Circuit Breaker

An automatic trading pause triggered by an extreme price move.

Also called: trading halt · limit up limit down · halted

In plain language

Exchanges halt trading in a security, or the entire market, when prices move beyond defined thresholds in a short window. The pause is meant to let information disseminate.

For an individual stock, a halt typically lasts several minutes. Market-wide breakers trigger at set percentage declines and can close markets for the day at the extreme.

During a halt you cannot trade at all. Your position is frozen and your stops cannot execute — and price often reopens far from where it stopped.

Worked through

A stock halted at $26, reopening at $19

Price when halted: $26.00
Stop resting at: $25.20
Duration of halt: 18 minutes
Reopening print: $19.00

For eighteen minutes nothing could be done. Orders could not execute, the position could not be closed, and the stop sat there as inert as a note on a desk. When trading resumed it did so at $19, and the stop filled there.

Halts exist for good reasons — to let news disseminate, or to interrupt a disorderly move — and the effect on an open position is the same regardless of the reason: every risk control you have is suspended by someone else, without notice, for an unknown length of time.

They also tend to arrive on exactly the instruments and the days where a position is most likely to be in trouble. A halt is not a random event sprinkled across a calm market; it is correlated with the move that triggered it.

Nothing in the order book protects against this. Only size does — a position small enough that reopening several points through the stop is an unpleasant day rather than a structural problem.

Why it matters

A halt is a period where your risk controls are switched off by the exchange. This is the clearest argument for sizing so that a single position cannot do serious damage.