US stock exchanges first triggered their market-wide circuit breakers in March 2020 during the 2020 stock-market crash.
The New York Stock Exchange and other US markets activated the mechanisms four times during March. The first occurred on March 9, after the S&P 500 fell at least 7% from its previous close. Further halts followed on March 12, March 16, and March 18 as investors reacted to the accelerating COVID-19 pandemic and the economic shock of lockdowns.
A Level 1 circuit breaker pauses trading when the S&P 500 falls 7% before 3:25 p.m. Eastern Time. Level 2 is reached at 13%, while a 20% fall triggers a Level 3 halt for the rest of the session.
These rules are designed to slow panic and give market participants time to assess information. They do not prevent prices from falling later, and they are different from ordinary individual-stock trading halts.