Circuit Breaker
A circuit breaker is a pre-established, automatic market-halting mechanism that temporarily suspends trading in a security or exchange when price movements exceed defined thresholds, designed to provide markets with a 'cooling-off period' to prevent panic selling and restore orderly trading conditions.
Key takeaways
- U.S. equity market-wide circuit breakers halt all trading when the S&P 500 declines 7% (Level 1), 13% (Level 2), or 20% (Level 3) from the prior day's closing price, with halts lasting 15 minutes for Levels 1 and 2.
- Individual stock circuit breakers (Limit Up-Limit Down, LULD) halt trading when a stock moves more than 5-10% from a reference price within a five-minute window, preventing mini flash crashes.
- Circuit breakers were introduced after the 1987 Black Monday crash; LULD was implemented following the May 6, 2010 Flash Crash, which saw the Dow drop nearly 1,000 points in minutes.
- Futures markets have circuit breakers called 'limit moves' — maximum daily price movement limits that stop trading when hit, allowing for orderly margin calls and position reassessment.
- Critics argue circuit breakers may create 'magnet effects' where prices are pulled toward trigger levels, or that they delay rather than prevent significant price adjustments.
Explanation
Market-wide circuit breakers in U.S. equities were first introduced by the NYSE in October 1988 following the Brady Commission's investigation of the 1987 crash, which identified the coordinated positive feedback loop between stock and futures markets (futures selling triggering cash index selling triggering more futures selling) as a primary amplification mechanism. The initial circuit breakers used fixed Dow Jones point levels, which became irrelevant as the market rose. The SEC updated the thresholds to percentage-based triggers in 2013 following the lessons of the 2010 Flash Crash.
The current LULD (Limit Up-Limit Down) mechanism for individual securities creates a price band around a rolling five-minute average. If a stock's price moves outside this band (5% for Tier 1 large-cap securities, 10% for Tier 2 smaller stocks, and 20% for securities priced below $3), a Limit State is triggered where market makers must post a bid or ask within the band. If the stock doesn't return to the band within 15 seconds, a Trading Pause of five minutes is imposed. This mechanism was specifically designed to prevent the type of erroneous trades ($0.01 and $100,000+ executions) observed during the 2010 Flash Crash.
Market-wide circuit breakers operate on three levels tied to the S&P 500's intraday decline from the previous close: Level 1 (7% decline) triggers a 15-minute halt if triggered before 3:25 PM ET; Level 2 (13% decline) triggers another 15-minute halt if occurring before 3:25 PM ET and the Level 1 halt has already been triggered; Level 3 (20% decline) halts trading for the remainder of the trading day. These thresholds have been triggered only a handful of times: March 9, 12, 16, and 18, 2020 during the COVID-19 market crash were the most recent activations.
The behavioral and market quality effects of circuit breakers are debated in the academic literature. Theoretical models (Greenwald and Stein, 1991) suggest circuit breakers improve welfare when they allow markets to aggregate information that would otherwise be swamped by panic. Empirical work by Kim and Rhee (1997) on Korean market circuit breakers found 'magnet effects': prices accelerate toward trigger levels as investors rush to execute before the halt. After the halt, there is evidence that price discovery improves and post-halt volatility is reduced. However, the halt also creates uncertainty about the mark-to-market of portfolios and can prevent investors from executing necessary rebalancing or hedging trades.
For derivatives and commodities futures, daily limit moves serve a similar circuit breaker function. CME Group's E-mini S&P 500 futures halt trading in a 'fast market' pause when prices move 5% in either direction in a single day. Commodity futures (agricultural, energy) have daily price limits that vary by contract; hitting these limits stops trading even if buyers and sellers exist at those prices, preventing extreme intraday moves in physical delivery contracts.
Formula
Level 1 Trigger = Prior Close × (1 − 0.07); LULD Band = Reference Price × (1 ± Percentage Threshold)
Example
On March 16, 2020, as COVID-19 fears intensified, the S&P 500 opened down 8% immediately, triggering the Level 1 circuit breaker within seconds of the opening bell. Trading was halted for 15 minutes. When trading resumed, selling pressure continued but at a more controlled pace: market makers returned to their posts, order book depth rebuilt, and prices ultimately stabilized approximately 12% below the prior close by end of day. The halts on March 9, 12, 16, and 18 represented the only circuit breaker activations since the rules were updated in 2013, providing empirical evidence that Level 1 halts were effective in giving liquidity providers time to reassess and return capital to the market.
Related terms
Cap Delivery Electronic Trading Exchange Hedging Iceberg Order Limit Move Liquidity Mark To Market Market Depth Order Book Price Discovery