What Happens When a Transformer Blows?

Transformers are devices that step up or step down the voltage of an electrical supply. In order to minimize transmission losses, utility companies transmit electricity at high voltages across overhead or buried wires, and a transformer steps down this voltage to make it suitable for household use. When a transformer fails, however, it can fail spectacularly, resulting in a fire or explosion.

The heart of an electrical transformer is two sets of coiled wires around a ferromagnetic core. The side connected to the high-voltage line has more coils than the side connected to the residential line. When power flows through the denser coil, it creates a magnetic field, transferring electrical current to the other side of the transformer at a lower voltage. Insulation protects the wires, and a coolant, such as mineral oil, keeps the entire assembly from overheating.

Transformer explosions can happen for a variety of reasons. One of the most common is a lightning strike from a storm forcing an overload of the transformer. Damage to wires or equipment elsewhere in the electrical grid can also cause too much electricity to flow into the transformer, causing it to blow. Transformers contain protective circuits designed to shut off the system if the voltage spikes, but these safety devices can take up to 60 milliseconds to trigger, and may not be fast enough to prevent the transformer from blowing. In addition, wear and corrosion over time can weaken the wire insulation, or other transformer components, increasing the possibility of a failure.

Whether the failure comes from corroded insulation or a lightning strike, the result can be spectacular. Overcharged or worn wiring creates heat and a spark, enough to ignite the mineral oil keeping the transformer cooled. When the mineral oil begins to burn, it creates a massive overpressure inside the sealed transformer, eventually causing the vessel to rupture with a loud bang and shower of sparks and flame. Due to the materials involved, the flash is frequently bluish-green in color, and can be seen at night from a very long distance.

When a transformer blows, it interrupts electrical service to any residences or businesses connected to the transformer. Electric service crews must replace the destroyed hardware, first shutting down the incoming electrical line to prevent damage and injury. Depending on the level of damage, service workers may have to shut off other electrical service in the immediate area temporarily, in order to prevent stress on the electrical grid while replacing the destroyed transformer.