How do US stock market circuit breakers function during panic selling?

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US Stock Market Circuit Breakers Function During Panic Selling

A trading halt was triggered in US markets on March 9, 2020, after the S&P 500 index fell 7 per cent shortly after opening, illustrating the function of market-wide circuit breakers designed to mitigate significant declines. These mechanisms were mandated by the Securities and Exchange Commission (SEC) following the Black Monday crash on October 19, 1987, when the Dow Jones Industrial Average plummeted by 22.6 per cent. The current system aims to prevent a recurrence of such drastic market drops by pausing trading under specific conditions.
Triggering and Pausing of Trading

The primary circuit breaker is activated if the S&P 500 index experiences a decline of more than 7 per cent before 3:25 p.m. New York time, leading to a 15-minute suspension of trading. Should the market continue to fall after trading resumes, a second pause is initiated if the index drops by 13 per cent before 3:25 p.m. If a 20 per cent decline is reached at any point, trading for the entire session is suspended.
Broader Market Impact of Circuit Breakers

Once the initial circuit breaker is triggered based on the S&P 500's performance, trading halts are applied across other major indices, including the Dow Jones Industrial Average and the Nasdaq. This coordinated suspension ensures a unified market response during periods of severe downturn, preventing isolated index movements from creating further panic or confusion.
Historical Context and Evolution of Market Disruptions

The implementation of circuit breakers evolved from past market crises. Following the 1987 crash, the SEC introduced these measures. More recently, markets have also experienced closures due to external events, such as Superstorm Sandy in October 2012, which led to the closure of the NYSE trading floor, and the September 11, 2001, terror attacks, which resulted in a shutdown until September 17 to avert panic selling.
Technological Preparedness and Resilience

US stock exchanges maintain contingency plans to ensure continuous and orderly market functioning, even amidst technological failures or external disruptions. While most trading is now electronic, with the NYSE capable of operating fully electronically if its physical floor is unavailable, past incidents like a technical glitch on the NYSE in July 2015 and a Nasdaq trading freeze in August 2013 have highlighted the need for robust systems. Regulation Systems Compliance and Integrity, introduced in 2014, further mandates higher emergency preparedness levels for exchanges, complemented by regular testing of backup facilities.

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How do US stock market circuit breakers function during panic selling?

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