A circuit breaker was the emergency trading safeguard first triggered in the United States during the March 9, 2020 market crash. The S&P 500 fell enough at the open to activate a Level 1 market-wide halt.
Trading on major U.S. exchanges paused for 15 minutes after the index dropped 7% from its previous close. The halt was designed to give investors time to absorb information and reduce disorderly, panic-driven selling.
The crash occurred as markets reacted to the global spread of COVID-19, worsening economic forecasts, and an oil-price shock caused by a dispute between Saudi Arabia and Russia. Additional U.S. circuit breakers were triggered on March 12, March 16, and March 18.
Circuit breakers do not prevent losses or guarantee a rebound. They temporarily interrupt trading according to preset thresholds. Their use in March 2020 demonstrated that the safeguards introduced after earlier crashes could operate during a modern global shock.