What are circuit breakers in markets?

Market-wide circuit breakers are automatic trading halts that trigger when the S&P 500 index declines by specified percentages from the previous day's closing level. The current rules, set by the SEC and exchanges, define three levels. Level 1 triggers at a 7% decline and halts trading for 15 minutes. Level 2 triggers at a 13% decline and halts for another 15 minutes. Level 3 triggers at a 20% decline and halts trading for the remainder of the day. Levels 1 and 2 only trigger before 3:25 PM ET; after that, only Level 3 applies.

Circuit breakers were introduced after the October 19, 1987 "Black Monday" crash, when the Dow Jones fell 22.6% in a single session. The original rules used percentage triggers based on the Dow Jones Industrial Average and were revised multiple times. The current S&P 500-based system was adopted in 2013, reflecting the S&P 500's status as the benchmark US equity index. Individual stocks also have their own "limit up/limit down" (LULD) mechanisms that pause trading when a stock moves more than a specified percentage from its rolling reference price.

The most significant recent activation occurred on March 9, 2020, when COVID pandemic fears triggered a Level 1 halt just minutes after the opening bell. The halt triggered again on March 12 and March 16. These halts were the first market-wide circuit breaker triggers since the current rules were adopted and demonstrated both the mechanism's function and its limitations: while the 15-minute pause allowed some information processing, the selloffs resumed immediately afterward.

The policy debate around circuit breakers involves tradeoffs between stability and price discovery. Proponents argue that halts prevent cascading panic selling and forced liquidations that push prices far below fundamental values. Critics argue that halts can actually increase selling pressure because traders rush to sell before the next halt, and the resumption of trading often features a gap down. The "magnet effect," where prices accelerate toward the trigger level as traders anticipate the halt, is a documented phenomenon that complicates the intended stabilizing effect.