The first U.S. stock-market circuit breakers triggered during the 2020 COVID-19 crash in March 2020.
On 9 March 2020, the S&P 500 fell enough at the opening of trading to activate a Level 1 market-wide circuit breaker. Trading stopped for 15 minutes. Similar halts followed on 12 March, 16 March, and 18 March as investors reacted to the pandemic, economic shutdowns, and an oil-price dispute.
These circuit breakers are designed to pause trading after exceptionally large declines, giving market participants time to assess information and reduce disorderly selling. They do not permanently prevent prices from falling, and trading resumes after the scheduled pause if conditions allow.
The March halts are sometimes described as the first ever U.S. circuit breakers, but that is incorrect. The system was introduced after the 1987 crash and was first used in 1997. March 2020 was notable because the safeguards were activated several times in a short period.