A circuit breaker was the emergency market safeguard triggered four times in the United States during the March 2020 crash. These rules temporarily halt or slow trading after exceptionally large declines, giving investors time to assess information and orders.
The S&P 500 triggered market-wide circuit breakers on March 9, March 12, March 16, and March 18, 2020. Each halt followed a 7% intraday drop from the previous close under the rules then in force. The dates fell during the global economic shock caused by the COVID-19 pandemic and an oil-price dispute.
Circuit breakers are not intended to prevent all losses. They pause trading at specified thresholds, while trading resumes if conditions allow. The U.S. system was introduced after the 1987 crash and later revised; the 2020 episodes were the first market-wide Level 1 halts since 1997.