U.S. stock exchanges first triggered circuit breakers during the COVID-19 market crash in 2020.
The first modern U.S. market-wide trading halt of that episode occurred on March 9, 2020, after the S&P 500 fell at least 7 percent shortly after opening. Similar halts followed on March 12, March 16, and March 18.
These circuit breakers temporarily stop trading when a market index falls through specified thresholds. Their purpose is to provide time for information to circulate and for investors to reassess orders instead of allowing an uncontrolled cascade of automated and human selling.
The 2020 crash reflected the economic shock of the COVID-19 pandemic, including lockdowns and uncertainty about business activity. A collapse in oil prices also intensified the March 9 sell-off after a dispute between major oil producers.