Circuit Breakers and Trading Halts

A circuit breaker is a rule that automatically stops trading for a period when price moves exceed a pre-set threshold, on the theory that a pause gives participants time to absorb news and reassess, rather than letting a cascade of automated selling or a liquidity vacuum drive prices further on momentum alone. Two distinct kinds operate in US equity markets.

Market-wide circuit breakers halt trading across the entire market when a broad index (the S&P 500) falls a set percentage from the prior close, 7% triggers a 15-minute pause, 13% another 15-minute pause, and 20% halts trading for the rest of the day. These are rare, blunt instruments meant for genuine market-wide panics, not routine volatility.

Single-stock halts, under the Limit Up-Limit Down mechanism, work differently: each stock has a price band around its recent average price, and if the stock's price tries to trade outside that band and stays there, trading in that one stock pauses briefly, typically five minutes, while the rest of the market keeps trading normally. This targets abnormal moves in an individual name (a fat-finger trade, a sudden rumor, an earnings-related air pocket) without disrupting anything else.

Both mechanisms end with a reopening auction that aggregates buy and sell interest to set a new price before continuous trading resumes, rather than simply flipping trading back on at the last traded price.