Circuit Breaker

A circuit breaker is a pre-defined, automatic market mechanism that temporarily halts or restricts trading once prices move beyond set thresholds within a short period, intended to curb extreme volatility, prevent disorderly cascades (such as panic selling and feedback loops driven by automated orders), and give participants time to reassess. In the United States, market-wide circuit breakers are governed by exchange rules under SEC Rule 80B and are keyed to the S&P 500 Index, with three tiers measured against the prior day's close: a Level 1 decline of 7% and a Level 2 decline of 13% each trigger a 15-minute halt if they occur before 3:25 p.m. ET (after that time, Level 1 and 2 no longer halt trading), while a Level 3 decline of 20% halts trading for the remainder of the day at any time it occurs. Alongside these are single-security mechanisms: the Limit Up-Limit Down (LULD) system pauses an individual stock when its price would trade outside a percentage band around a recent reference price, and futures and commodity markets use their own "price limits" or "limit up/limit down" rules. Circuit breakers were introduced in US equities after the October 1987 "Black Monday" crash and have been refined over time, including the 2013 shift to the current 7/13/20 percentage framework and the adoption of LULD following the May 2010 "flash crash". They exist on many exchanges worldwide, though thresholds, durations, and designs differ by market and by instrument; some implementations, such as China's short-lived 2016 mechanism, were withdrawn after they appeared to worsen rather than calm volatility. A circuit breaker affects when and how fast trading occurs, not the fundamental value of any asset.

In plain English — A circuit breaker is a built-in "emergency stop" for a market. When prices drop sharply within a short window, the exchange automatically pauses trading for a set time, or closes it for the rest of the day if the fall is extreme. The idea is borrowed from electrical circuit breakers that cut the current before a surge causes damage: instead of letting a panic feed on itself, the market is given a forced timeout so buyers, sellers, and information can catch up. Circuit breakers come in two main flavours. Market-wide breakers halt every stock at once when a benchmark index (such as the S&P 500) falls past set percentage thresholds. Single-stock or single-contract breakers, often called Limit Up-Limit Down (LULD) bands or price limits, pause just one instrument when it moves outside a band around its recent price. They do not change the underlying news or value of anything; they only control the timing and pace at which trading is allowed to continue.

Imagine a broad equity index such as the S&P 500 closed yesterday at 5,000. Under the US market-wide rules, today's Level 1 threshold (a 7% decline) sits at 4,650, Level 2 (13%) at 4,350, and Level 3 (20%) at 4,000 — these levels are recalculated each day from the prior close. Suppose heavy selling drives the index down through 4,650 at 11:00 a.m. Because that is a 7% fall before 3:25 p.m., a Level 1 circuit breaker triggers and all market-wide trading is halted for 15 minutes. During the pause, a trader who was holding a position cannot buy or sell that instrument on the exchange, and resting stop-loss orders cannot be filled until trading resumes — so the price the trade actually fills at on reopen can be very different from where the halt began (this is one form of slippage). When trading restarts, if selling continues and the index reaches 4,350 (down 13%), a second 15-minute Level 2 halt occurs. If it later falls to 4,000 (down 20%), a Level 3 breaker closes the market for the rest of the day. A real-world illustration of multiple triggers happened in March 2020, when sharp pandemic-driven declines set off market-wide Level 1 halts on several separate trading days. All figures here are illustrative and for education only — they are not a forecast or a recommendation to take any action.