Electrical utilities use large electrical transformers to drop the voltage coming from power lines. These transformers can be huge; the size of a car or greater. The transformers are sealed with an oil bath surrounding all of the electrical coils inside. This oil bath acts as both a coolant and an insulator. At the top of the transformer, there is a headspace that has no oil and instead is filled with nitrogen, generally under a slight positive pressure. For a new transformer, this nitrogen blanket will remain “clean” for a long time. As the transformer ages, and the insulation between the wire coils starts to break down, the high voltage can arc between two adjacent coils of the transformer. When this occurs, the high voltage passes through the oil blanket, and causes the oil to break down. This causes small amounts of flammable vapors to form from the oil, and rise to the top of the transformer to mix with the nitrogen blanket. These flammable vapors consist of a variety of gases, but generally hydrogen is predominant. Periodic testing of the nitrogen blanket for flammable vapors is a good indication of the health of the transformer. If the testing reveals a buildup of flammable vapors, the transformer can be removed from service in a planned manner instead of a catastrophic manner (they can blow up if undetected).