When a transformer fails, the immediate result ranges from a quiet loss of power to a violent explosion, depending on the type and severity of the fault. Inside the unit, insulation breaks down between windings, creating electrical arcs that superheat the surrounding oil and generate intense pressure. What follows can affect everything from a single neighborhood to thousands of square miles of grid, and the replacement process now takes longer and costs more than at any point in recent history.
What Happens Inside the Transformer
Transformers rely on layers of insulation, usually paper and oil, to keep high-voltage windings separated. Over time, that insulation degrades through a combination of chemical reactions: oxidation from dissolved oxygen, hydrolysis from moisture, and thermal decomposition from heat. Partial electrical discharges, tiny sparks that jump through weakened insulation, accelerate the damage further. Once the insulation can no longer hold back the voltage difference between windings, a full arc forms.
That internal arc is where things escalate. An electrical arc inside a transformer can reach temperatures above 20,000°F, rapidly vaporizing the mineral oil that fills the tank. The vaporized oil produces a massive volume of combustible gas in milliseconds, and the pressure inside the sealed steel tank spikes dramatically. Transformers are equipped with quick-acting pressure relief devices designed to vent this buildup in a controlled way. When the fault develops faster than the relief valve can respond, the tank itself can rupture, sending hot oil and metal fragments outward.
Fires, Explosions, and Oil Spills
A ruptured transformer tank often leads to fire. The mineral oil inside is flammable, and the electrical arc provides an ignition source. Large utility transformers can hold thousands of gallons of oil, so a fire can burn intensely and spread to adjacent equipment at a substation. Videos of transformer explosions that circulate online, showing bright flashes and fireballs lighting up the night sky, typically capture this sequence: internal arc, pressure buildup, tank failure, oil ignition.
Even without an explosion, oil leaks from a failed transformer create environmental problems. Older units may contain oil contaminated with PCBs, industrial chemicals that persist in soil and water for decades. Federal regulations under the EPA set specific cleanup standards for any spill of material containing PCBs at concentrations of 50 parts per million or greater, with even stricter requirements when spills reach surface water, drinking water supplies, sewers, grazing land, or vegetable gardens. Cleanup can involve excavating contaminated soil and disposing of it at specialized facilities.
Electrical Hazards Near a Failed Transformer
A transformer failure can create invisible dangers on the ground nearby. During a fault, electricity flowing into the earth creates a voltage gradient radiating outward from the grounding point. This produces two specific hazards. Step potential is the voltage difference between your two feet as you walk near the fault. If one foot is closer to the energized ground than the other, current flows up one leg and down the other. Touch potential occurs if you’re physically contacting a metal structure, like a utility pole or fence, that has become energized. Current enters through your hand, passes through your torso, and exits through your feet.
Both hazards are particularly dangerous because of how utility systems try to restore power automatically. Devices called reclosers will re-energize a faulted line after a brief pause, sometimes multiple times. Someone who has been knocked down by an initial shock may be lying on the ground, increasing their contact area with the energized earth, when the recloser sends power back through. This is why utility workers advise shuffling away from downed equipment with small steps rather than walking normally, keeping both feet close together to minimize the voltage difference between them.
How One Failure Cascades Across the Grid
Power grids are more fragile than most people realize. Research from Idaho National Laboratory has modeled what happens when a major transformer goes down along with nearby transmission lines. In one scenario, the loss of a single large transformer and two transmission lines caused four parallel lines to become overloaded as electricity rerouted through them. Those four lines tripped offline, forcing 14 more circuits into overload. The chain reaction continued until 25 circuits had been knocked out and roughly 4,500 megawatts of load was lost, enough to power several million homes.
The physics behind this are straightforward. Power grids are sparsely connected, meaning each node has relatively few links to other nodes. When one path for electricity disappears, the remaining paths must carry more current. If any of those paths exceed their rated capacity, protective systems automatically disconnect them, pushing even more current onto the shrinking number of surviving lines. The process can become self-sustaining. Grid operators fight cascading failures by deliberately cutting power to some customers (load shedding) to reduce the burden on remaining lines before the entire system fragments into disconnected islands, some of which black out entirely.
Warning Signs Before Failure
Transformers give off audible clues as they deteriorate. A healthy transformer produces a steady, low-pitched hum caused by the vibration of its steel core at the frequency of the electrical current (60 Hz in North America). Changes in that sound often signal trouble. A sharp or irregular buzzing can indicate loose core clamps or bolts. Rattling or metallic clicking suggests loose sheet steel or mounting hardware. A sudden increase in volume or a shift in pitch points to overloading, partial discharge activity, or insulation breakdown.
Utilities also monitor dissolved gases in transformer oil, since different types of internal faults produce characteristic gas signatures. Elevated levels of certain gases can reveal overheating, arcing, or partial discharge months before a catastrophic failure. Temperature sensors, moisture monitors, and periodic oil sampling round out the diagnostic toolkit. Despite all of this, some failures still occur without warning, particularly when caused by external events like lightning strikes, falling trees, or animals bridging energized components.
Replacement Costs and Wait Times
Replacing a failed transformer has become significantly harder and more expensive. Since 2019, prices for large power transformers have increased 77%, and some classes of smaller distribution transformers have nearly doubled in cost. The underlying causes include years of underinvestment in domestic manufacturing, a surge in demand from data centers, electric vehicle charging infrastructure, and post-pandemic construction, plus volatile prices for the specialized steel and copper these units require.
Lead times for large power transformers now stretch well beyond historical norms, often into multi-year territory. A utility that loses a major transformer today may wait 12 months or longer for a replacement, and that assumes the unit can be sourced at all. Nearly $1.8 billion in new North American manufacturing capacity has been announced, but analysts at Wood Mackenzie warned in mid-2025 that the U.S. transformer market remains structurally out of balance, with demand continuing to outpace any realistic near-term increase in production. Trade policies, including expanded steel and aluminum tariffs, are adding further upward pressure on prices for both imported and domestically built units.
For communities, this means a failed transformer at a critical substation can result in extended outages or reliance on temporary, lower-capacity equipment while a permanent replacement is built and delivered. Utilities have begun ordering transformers years in advance of projected need, treating early procurement as a strategic priority rather than a routine purchase.