When I was 22, fresh out of electrical engineering school and working in the back of my family’s small distribution shop, I watched a friend of ours—an older lineman named Ray—have a close call with a high voltage switch gear failure. He was troubleshooting a 13.8kV panel in a rural substation, and when he went to flip the breaker, there was a loud crack, a flash of blue light, and the whole panel went up in smoke. Ray walked away with second-degree burns on his arms, but that incident stuck with me. That’s when I learned that high voltage switch gear isn’t just metal boxes and switches hanging on a wall; it’s a safety net. And the first line of defense in that net is something called breaking capacity. If you’re a facility manager, an electrical engineer, or someone who’s tasked with keeping a power system running, understanding breaking capacity isn’t just a technical detail—it’s what keeps your team safe and your operations online. High Voltage Switch Gear
Let’s start with the basics. High voltage switch gear is the set of electrical devices that control, protect, and isolate equipment in power systems rated above 1,000 volts. Think circuit breakers, load break switches, fuses, disconnectors—all of these fall under the switch gear umbrella. Their number one job? Stop faults. A fault is an unintended short circuit, and when that happens, massive amounts of current (sometimes tens of thousands of amps) surge through the system. If that current isn’t interrupted quickly, it can melt conductors, explode equipment, and cause fires. That’s where breaking capacity comes in. It’s the maximum amount of fault current a switch gear device can safely interrupt without failing, right there in the moment when you need it most.
But here’s the thing: breaking capacity isn’t a one-size-fits-all number. It’s calculated based on two specific points. The first is the symmetrical breaking capacity, which is the rms (root mean square) value of the alternating current (AC) that the device can cut off. The second is the asymmetrical breaking capacity, which accounts for the extra current caused by the DC component in a fault. In the first few cycles of a short circuit, the current isn’t a clean sine wave—it has a DC offset that adds to the total current, making it harder to interrupt. Good switch gear has to handle both of these, especially in systems with long cables or heavy loads where that DC component is stronger. I’ve seen too many times where a client ordered a switch gear with only the symmetrical rating, and it failed the first time a real fault hit because the asymmetrical current was higher than they expected.
Over the 15 years I’ve been selling and troubleshooting high voltage switch gear, I’ve learned that choosing the right breaking capacity comes down to knowing your system’s fault level. Fault level is calculated based on the total capacity of the power source—transformers, generators, and the impedance of the lines between the source and your switch gear. For example, a 24kV substation fed by a 50MVA transformer will have a different fault level than the same voltage system fed by a 100MVA generator. If you undersize your breaking capacity, your breaker won’t put out the fault—it will weld its contacts together, turning a manageable problem into a total outage. I still remember a client in Ohio who skipped getting a fault study for a new manufacturing plant. They installed switch gear rated for 25kA breaking capacity, but their plant’s fault level was 32kA. Six months later, a short circuit in a conveyor system caused the breaker to fail; it didn’t just shut down the conveyor—it took out the entire plant’s production line for three days, costing them over $200,000 in lost revenue. That’s the cost of cutting corners on breaking capacity.
Another common mistake I see is mixing up breaking capacity with making capacity. Making capacity is the maximum current a switch gear can withstand when it’s closed onto a fault, which happens when you accidentally flip a breaker into a faulted circuit. That’s different from breaking capacity, which is about interrupting current. A device might have a 40kA breaking capacity, but its making capacity could be 100kA because the mechanical stress when closing into a fault is higher than when interrupting it. You can’t use one to calculate the other, and relying on the wrong one is a fast track to a safety incident. Ray’s incident? The breaker he was using had an outdated making capacity rating, not a proper breaking capacity, and that’s why the arc flash happened. It’s a lesson we drilled into our team from day one: never skimp on either, but know the difference.
Now, how do manufacturers rate breaking capacity? It’s not just a number pulled out of thin air. Every switch gear we supply goes through rigorous testing in certified labs, following standards like IEC 62271-100 or ANSI C37.06. These tests involve injecting a known fault current into the device, closing it onto the fault, then opening it to interrupt the current. The lab measures if the contacts remain intact, if there’s excessive arc damage, and if the device can be reset or reclosed after the test. For example, a 12kV vacuum circuit breaker we offer is rated for 31.5kA breaking capacity. That means it can safely interrupt 31,500 amps of fault current at 12kV, multiple times, without failing. It’s not just a test for one time—breakers are designed to handle multiple fault interruptions, though we always advise replacing a breaker that’s interrupted a severe fault, just to be safe.
Arc rating is also tied closely to breaking capacity, and this is something facilities managers often overlook. When a breaker interrupts a fault, it creates an arc between the contacts. The higher the breaking capacity, the more controlled that arc should be—but if the device isn’t rated for the arc energy in your specific system, that arc can rupture the switch gear enclosure, sending molten metal and hot gas into the workspace. We recently worked with a pharmaceutical plant that didn’t realize their switch gear had a 20kA breaking capacity, but their fault level was 28kA. The arc flash incident that followed sent a technician to the hospital with burns, and the plant had to shut down a critical production line for a week. Since then, we’ve made it a point to not just sell switch gear, but to sit down with clients and walk them through a fault study, so they don’t have to learn the hard way.
What about different types of switch gear? Breaking capacity varies by device, too. Load break switches, which are used for routine switching, not fault interruption, have lower breaking capacity ratings—usually 10kA or less. Fuses, on the other hand, have very high breaking capacity because they’re designed to melt quickly when a fault occurs, cutting off the current. Vacuum circuit breakers, which are the most common in medium to high voltage systems today, have high breaking capacity because the vacuum extinguishes the arc fast—usually in a few milliseconds. SF6 (sulfur hexafluoride) breakers also have high breaking capacity, since the gas is a great arc quencher. We’ve found that vacuum breakers are more popular now because they’re easier to maintain and don’t have the environmental concerns of SF6, but both are reliable if rated correctly.
I get a lot of questions from new clients: “Can I use a higher breaking capacity than my system needs?” Technically, yes—but it’s not always cost-effective. Switch gear with higher breaking capacity is more expensive, heavier, and needs more robust mounting hardware. If your fault level is 25kA, a 31.5kA rated breaker might work, but you’re paying for capacity you’ll never use. That’s why a detailed fault study is non-negotiable. We offer free fault study consultations to all our potential clients, because we’d rather walk someone through the numbers than sell them something that’s either too cheap or overkill. Last year, a solar farm came to us looking for switch gear for their new 34.5kV array. They initially wanted a 40kA breaker, but their fault study showed their actual level was 28kA. We sold them a 31.5kA breaker, saved them 15% on the cost, and they’ve had zero issues in the year since installation.
Another thing to consider is future expansion. If you’re building a new facility, don’t just size your switch gear for today’s load—plan for future upgrades. I once worked with a data center that installed switch gear for their initial 5MW load, but they left extra space in the enclosure. Two years later, they expanded to 10MW, and their fault level doubled. Instead of ripping out old switch gear and replacing it, they just upgraded the breakers to a higher breaking capacity. That was a smart move, and it saved them tens of thousands in installation costs. So when we quote a job, we always ask clients about their long-term plans—even if they don’t have details yet. It’s our job to plan for them, not just for today.
Now, let’s talk about common myths. One myth is that breaking capacity is only important for big industrial facilities. That’s not true. Hospitals, schools, even large retail centers have high voltage systems, and a fault there can disrupt critical services. A hospital backup generator system, for example, needs switch gear with proper breaking capacity because a failure there could mean life support systems going down. Another myth is that new switch gear always has the right breaking capacity. We’ve encountered brand new switch gear that was incorrectly rated by the manufacturer, or mislabeled during installation. That’s why it’s always good to have a third-party inspector check your switch gear’s ratings, especially if you’re buying used equipment or retrofitting an old system.
I’ve been in this business long enough to have seen hundreds of switch gear failures, and almost every single one came down to a misunderstanding of breaking capacity. It’s not a technical jargon term—it’s the difference between a fault that’s contained and one that causes injury, downtime, or even death. That’s why when I sit down with a client, I don’t just talk about kA ratings. I talk about Ray’s incident, the Ohio manufacturing plant, the pharmaceutical technician. I make sure they understand that breaking capacity is about protecting their people and their business, not just a box on a spec sheet.
If you’re in the market for high voltage switch gear, or if you’re not sure if your current system has enough breaking capacity, don’t guess. Reach out to our team for a consultation. We can walk you through a fault study, recommend the right equipment, and make sure you’re not cutting corners on safety. We’ve worked with facilities across the country, from small rural substations to large industrial plants, and we know how to tailor solutions to your specific needs. You don’t have to navigate the complexities of high voltage switch gear alone—we’re here to help.
Electric Vehicle Charger References
- IEC 62271-100: High-voltage switchgear and controlgear – Part 100: Alternating-current circuit-breakers
- ANSI C37.06: American National Standard for Switchgear – AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis – Preferred Ratings and Related Required Capabilities
- IEEE Std 242-2019: IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems
- Electrical Construction and Maintenance, “Understanding Breaking Capacity in High Voltage Switchgear” (October 2022)
- National Fire Protection Association (NFPA) 70E: Standard for Electrical Safety in the Workplace
Zhejiang Dongyu Electric Co., Ltd.
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