Ferroresonance is a complicated issue, one that industry workers must be educated about. That’s because as the number of URD system installations grows and systems age, instances of ferroresonance increase – as do threats to worker and customer safety, equipment and service reliability. I first became acquainted with ferroresonance in the 1980s while troubleshooting a pad-mounted, three-phase transformer at night. The pad fed a chemical plant, closed for the evening, in the middle of nowhere. The 480-volt, 2000-amp main was single-phasing, so plant electricians had dropped all the 480-volt sub-feeds except one: a single-phase, 240-volt sub-panel that fed the plant’s fire control systems plus some lighting that was barely functional (keep this information in mind – it will be important a little later). The underground radial feed was long, and the A-phase pothead fuse had blown; my first thought was a bad cable. We pulled the radial-fed elbows at the transformer, opened the 2000-amp main and re-fused the pothead. It held – so why had the other fuse blown? Next, we checked the meter’s kilowatt demand, which was barely more than half the peak-load capacity of the 1500-kVA transformer. Then we smoke-tested the transformer, closing it in from the potheads, opening the potheads with a load-break tool, and finally closing in the elbows on the de-energized pad. The 23-kV potheads were dead-ended on a 10-foot heavy-duty double arm. The fuse for A-phase was on the far side of the arm, on the other side of the neutral from where the bucket truck had been set up. I wasn’t going to side-sling the fuse barrel feeding a 1500-kVA pad, so I boomed over the neutral positioning for the A-phase fuse and closed it. As I was booming back over the neutral to close in B-phase and C-phase, I heard what sounded like a car crash coming from the vicinity of the pad, followed by a flash and something going to ground. The A-phase pothead fuse erupted behind me and the feeder relayed. Back at the pad, A-phase and B-phase elbows were blown off the bushings. The A-phase polymer arrester that was plugged into the feed-through bushings had split down the middle and was still smoking. Additionally, the current transformer cabinet wiring and polyphase Class 10 meter were on fire. What Happened? Given that the CEO of the chemical plant was on the utility company’s advisory board, some plant personnel were alerted when we blew up their transformer. Three crew members stayed at the plant to pull the bad transformer and ready a spare three-phase. An apprentice and I went to retrieve a new transformer. On our way to the yard, an engineer on-site at the plant radioed us, requesting that we bring back a 2000-kVA transformer. I asked if he thought the damaged transformer had been overloaded; he said no and told us he would explain more later. His eventual explanation? You guessed it: ferroresonance. As it turned out, during our troubleshooting, we had created perfect conditions for the loud noise and fire. No one had known that the capacitive reactance of the cable on A-phase and B-phase was nearly equal to – and in series with – the inductive reactance of the 1500-kVA transformer windings. In the evenings during the off-season, the plant reduced operation and electrical loads. With the matching series reactance of the cable and transformer impedance, the lightly loaded transformer would begin to react, creating low-level ferroresonance that overheated and prematurely aged the transformer until the pothead fuse blew. When we began troubleshooting and opened the main, conditions became ideal for runaway core excitation or ferroresonance. The fact that it had taken me a couple minutes to get from the A-phase fuse to the B- and C-phase fuses – combined with the open main and no secondary load – triggered everything needed to blow up that $60,000 installation. You may still be wondering why the engineer requested that 2000-kVA transformer. The answer: in observing damage from the incident and speaking with the crew, he recognized the problem and opted for the new transformer to raise impedance. That way, the cable capacitance and transformer inductance would no longer be almost equal. Need-to-Know Info Ferroresonance is a rare condition most likely to occur with three-phase, pad-mounted, delta-connected transformers. Not nearly as often, ferroresonance has been documented in wye-wye transformers as well as in aerial three-pot banks served by long-dedicated aerial circuits. When ferroresonance occurs in a transformer, high voltages three to five times the rated primary can appear on the primary and secondary and in the core. Oil heats to temperature extremes in minutes, blowing out of vents and bubbling paint on top of the transformer. Surge arresters – not designed to clamp sustained overvoltages – can be cooked to destruction and potentially fragment during failure. The rise in primary also increases the secondary voltage, sometimes blowing up meters like bombs. Other times, purely coincidental yet ideal conditions create low-level ferroresonance that can boil the life out of a transformer with barely a whimper. I know of one case in which a 1000-kVA transformer was replaced three times in five years. Finally, the utility realized a low-level resonant circuit – yes, without an open phase – was killing the transformer every night when the commercial building load dropped to about 6% of the transformer’s rating. URD cables are capacitors; transformer coils are magnetic inductors. To create resonant circuits, there must be capacitive reactance and inductive reactance of almost equal value in series with each other, and the inductor must have very little to no load. The most likely situation is a three-phase transformer fed by a long underground circuit. In some of the most dramatic events, a pothead fuse was opened or blew, allowing a still-energized primary cable (capacitance) to be more or less in series with a coil (inductance). If a series-connected phase-to-coil connection is allowed to remain energized, and there is low loading on the transformer’s secondary or the customer’s mains are open, no impedance exists in the primary circuit. Current is free to flow, and runaway voltage rises in the laminated core – hence the “ferro-” component of ferroresonance. In testing, loading the secondary above 20% has proven sufficient to prevent resonance. The first indication of the condition is typically a loud rattling noise – often described as shaking a coffee can full of marbles – emanating from a transformer due to magnetostriction in the laminated core (note that normally, magnetostriction causes the 60-Hz hum in transformers). The noise is wicked enough that almost no one who hears it stands around to see what is going to happen next. The two other ferroresonance cases I have worked on both involved amorphous-core, three-phase, pad-mounted, wye-delta transformers. One was 1,700 feet of 1/0 to a 1000-kVA; the other was 2,000 feet of 1/0 primary to a 1500-kVA pad-mount. These are not formulas for determining the potential for ferroresonance but examples of the conditions present when it has occurred. Preventive Efforts So, what’s the best way to avoid suspected resonant circuits? Never open three-phase transformers one phase at a time from potheads or lateral taps. Some utilities are using an air-break or AB switch to isolate the coil before switching potheads. Other methods include shortening the primary run to change capacitance or replacing transformers to ensure different inductive impedance values. Rural electric association specs add a fourth dropout to temporarily ground the high-side floating neutral when energizing or de-energizing three-pot banks. The goal is to split the series path between the primary feed and the transformer coils, removing the series reactance, which is a prime condition necessary to create the resonant circuit. The last preventive effort is leaving some or all of the secondary load connected. This, of course, contradicts what we have always been told (i.e., “Don’t single-phase the customer”), but not to worry. You won’t kill the three-phase customer equipment in the time it takes to close three pothead switches – and you’ll almost certainly avoid blowing up a costly transformer. About the Author: After 25 years as a transmission-distribution lineman and foreman, Jim Vaughn, CUSP, has devoted the last 28 years to safety and training. A noted author, trainer and lecturer, he is a senior consultant for the Institute for Safety in Powerline Construction. He can be reached at jim@ispconline.com. Editor’s Note: This is an update to Jim Vaughn’s article “Ferroresonance Explained,” first published by Incident Prevention magazine in 2012.

In this installment of “Voice of Experience,” I am going to share a safety perspective that struck a real chord with me when someone offered it during a recent meeting. My goal in passing it along during this season of reflection is to prompt readers to contemplate and continue refining their safe work practices. From the time we are born, we learn about the world in various ways, often in the form of instructions about what we should and should not do. Our parents forbid us from crossing the street without their assistance. We are warned to keep our hands away from hot stovetops. Even before learning to read, we begin to observe and understand visual cues – for instance, a child riding in a car seat might notice different types of lines on the roadways (e.g., a single dotted white line, two solid yellow lines). Such instructions and cues provide details about our surroundings to help us make safer, more informed choices. As teenagers, many of us learn to operate a car and undergo testing to earn a driver’s license. It would make sense, then, for us to increasingly respect the rules of the road – including those white and yellow lines we noticed well before we were permitted behind the wheel – as we accrue more driving experience. After all, we must protect not only our personal safety but the well-being of our passengers, pedestrians and other motorists. Yet in 2023, U.S. Department of Transportation data tells us that more than 40,000 people were fatally injured on American roadways (see www.transportation.gov/NRSS/SafetyProblem). Safety vs. Potential Peril Let’s talk about a different type of line for a moment. Have you ever heard someone in politics or business warn others against crossing a proverbial line in the sand? That is highly likely; it is not an uncommon statement. When the individual at the meeting I attended shared his safety perspective, what sprang to mind was the industry-specific line in the sand that we can almost always spot on any given jobsite. It is the line that divides a worker’s physical safety from their potential peril. In terms of line work, where do you think that line in the sand should be, particularly for energized work? A student enrolled in an accredited lineworker development program must complete four to seven years of training to graduate; by the time they finish, they almost certainly know the line’s precise location. But all lineworker development programs are not created equal, nor do testing and proficiency demonstrations always adequately assess whether an employee has received the requisite training. These inconsistencies are significant contributors to incidents that continue to seriously injure and kill our brothers and sisters. Employees who had served the industry for less than three years at their time of death account for far too many of the cases I have investigated during my career. As I noted earlier, the dividing line between worker safety and potential peril is visible at nearly every jobsite – but only if we are willing to look. On one side of the line, we are guided by regulatory standards and industry best practices; on the other, employees engage in hazardous and even illegal work practices. Here is my question to you: In recent months, how close to that line have you been standing? If you were to cross it, do you truly understand what could happen? Would you be prepared to handle the full weight of any adverse consequences? Because the fact is, workers cross the line every day. And each year, dozens of our brothers and sisters pay the price with their lives. You may be wondering if there is a specific reason why so many employees cross the line into unsafe work practices. Investigations often determine that an incident stemmed from intentional choices made by an inadequately trained worker or crew who lacked sufficient management oversight. This is why it is paramount for every worker to be appropriately trained on each task they will be required to perform, including the prescribed way to complete it, the reasons it must be done in that specific order, and what happens if any steps are transposed or omitted. Conclusion Legally, morally and ethically, standing on the safety side of the line is the right thing to do. Let’s vow to step as far back from that line as possible rather than standing the closest we can without crossing over. We are human, so mistakes will be made; we must give ourselves room to fail safely instead of teetering on the edge of possible catastrophe. As we head into 2026, I urge you to reflect on the past 12 months and evaluate your performance. Did you cross the line between safety and potential peril, or perhaps find yourself working right up against it? If so, what will you do differently tomorrow to increase your distance – and your margin of safety? About the Author: Danny Raines, CUSP, is an author, an OSHA-authorized trainer, and a transmission and distribution safety consultant who retired from Georgia Power after 40 years of service and now operates Raines Utility Safety Solutions LLC.

Learn more from Danny Raines on the Utility Safety Podcast series. Listen now at https://utilitysafety.podbean.com!