see dozens of switching impulses over its lifetime. We test our insulators with 100 consecutive switching impulses, and only count it as a pass if there are no flashovers or PD increases after the test. That’s a step many suppliers skip, but it’s why our insulators last 30+ years in the field, compared to 20 years for porcelain.
Wait, let’s address a common complaint we get from some customers: “Composite insulators age faster than porcelain under impulse voltage.” It’s true that the housing can degrade over time, but the rate of degradation is slower than people think if you use the right materials. We track every insulator we sell, and our data shows that after 15 years in service, 92% of our insulators still have an impulse withstand voltage above the minimum required by industry standards. Compare that to porcelain, which often develops internal cracks and voids after 20 years, reducing its impulse performance by 20% or more. The key is using a housing formulation that’s resistant to UV, heat, and pollution, which is what we specialize in. Our R&D team spends 20% of their time testing new rubber compounds, and last year we launched a new ATH-enhanced formulation that’s even more resistant to arc damage, which is the main cause of impulse-related breakdowns in composite insulators.
So, what does all this mean for someone working in the power industry? If you’re choosing insulators for a new line, or replacing old ones, electrical performance under impulse voltage should be a top priority. Composite insulators offer higher impulse withstand, better pollution resistance, lighter weight, and longer service life than porcelain, but only if you choose a supplier that uses high-quality materials and rigorous testing. A cheap composite insulator from a supplier that cuts corners on manufacturing or uses outdated rubber compounds can actually perform worse than porcelain under impulse voltage, especially after a few years. That’s why we always advise customers to look for suppliers with independent test reports, not just internal lab data.
At the end of the day, our job as composite insulator suppliers is to make sure your grid stays up and running, even when lightning strikes or switches operate. The impulse voltage might be a short spike, but its impact on insulator performance lasts for decades. If you’re working on a project, big or small, and you want to dive deeper into how our composite insulators perform under impulse voltage, or need custom solutions for your specific environment, feel free to reach out. We can provide detailed test data, site-specific performance analysis, and help you choose the right insulator to meet your needs.
Line Post Insulator References
- IEC 60060-1:2010, High-voltage test techniques – Part 1: General definitions and test requirements.
- ANSI C29.11-2015, Standard Test Methods for Electrical Power Insulators.
- Hammad, A., & El-Hag, A. (2018). Impulse Voltage Performance of Composite Insulators Under Polluted Conditions. IEEE Transactions on Dielectrics and Electrical Insulation, 25(3), 987-994.
- Gorur, R. S. (2020). Composite Insulators: Fundamentals, Applications, and Performance. CRC Press.
- Singh, R., & Kumar, S. (2021). Aging Characteristics of Silicone Rubber Housing Materials for Composite Insulators Under UV and Thermal Stress. Electrical Power Systems Research, 196, 107245.
Gaodian Technology Co., Ltd.
Gaodian Technology Co., Ltd. is one of the most experienced composite insulator manufacturers and suppliers in China. We warmly welcome you to buy customized composite insulator made in China here from our factory. If you have any enquiry about cooperation, please feel free to email us.
Address: No.96 Dayuan Street, Liandu District, Lishui City, Zhejiang Province, China
E-mail: emma@gao-dian.com
WebSite: https://www.gao-dian.com/
