A circuit breaker was the emergency trading safeguard triggered four times during the U.S. stock-market turmoil of March 2020.
Circuit breakers temporarily halt trading after exceptionally rapid declines. In March 2020, U.S. markets experienced extreme volatility as the COVID-19 pandemic disrupted economic activity and investors feared recession. The S&P 500 triggered its Level 1 circuit breaker on March 9, March 12, March 16, and March 18.
The mechanism was designed to slow panic selling and give investors time to absorb information. It replaced the older practice of allowing prices to move continuously during a sudden collapse, although trading halts cannot eliminate underlying economic risk.
A circuit breaker is not the same as a margin call. A margin call demands additional collateral from an investor whose borrowed position has lost value; a circuit breaker is a market-wide trading pause based on index movements. The 2020 episode was the first time the modern U.S. system activated four times in one month.