Circuit breakers halted U.S. exchange trading after extreme intraday declines during the March 2020 stock-market crash.
On March 9, March 12, March 16, and March 18, 2020, the S&P 500 fell enough to trigger Level 1 market-wide circuit breakers. Each activation paused trading for 15 minutes. The rules are based on percentage declines in the S&P 500 from the previous day's closing level and are intended to give investors time to absorb information rather than react in an uninterrupted cascade.
The 2020 crash occurred as the COVID-19 pandemic spread and an oil-price dispute intensified. It followed a long bull market and produced exceptionally rapid losses in major U.S. indexes. The trading pauses did not prevent the bear market, but they temporarily interrupted extreme selling.
Circuit breakers are sometimes confused with individual-stock trading halts. Market-wide breakers affect the broader exchange session, while a single security can be paused under separate rules for company-specific or price-movement reasons.