Causes and Preventive Measures of Fire and Explosion in Oil Circuit Breakers

Causes of Fire and Explosion in Oil Circuit Breakers

• When the oil level in an oil circuit breaker is too low, the oil layer covering the contacts becomes too thin. Under the effect of the electric arc, the oil decomposes and releases flammable gases. These gases accumulate in the space beneath the top cover, mixing with air to form an explosive mixture, which can ignite or explode under high temperature.

• If the oil level inside the tank is too high, the released gases have limited space to expand, leading to excessive internal pressure that may cause the tank to rupture or explode.

• Excessive impurities and moisture in the oil can cause internal flashover within the circuit breaker.

• Improper adjustment or malfunction of the operating mechanism may result in slow operation or poor contact after closing. If the arc cannot be promptly interrupted and extinguished, excessive flammable gas builds up inside the tank, potentially causing fire.

• The interrupting capacity of an oil circuit breaker is a critical parameter for power systems. If this capacity is lower than the system’s short-circuit capacity, the breaker cannot interrupt the high short-circuit current effectively. The sustained arc then leads to fire or explosion of the breaker.

• Poor sealing between the bushings and the tank cover, or between the cover and the tank body, can allow water ingress and moisture accumulation. Additionally, a dirty tank interior or mechanically damaged bushings may cause ground faults, leading to fire or explosion.

Preventive Measures Against Oil Circuit Breaker Fires

(1) The rated interrupting capacity of the oil circuit breaker must match the short-circuit capacity of the power system.

(2) Regular monitoring and routine inspections of oil circuit breakers should be strengthened—especially during peak load periods, after each automatic trip, and under adverse weather conditions—by increasing patrol frequency to continuously assess operational status.

(3) During routine inspections, special attention should be paid to:

• Oil level indicated on the oil gauge,

• Signs of oil leakage,

• Condition of insulating bushings (checking for dirt, cracks),

• Presence of abnormal noises or flashover phenomena.

(4) Indoor oil circuit breakers must be installed in fire-resistant buildings with adequate ventilation. Indoor bulk-oil breakers should be equipped with oil-containment facilities. Pole-mounted oil breakers should be protected by lightning arresters.

(5) Regular minor and major maintenance, along with electrical performance tests and oil sample analyses, must be conducted to ensure the oil circuit breaker remains in optimal operating condition.