If you’ve ever worked with pad-mounted transformers—those often-units tucked on concrete slabs at commercial parks, apartment complexes, and even busy urban sidewalks—you know they’re the quiet workhorses of the distribution grid. For my team, as a pad-mounted transformer supplier, we spend a lot of time talking to clients about things that go wrong with these units, and one of the most underdiscussed issues we see over and over is open-circuit faults. Most people assume a transformer fault means something catastrophic—an explosion, a fire, a total shutdown that knocks out an entire neighborhood. But an open-circuit fault is different: it’s often subtle, easy to miss at first, but it can cause long-term damage that costs our clients way more than they expect, especially when left unaddressed. Pad Mounted Transformer
Let me start with the basics, just so we’re all on the same page. An open-circuit fault in a pad-mounted transformer isn’t the same as a short circuit, where two live wires touch each other and create a surge of electricity that blows fuses or trips breakers. An open circuit happens when a path that’s supposed to carry current gets broken, like a loose connection, a corroded wire that snaps, a burned-out fuse in one phase, or a faulty terminal on the transformer’s primary or secondary side. A lot of our clients brush off early signs of open-circuit issues because it doesn’t cause a big, obvious outage. They’ll say, “The lights are on, just some outlets are dead” or “My HVAC is running but it’s not blowing cold air” and assume it’s a minor electrical problem, not a transformer issue. That’s where the trouble starts.
The first impact of an open-circuit fault is uneven load distribution across the transformer’s windings. Most pad-mounted transformers are three-phase units, right? They power three separate legs of electricity that split the load evenly: one for lighting, one for heavy equipment, one for HVAC or other high-draw systems. When one phase opens up, the remaining two phases have to pick up all the load that was meant for three. Let’s do a quick real-world example I saw last month at a small shopping center our team equipped. The transformer for that center was a 150kVA pad unit installed three years prior, and one of the secondary phase wires corroded at the connection point inside the transformer’s enclosure, causing an open circuit. At first, the only noticeable issue was that one of the storefronts’ cash registers wouldn’t turn on, and the security system cameras on that side of the lot were glitchy. The building’s electrician checked and blamed the building’s wiring, so they fixed that store’s outlet, skipped checking the transformer, and kept operating. Six months later, the transformer overheated so badly that it melted a winding, and we had to replace the entire unit at a cost of more than $20,000 for the client, plus an extra $12,000 in lost revenue from the two-day outage while we installed the new pad unit. That’s the kind of hidden cost no one plans for.
Uneven load isn’t just a minor inconvenience for the transformer. Pad-mounted transformers are engineered to run within a tight temperature range, and when two phases are carrying 150% to 175% of their designed load (since the third phase is offline), the core and windings heat up. That heat accelerates the breakdown of insulation on the copper windings. Insulation is the most important part of a transformer—without it, the windings touch each other, causing a short circuit that can destroy the unit. The open-circuit fault speeds up insulation degradation so much that a transformer that’s supposed to last 25 years might fail in half that time. I’ve seen this with clients who opt for cheaper maintenance checks: they do a quick visual once a year, but they never test phase current balance, so the open circuit flies under the radar until it’s too late.
Another big impact we warn clients about is voltage imbalance. When a phase opens, the voltage across the remaining two phases shifts, leading to unbalanced voltages. Most commercial and industrial equipment—motors, compressors, HVAC systems, even modern commercial kitchen appliances—are built to handle a small amount of voltage imbalance, usually under 1%. If it gets higher than that, which is common with an open-circuit fault, motors start to overheat. Let’s take the apartment complex example: our team installed a 300kVA pad unit for a 20-unit building in 2021. Last year, the main secondary cable to the building’s electrical panel had a broken neutral conductor, which created an open circuit condition in one of the phases. The property manager noticed that half the units had lights that were too dim, and the other half had lights that were too bright, but they thought it was a utility issue and waited a month to call in an electrician. By the time they fixed the open neutral, 12 of the building’s HVAC air handlers had burnt out motors, and they had to replace them at a total cost of $18,000. That’s on top of the potential damage to the pad-mounted transformer itself from the extra heat. We get calls like this every week from property managers, small business owners, and municipal electric departments that didn’t realize an open-circuit fault in their pad transformer was the root cause.
Wait, I should clarify that open-circuit faults can happen on either the primary or secondary side of the pad-mounted transformer, and each has different impacts. Primary-side open circuits are often caused by issues on the utility side: blown fuses, loose connections at the pole that feeds the transformer, or a damaged overhead conductor leading to the pad unit. Primary-side open circuits are a bit easier to detect because they usually cause the entire transformer to be dead—no power at all to the secondary side. That’s the obvious outage that people notice right away. The trouble with primary-side open circuits is that once you reset the fuse or fix the line, you might not check if the transformer itself was damaged during the time it was offline. If the core was stressed during the outage, or if moisture got into the enclosure while power was off, that can lead to latent failure later.
Secondary-side open circuits are trickier, as we saw with the shopping center and apartment examples. These happen on the low-voltage side, inside the pad unit’s enclosure or the wiring that runs from the transformer to the building or load. Secondary-side open circuits often only affect part of the load, so people dismiss the symptoms. A lot of our clients will call us saying, “My pad transformer is working but my parking lot lights are half out” or “My loading dock power is not working” and assume it’s a fixture or outlet problem, not the transformer. We always tell them to check phase current balance when that happens, because an open-circuit fault in a secondary phase is one of the most common causes of those partial outages.
I also want to talk about another impact that’s often overlooked: safety risks. Pad-mounted transformers are enclosed in steel cases, but that enclosure isn’t perfect. When a winding overheats because of uneven load from an open-circuit fault, the internal temperature can get high enough to melt insulation and even create small arc flashes inside the unit. If the steel enclosure isn’t properly grounded, that arc can escape, or overheated oil (wait, yes, pad-mounted transformers use either oil or cast resin, right? Most medium-voltage pad units use oil for better heat dissipation) can leak out, creating a fire hazard. Last year, we worked with a convenience store chain that had a small fire in one of their pad units after an open phase caused overheating. The fire was contained by the unit’s pressure relief valve, but it damaged the transformer, and the store was closed for 12 hours. The cost of the lost sales for that convenience store was way higher than the cost of fixing the transformer and checking for other open-circuit risks.
So, what can our clients do to avoid these issues? As a pad-mounted transformer supplier, we don’t just sell units—we also offer maintenance recommendations and testing services that help catch open-circuit faults early. The biggest thing is regular, proactive testing, not just visual inspections. A qualified electrician should test phase current balance every six months, even if there are no obvious issues. They should check all connections inside the pad unit’s enclosure for corrosion or looseness, because that’s the most common cause of open-circuit faults. We also recommend installing overcurrent protection devices that can detect phase imbalance, which will trip the transformer before it gets damaged by uneven load.
Wait, I should make sure this is accurate, right? I’ve been working in this industry for eight years now, and I’ve seen too many clients cut corners on maintenance, only to end up paying for a new transformer, replacement equipment, and outage costs that add up to way more than the cost of regular testing. One of my first clients was a hotel in downtown Austin that skipped their annual maintenance check for two years because of budget cuts. Their pad-mounted transformer developed an open circuit in one secondary phase, leading to a voltage imbalance that burned out 15 elevators and a dozen commercial refrigeration units in the hotel’s kitchen. The total cost of that failure was almost $100,000, which was way more than the cost of testing and maintaining the transformer each year. That’s a story I share with every new client, because it sticks with people.
Another thing to note: not all pad-mounted transformers are the same. When we recommend units to clients, we factor in the type of load they’re powering, because some loads are more sensitive to open-circuit faults than others. For example, a pad unit powering a small retail space with mostly lighting will handle a temporary open phase better than a unit powering a hospital wing with critical care equipment, or an industrial facility with large motors. For sensitive loads, we recommend installing three-phase monitoring systems that alert facility managers the second a phase imbalance or open circuit is detected. That way, they can fix the issue before it causes damage, not after.
I think a lot of people outside our industry underestimate how much a small fault like an open circuit can impact a pad-mounted transformer. We’re often asked by clients, “Why can’t I just fix the broken wire and keep using the old transformer?” And the answer is: if you wait too long, that old transformer might already have insulation damage, winding stress, or latent issues that will cause it to fail soon after you fix the open circuit. That’s why testing is so important. When we inspect a pad unit, we don’t just check for open circuits—we check for signs that the transformer has already been stressed by past open or short faults, so we can recommend replacements or repairs before it fails unexpectedly.
As a supplier, our goal isn’t just to sell you a pad-mounted transformer. It’s to make sure that transformer works for you for its full 25 to 30-year lifespan, without causing unexpected costs or downtime. Open-circuit faults are one of the most avoidable causes of premature transformer failure, but they’re also one of the most common if you skip regular maintenance. If you’re noticing partial outages, dim or erratic lighting, or equipment that’s running too hot or not at full power, it’s worth checking your pad-mounted transformer for open-circuit faults. Don’t wait for a full outage or a total transformer replacement to address the issue.
If you have questions about open-circuit faults, transformer maintenance, or need to upgrade your pad-mounted transformer to better handle your load, feel free to reach out for a consultation. We’re happy to walk through your specific needs, answer any questions you have about our units, and help you find a solution that fits your budget and your timeline.
References
Box Type Substation IEEE Std C57.12.00, Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers
NEMA Standard TP 1, Guidelines for the Application, Installation, Operation, and Maintenance of Pad-Mounted Transformers
Electrical Construction and Maintenance Magazine, “Common Faults in Pad-Mounted Transformers and How to Prevent Them,” 2022
Underwriters Laboratories (UL) 1561, Standard for Safety for Pad-Mounted Equipment, Distribution-Type, 600 Volts and Less
Jiangsu Yuantong Electric Co., Ltd.
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