A transformer usually “blows” when heat, stress, or insulation failure pushes it past what it was designed to handle, often ending in an internal short, arcing, and sometimes an explosion or fire.

Key causes: what makes a transformer blow?

Think of a transformer as a box that must keep electricity, heat, and insulation in delicate balance. When one of these gets out of control, it fails. 1. Overloading (too much load for too long)

  • Drawing more power than the transformer’s kVA rating makes current rise in the windings.
  • Higher current means higher I2RI^2RI2R losses in the copper, so temperature climbs quickly.
  • If this runs on for hours or days, insulation cooks, becomes brittle, and eventually breaks down, causing short circuits and “blowouts.”

2. Overheating and poor cooling

  • Blocked ventilation, dust on cooling fins, failed fans, low oil level, or degraded cooling radiators all trap heat inside.
  • Even at normal load, poor cooling pushes internal temperature above design limits, accelerating insulation aging and leading to failure.
  • High ambient temperatures (heat waves, enclosed rooms) make it worse because the transformer can’t dump heat into already hot air.

3. Insulation breakdown inside the transformer

  • Overheating, age, moisture, contamination, and oxidation gradually weaken the paper/oil or solid insulation between windings and parts at different voltages.
  • Once insulation strength drops below the operating voltage stress, internal arcing starts: that arc can vaporize oil or materials, rapidly create gas and pressure, and the tank may rupture or “blow.”
  • Oil that has degraded or become contaminated loses both dielectric strength and cooling ability, making blowouts more likely.

4. Short circuits and faults on the grid

  • External faults (downed lines, phase‑to‑ground faults, shorted loads) send very high fault currents through the transformer.
  • Even for a short time, these currents create enormous mechanical forces on the windings and severe heating.
  • If protection relays or fuses don’t clear the fault fast enough, windings can deform, insulation fails, and the transformer can violently fail.

5. Lightning strikes and surge events (especially in storms)

  • Lightning or switching surges can impose extremely high transient overvoltages on distribution transformers.
  • These spikes punch through insulation, cause internal flashover, and may ignite oil, leading to the classic loud bang and bright flash you see during storms.
  • This is why you often hear about transformers blowing during thunderstorms or in areas with poor surge protection.

6. Sustained overvoltage and harmonics

  • Operating with voltage above design for long periods over‑excites the core, causing excessive iron (core) losses and overheating.
  • Nonlinear loads (lots of electronics, VFDs, LED drivers, data centers) inject harmonics into the system.
  • Harmonics increase core and copper losses, raise neutral currents, and can overheat the transformer even when the kVA looks “within rating,” eventually leading to failure.

7. Mechanical and connection issues

  • Loose or corroded terminals, poor internal connections, or manufacturing defects create high‑resistance “hot spots.”
  • These spots heat much faster than the rest of the winding, damaging insulation locally until a short develops.
  • Vibration, improper mounting, or damage during transport can also weaken internal components so they fail under normal stress later.

8. Aging and lack of maintenance

  • Over years, thermal cycles, moisture ingress, oxidation of oil, and general wear reduce the dielectric strength of insulation.
  • Without oil testing, dissolved gas analysis, thermal imaging, and tightening/checks, incipient problems go unnoticed until they turn into a blowout.
  • Aging by itself doesn’t “explode” a transformer, but it lowers the margin so that a moderate surge or overload that a new unit would survive instead causes failure.

What actually happens when a transformer “blows”?

When people say a transformer blew, they usually mean one of two things:

  • Burnout:
    • Internal windings or connections overheat and burn, often filling the tank with gases and carbon.
    • Protection devices trip, power goes out, but there may be little external drama beyond smoke and a bad smell.
  • Explosion / violent failure:
    • A severe internal arc in oil rapidly vaporizes oil and creates gas, dramatically increasing pressure.
    • If pressure-relief devices and protection don’t act quickly, the tank can rupture, expelling burning oil, accompanied by a very loud bang, bright flash, and sometimes fire or smoke plume.

From the outside, people often just see: lights flicker, a loud boom, maybe a blue or orange flash near a pole‑mounted transformer, and then a power outage.

Why transformers blow a lot during storms or “bad power” conditions

In recent years (and especially with more electronics on the grid), several trends have made transformer blowouts feel more common:

  • More lightning and extreme weather events stressing overhead lines and transformers.
  • Higher penetration of nonlinear electronic loads in homes and industry, increasing harmonics and neutral currents.
  • Aging infrastructure in many regions, where older transformers stay in service beyond their ideal life and are more vulnerable to surges and overloads.

During a storm in a typical neighborhood, a single lightning strike or tree‑caused fault can produce:

  • A surge that punctures insulation.
  • A severe fault current that deforms windings.
  • Repeated reclosing attempts from automatic breakers, subjecting the same stressed transformer to multiple jolts, until it finally fails violently.

How failures are usually prevented

Professionals try to keep transformers from ever reaching that “blow” point by:

  • Designing correctly: Adequate kVA rating, surge arresters, proper cooling, and harmonic‑tolerant designs where needed.
  • Operating within limits: Avoiding sustained overloads, monitoring temperature and load, and controlling harmonics.
  • Maintaining regularly: Oil testing, dissolved gas analysis, thermal imaging for hot spots, tightening connections, cleaning cooling surfaces.
  • Using proper protection: Fuses, relays, pressure relief devices, Buchholz relays (on some types), and surge protection to clear faults before catastrophic damage occurs.

A simple example: if a small pole‑mounted transformer is repeatedly overloaded by new air conditioners and EV chargers added on the same street, it runs hotter and hotter, its insulation ages faster, and the first big summer storm with a surge is much more likely to make it “blow” dramatically.

TL;DR:
A transformer blows when heat, stress, and weakened insulation come together—typically because of overloads, poor cooling, surges (lightning or faults), or aging and neglected maintenance, which trigger internal arcing, rapid gas/pressure build‑up, and sometimes an explosion.

Information gathered from public forums or data available on the internet and portrayed here.