When I first started working in the medium voltage switchgear industry 12 years ago, I didn’t realize how much dielectric strength sits at the heart of every reliable installation. I remember a customer reaching out to me in 2014, panicking because their new switchgear had failed a routine insulation test on-site. They’d sourced units from a cheap overseas supplier, and the dielectric strength was far below what their local utility required. That experience stuck with me, and it’s why every time we design or supply medium voltage (MV) switchgear now, we don’t just check boxes on a spec sheet—we prioritize dielectric strength as the non-negotiable foundation of our work. Medium Voltage Switchgear
First, let’s define what dielectric strength actually means here, because it’s easy to mix it up with general insulation performance. For MV switchgear, dielectric strength is the maximum voltage an insulating material can withstand before it breaks down, allowing electrical current to leak or arc across what’s supposed to be an insulating gap. It’s measured in kilovolts per millimeter (kV/mm), but in real-world switchgear terms, we care far more about how that translates to safe performance across the entire range of operating conditions: from temperature swings in a steel mill in Ohio to humidity levels in a coastal power plant in Singapore, even during lightning strikes or sudden load surges.
Medium voltage switchgear sits between low voltage (under 1kV) and high voltage (above 52kV), so our baseline range is typically between 1kV and 52kV, with most common installations falling between 15kV and 35kV. The dielectric strength requirements for these systems aren’t arbitrary—they’re set by two core sets of standards: international IEC standards and regional ones like ANSI in North America, since those are the frameworks our customers around the world rely on. Let’s break down the key requirements by voltage class, because that’s where most of our customers start when specifying gear. For a 15kV switchgear unit, the standard required power frequency dry withstand voltage is 42kV, while wet withstand is 38kV. For a 35kV unit, that jumps to 95kV dry and 85kV wet. These numbers aren’t pulled from thin air; they account for the maximum temporary overvoltage a system might see during a fault, plus a safety margin that’s critical for preventing catastrophic failures.
But here’s what many new customers don’t realize: dielectric strength requirements don’t end at the factory test. The insulation inside switchgear isn’t just a solid material—we use a combination of materials: epoxy resin for busbars, silicone rubber for insulators, even air or sulfur hexafluoride (SF6) gas for enclosed units. Each of these materials has its own dielectric strength, but they also interact with each other and the environment inside the switchgear enclosure. That’s why one of the most important tests we run in our facility is the partial discharge (PD) test, which checks for tiny, invisible leaks in insulation that don’t show up during standard voltage tests. A unit might pass the 42kV dry withstand test, but if there’s a tiny void in the epoxy busbar insulation, that void will cause PD over time, eroding the insulation until it fails. For us, we cap PD levels at less than 10 picocoulombs for all our MV switchgear, well below the 50 picocoulomb limit set by standards, because we want our customers to go years without having to troubleshoot insulation issues.
Another critical factor is the ambient conditions the switchgear will operate in, which changes the required dielectric strength. For example, customers installing switchgear in desert climates with temperatures that swing from -10°F to 120°F can’t use the same insulation as someone installing in a tropical rainforest with 95% humidity year-round. At high temperatures, insulating materials like epoxy can lose up to 20% of their dielectric strength, so we adjust our design to use higher-grade epoxy with a thermal rating of 155°C, which maintains its dielectric properties even when operating at 100°C internal temperatures. For coastal installations where salt spray is a problem, we use silicone rubber insulators with hydrophobic coatings, which repel water and prevent the formation of a conductive layer on the surface that would lower the effective dielectric strength. We had a customer in Dubai a few years ago who told us their old switchgear failed every summer because of high humidity—when we upgraded their insulation materials and adjusted the dielectric test voltage to account for their local conditions, they haven’t had an issue in six years.
Then there’s the issue of transient overvoltages—think lightning strikes or switching surges that happen in milliseconds. These overvoltages can be two to three times the normal operating voltage, so the dielectric strength of the insulation has to handle that spike without breaking down. That’s why our switchgear is designed with impulse withstand voltage requirements too. For a 15kV unit, the impulse withstand is 95kV, and for 35kV it’s 200kV. This test simulates a lightning strike hitting the power line, and we’ve had customers tell us these impulse tests made all the difference when a nearby storm took out a competing unit but theirs kept running. I still get a message every year from a food processing plant in Iowa that told us their switchgear kept their production line running during a derecho in 2020, when thousands of other units failed due to insulation breakdown from surge voltages.
A common mistake we see new customers make is cutting corners on dielectric testing to save money. A few years ago, a mining company approached us after switching to a cheaper brand of 15kV switchgear that kept failing during their night shifts, when load surges were highest. They’d only paid for the power frequency withstand test at the factory, not the impulse or partial discharge tests. When we tested the units they had on-site, their dielectric strength was 30% below what was required for impulse voltage, and the PD levels were over 200 picocoulombs. We worked with their engineering team to replace the insulation in those units and supplied our own gear that met all the required test thresholds, and they’ve been a loyal customer ever since. That experience taught me that dielectric strength isn’t just a technical spec—it’s a safety and reliability issue that can make or break a customer’s business.
So what does all this mean for our customers when they’re specifying medium voltage switchgear? It’s not just about picking the right voltage class—it’s about making sure the dielectric strength is tailored to their specific operating environment, that all required tests are performed, and that there’s a margin for unexpected conditions. Here at our company, we don’t just sell you switchgear and walk away. We work with each customer to assess their site conditions, their load requirements, and any local regulations before we even design the unit. For example, a substation in a remote area with limited maintenance access needs higher dielectric strength and more robust insulation than a unit in a facility with a full-time electrical team. We also provide full test reports with every unit, including dielectric strength test results, PD levels, and impulse test data, so our customers can show compliance to their utilities and insurance providers.
I know that for many people in the industrial or power sector, dielectric strength sounds like a technical buzzword that’s hard to wrap your head around. But the bottom line is this: if your MV switchgear doesn’t meet the required dielectric strength for your application, you’re putting your team at risk of arc flashes, your equipment at risk of downtime, and your business at risk of costly repairs or lost production. In the 12 years I’ve been in this industry, I’ve seen more avoidable issues come from insufficient insulation and low dielectric strength than from any other technical failure. And with more facilities adopting renewable energy sources like solar and wind, which add variable load to the grid, the demand for reliable, high-dielectric switchgear is only going to grow.
If you’re in the market for medium voltage switchgear, or if you’re experiencing issues with current insulation performance, we’d be happy to walk through your specific needs with you. Our team has decades of experience working across industries, from manufacturing and mining to power utilities and renewable energy projects, and we can help you specify the right dielectric strength requirements for your site. Don’t settle for units that just meet the minimum base specs—insist on testing that accounts for your environment, your load, and your long-term reliability needs. Reach out to our team to discuss your project, and we’ll provide a customized solution tailored to your requirements.
Step Up Transformer References
IEC 62271-100, High-voltage switchgear and controlgear – Part 100: AC switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV
ANSI C37.06, American National Standard for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis
NEMA SG-6, Standard for Switchgear, Medium Voltage, 1 kV through 38 kV, AC
Huachi Electric Co., Ltd.
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