If you’ve ever stood inside a building during a summer thunderstorm, counting the seconds between a flash of lightning and its thunder, you’ve relied on lightning arresters to keep the electricity running—and your property, equipment, and loved ones safe. As someone who’s supplied lightning arresters for over a decade, I’ve seen first-hand how a unit that’s worked perfectly for years can suddenly fail, often without warning. The last thing anyone wants is a lightning strike damaging critical infrastructure, because the arrester wasn’t replaced when it needed to be. Over the years, I’ve talked to electricians, facility managers, and industrial plant operators who missed small, easy-to-miss signs their arresters were on their last legs—signs that could’ve saved them thousands in repairs and downtime. Today, I want to pull back the curtain on the real, practical signs that an arrester needs replacement, not just the technical jargon you might find in a manual. This is the stuff I wish every property owner knew, from someone who’s installed, tested, and replaced hundreds of these devices. Lightning Arrester
First, let’s get one thing straight: lightning arresters aren’t “set-it-and-forget-it” components. A lot of people buy a unit, bolt it to their transformer or power line, and assume it’ll work forever. But the truth is, they degrade over time, thanks to exposure to extreme temperatures, UV radiation, rain, pollution, and even small, repeated surges that build up year after year. Think of them like a fire extinguisher: you don’t wait until there’s a blaze to check if it works. Same with arresters. The most common, and often easiest to spot, sign of a failing arrester is physical damage to the unit itself. I once visited a small manufacturing plant where their main line arrester had a crack running down the side of its porcelain housing—everyone had just walked past it every day, assuming it was fine. When I tested it, it was so degraded it wouldn’t have diverted a single surge, let alone a lightning strike. Cracks, chips, or any kind of visible damage to the housing aren’t just cosmetic. Porcelain, the most common material for outdoor arresters, is strong, but it’s brittle. A small crack can let in moisture, which seeps into the internal components and causes the arrester to leak current when it shouldn’t. I’ve also seen arresters with broken mounting brackets, bent terminal connections, or even discoloration on the housing from years of sun exposure—all red flags that the unit has lost its structural integrity. If you can see damage from the ground, don’t wait for a professional to confirm it’s bad. Order a replacement to be safe.
Next up: abnormal heating around the arrester during normal operation. This is a sign that’s easy to miss if you don’t know what to look for, but it’s one of the most reliable indicators of internal failure. Modern arresters are designed to run cool under normal operating conditions—you shouldn’t be able to feel a significant temperature difference between the arrester and the surrounding power line or transformer. But if you notice that an arrester is consistently warm to the touch, or even hot, that’s a sign it’s leaking “stray current.” Stray current isn’t the surge it’s supposed to divert; it’s a small, continuous flow of electricity that shouldn’t be there, caused by worn-out internal metal oxide discs (the core of the arrester) that can no longer insulate properly. I learned about this the hard way early in my career. I was out testing an arrester at a small retail park, and I used an infrared thermometer to check its temperature (a tool I now recommend to every customer). The unit was 40 degrees hotter than the line it was connected to. When I removed it, the internal discs were so corroded they were crumbly. That arrester was hours away from failing completely, which would’ve knocked out power to the entire strip mall. The thing is, this heating doesn’t just happen suddenly. It builds over months, so regular checks—even a quick visual scan during a site walkthrough—can catch it before it’s too late. If your maintenance team is noticing unusual heat around your power components, don’t just assume it’s your transformer. Rule out the arrester first.
Then there’s the indicator light or status window, if your arrester has one. Many modern metal oxide arresters come with a built-in indicator that shows whether the unit is still functional. The design is simple: a small pin or color-coded window that pops out or changes color when the arrester has absorbed too much energy, usually from a surge or a lightning strike. But here’s the catch: even if the indicator looks fine, that doesn’t mean the arrester is in good shape. I’ve seen dozens of arresters with intact indicators that failed during a routine test. Why? The indicator only triggers after the arrester has absorbed a large surge, but if the unit is slowly degrading from age or minor pollution, the indicator might never activate—until it’s too late. Conversely, an indicator that’s already popped out or changed color is a clear sign the arrester has done its job, and it’s time to replace it. That’s a no-brainer, but I’ve had customers tell me they “reset” the indicator pin instead of replacing the unit. That’s a huge mistake. The indicator is just a warning; it doesn’t fix the internal damage the arrester took. Resetting it and keeping a failed unit in place is like hitting the reset button on a smoke detector that’s already gone off—you’re ignoring a critical warning.
Another sign that’s often overlooked: repeated surge events or power interruptions that can’t be traced to another issue. Let’s say you have a generator backup, and it kicks on during a storm, but your office computers still get fried. Or you notice your security cameras flickering after a small thunderstorm, or your HVAC system tripping a breaker for no apparent reason. A lot of people blame the power company, or wiring in their building, but often the culprit is a failing arrester. I worked with a data center a few years ago that was dealing with dozens of tiny power surges every month, even when there wasn’t a single lightning strike nearby. Their internal testing pointed to their arrester, which we pulled and found had lost 80% of its surge capacity. It couldn’t divert the small, constant surges from nearby construction, solar panels, or even the power grid’s normal load changes. Those small surges might not cause a big outage, but over time, they add up—they can damage server hard drives, shorten the lifespan of motors, and even create fire risks in wiring. If you’re having unexplained surge-related issues, especially after replacing breakers or wiring, get your arrester tested. It’s almost always a cheaper fix than replacing damaged equipment.
There’s also the age of the arrester itself, which isn’t just a number on a label. I can’t tell you how many times I’ve had a customer bring me an old arrester, still working, and ask, “How long do these actually last?” The short answer is: most well-maintained arresters should last between 15 and 25 years, but that timeline can shrink in harsh environments. If your property is near the ocean, for example, salt air eats away at the internal components much faster. If you’re in a industrial area with high pollution—like a factory that releases chemicals, or a construction site with constant dust—arresters degrade even faster. I’ve seen arresters in coastal areas fail after just 8 years, while ones in a clean rural setting still work perfectly at 20. The problem is, most people don’t write down the installation date of their arrester. That’s why every customer I work with gets a little sticker on the arrester’s mounting bracket with the install date, so they can track its age. If your arrester is over 15 years old, even if it looks fine, you should schedule a professional test to check its capacity. Don’t wait for it to fail; replacement is much cheaper than emergency repairs after a lightning strike.
I also want to talk about what happens during a professional electrical test, because so many of my customers wait for an annual test before checking their arresters. When we test an arrester, we measure its leakage current—the small amount of current that flows through the unit when it’s not diverting a surge. A healthy arrester has a very low leakage current, usually under 1 mA. If that number goes up, that’s a sign the internal components are breaking down. I tested an arrester at a farm last year that had a leakage current of 12 mA—way too high. The farmer had no idea, because he never had an outage, but that high leakage current was slowly eroding the unit and could’ve caused a fire if it had kept going. Testing is crucial, but even between tests, there are things you can do to spot issues. For example, after a major storm, take 10 minutes to walk around your property and check every arrester. Look for cracks, check the indicator, and feel for unusual heat. A quick check after a storm can save you from a lot of headaches later.
Wait a minute—what about indoor arresters? A lot of people think arresters are only for outdoor power lines and transformers, but indoor arresters (used for electrical panels, HVAC units, and sensitive electronics) are just as important, and they have the same signs of failure. Indoor units can degrade from the heat inside a building, humidity, and even age. I’ve had customers come to me after their home office computers got damaged during a storm, and when we checked their indoor arrester, it had a cracked housing (from being in a hot attic) and a popped indicator. A lot of people forget about indoor arresters, because they’re hidden inside panels. Make sure you’re including them in your maintenance checks, too.
Let me wrap this up with a few common myths I hear all the time. Myth number one: “Arresters only need to be replaced if they get hit by lightning.” No—most arresters that fail never get hit by a direct strike. They fail from age, repeated small surges, or environmental damage long before a lightning event. Myth number two: “If an arrester looks fine, it’s fine.” As I mentioned earlier, internal damage doesn’t always show on the outside. A unit can look perfect and still have lost 50% of its surge capacity. Myth number three: “I can test it myself with a multimeter.” Most basic multimeters aren’t sensitive enough to measure the leakage current or surge capacity of an arrester. You need specialized tools, and more importantly, you need to know how to use them safely around high-voltage power lines. That’s why professional testing is worth the small cost.
At the end of the day, lightning arresters are one of the most underrated safety components in any electrical system. They don’t make headlines, they don’t add to your building’s design appeal, but they’re the difference between a minor thunderstorm and a costly disaster. Whether you’re a small business owner, a property manager, or a homeowner with sensitive electronics, paying attention to these signs will help you avoid unnecessary risk. If you notice any of the things I’ve talked about today—physical damage, unusual heat, a popped indicator, unexplained surges, or an arrester over 15 years old—don’t put it off. Lightning doesn’t wait for a convenient time, and neither should your replacement. If you’re not sure how to inspect or test your arrester, or if you need a high-quality, reliable replacement that’s built to last in your environment, reach out to our team to discuss your needs. We’ve worked with thousands of customers across industries, and we can help you find the right solution for your property, on your timeline.
Meteorological Sounding System References
- IEEE Std C62.22-2017, IEEE Guide for the Application of Metal-Oxide Surge Arresters for Alternating-Current Systems
- National Fire Protection Association (NFPA) 70®, National Electrical Code® (NEC®)
- Lightning Protection Institute (LPI) Standards for Surge Arresters
- International Electrotechnical Commission (IEC) 60099-4:2020, Surge Arresters – Part 4: Metal Oxide Surge Arresters Without Gap for AC Systems
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