A circuit breaker interrupts trading under a specified market rule. Understand the trigger, the halt and the reopening without mistaking a pause for a price floor.
Circuit breaker
A circuit breaker is a rule that interrupts trading when a specified market condition is reached, often an unusually large price move. For an execution model, the immediate consequence is a change in what can trade: a strategy may still hold its position while its usual route to selling or hedging is paused.
The rule belongs to a particular market and instrument. Its reference price, observation window, trigger, trading hours and reopening procedure are part of the definition in use. “The market fell a lot” is not enough to determine whether a halt should occur.
The trigger stops trading; it does not stop losses
Consider a wholly fictional index rule: the first observed decline of at least 10% from a fixed reference of 100 index points triggers a trading pause in the covered market. This is an arithmetic illustration, not a threshold used by any real venue.
For an observed index level I and reference I0, the decline fraction is
D=I0I0−I.
At 92, the decline is 8%, so this invented rule has not triggered. At 90, it is 10%, so the threshold is reached. Equivalently, the trigger level is 100(1−0.10)=90 index points.
Wholly fictional index sequence: reference 100, observed 92, trigger at 90 after a 10% decline, a trading pause with no executable index price implied, then illustrative reopening at 88, 12% below the reference. Event order only; no actual market rule or pause duration.
Wholly fictional mechanism, in index points. The 10% rule and reopening at 88 are invented; event positions show order, not elapsed time. A pause changes the trading state and does not guarantee the reopening level or any order fill.
The diagram allows an illustrative reopening at 88, a 12% decline from the reference. There is no contradiction: a temporary pause is not a promise that the next price will be at least 90. Reopening conditions must be satisfied under the relevant rule, and buy and sell interest may have changed during the interruption. The index itself is a measure, not an order that an investor can execute; a position in a related security has its own price, exposure and execution outcome.
A stop-loss rule also cannot turn a halt threshold into a guaranteed exit price. A stop instruction, an accepted order and a confirmed fill are separate events. If execution resumes at a worse price, the resulting slippage is economically real.
Two U.S. mechanisms with different references
The following is a U.S. equity-market illustration, checked against official sources on 2026-10-26. Other jurisdictions and derivatives contracts have their own rules.
Market-wide circuit breakers.NYSE identifies declines in the S&P 500 from its previous day's closing level as the reference. The Cboe MWCB FAQ gives the normal-session timing:
Level
Decline from previous close
Halt timing
1
7%
Before 3:25 p.m. Eastern Time: at least 15 minutes
2
13%
Before 3:25 p.m. Eastern Time: at least 15 minutes
3
20%
During the trading day: remainder of that trading day
Levels 1 and 2 do not cause a halt at or after 3:25 p.m. Each level can trigger only once per day. Reopening is a process: for example, BZX-listed stocks use a reopening auction whose scheduled restart can be extended. A minimum halt duration is not a guaranteed resumption time. This summary does not assign equity-session timings to futures or other products.
Individual-stock price bands. The Limit Up–Limit Down Plan covers U.S. NMS stocks during regular hours, with rights and warrants excluded. It uses a reference based on eligible transactions over the preceding five minutes, rather than the market-wide previous-close comparison; the applicable band depends on the security and time of day. Under Cboe's explanation, an unresolved 15-second Limit State leads to a five-minute Trading Pause. Reopening depends on the primary listing market's auction and price-band dissemination, with extensions and special closing procedures possible. Merely touching a band is not the same as a market-wide halt. These changing bands constrain eligible executions while in force; they do not establish a permanent floor under the stock's value.
Carry the trading state into the backtest
An unchanged last price during a halt is not evidence that an order can fill there. A replay should retain the halt and resumption messages, the instrument and venue scope, the rule version, and the information actually available when the strategy decided to trade. It should also model what happens to resting orders, cancellations and new submissions under that venue's rules. Do not interpolate a tradable path through the pause.
The owned lesson Trades, Quotes and Market State develops the distinction between historical trades, observable quotes and valid execution state, including event versus receipt time. The full lesson requires membership; its public preview shows the scope and prerequisites.
Finally, trading firms sometimes call their own exposure cutback or algorithm shutdown a “circuit breaker.” That is an internal risk control. It may stop the firm's orders or reduce desired exposure without halting the exchange, and it still depends on available liquidity to complete any liquidation.
Rishi K. Narang (2013), Inside the Black Box: A Simple Guide to Quantitative and High-Frequency Trading, 2nd ed., Wiley, Chapter 16, “Regulatory Considerations,” p. 298, for the market-structure rationale. The exchange sources above supply the current rules.
