A standard household 110V or 220V circuit breaker is a relatively simple device, it breaks the circuit by pulling the electric contacts apart. Low household voltage generally won't create a persistent spark across a small air gap. However, power transmission lines operate at hundreds of thousands of volts, and they need circuit protection, too. A small air gap isn't going to stand in the way of a very large potential voltage difference, it will ionize the air between the contacts and create a long-lasting spark to bridge the gap, which will severely damage the contacts.

Several methods are therefore used to safely break high-voltage circuits:

  • Oil-filled: Oil has a higher dielectric constant than air, so it is more resistant to spark formation.

  • Vacuum bottle: A spark requires some kind of medium to travel across, it cannot jump across a vacuum. However, very high voltages can vaporize some of the metal on the contacts themselves, creating a path to form a spark.

  • Compressed air: These work by blowing out the spark with a blast of compressed air, extinguishing it like a birthday candle.

  • Gas insulated: Similar to oil-filled, these are filled with a gas which has a high dielectric constant, usually SF6 (Sulfur Hexaflouride). Care must be taken to prevent the gas from escaping, as it is a greenhouse gas and harmful to the environment.

  • Air break: This version physically slides an insulating shield between the contacts to break the circuit.

High voltage circuit breakers are designed with a specific time delay based on the predicted magnitude of a short circuit and its distance down the power line. This is to ensure that problems in the line affect as small an area as possible. Only the closest breaker to the problem should trip.