Circuit Breaker

United States market-wide circuit breakers reference the S&P 500 and are calibrated off the prior session's closing value. Three levels apply: a decline of 7 percent, 13 percent and 20 percent from that close. The trigger point is computed in words as the previous close multiplied by one minus the threshold percentage, rounded to the nearest index point, and the resulting values are published before the session begins. A Level 1 or Level 2 breach halts all listed equity trading for fifteen minutes, but only if it occurs before 3:25 p.m. Eastern; after that cut-off the market runs to the close unless a Level 3 breach occurs. Level 3 closes trading for the remainder of the day, at any hour. Levels 1 and 2 can each be triggered only once per session, so a second 7 percent breach after a reopening does not produce a second pause.

Reopening after a market-wide halt runs through an auction process at the primary listing venue, which aggregates accumulated orders into a single clearing price rather than resuming continuous trading at the last print.

Individual stocks and ETFs in the US are governed by the Limit Up-Limit Down mechanism rather than index-based rules. A reference price, typically the average trade price over the preceding five minutes, is surrounded by a percentage band that widens for lower-priced and less liquid names and doubles during the opening and closing periods. Quotes may not execute outside the band. If the market cannot move back inside the band within roughly fifteen seconds, the security enters a trading pause of about five minutes, followed by a reopening auction.

These volatility pauses are distinct from regulatory halts, which an exchange imposes when material news is pending or when there is doubt about a security's compliance status. A volatility pause is mechanical and short; a news-pending halt lasts until information has been disseminated and can persist for hours. Candidates should keep the two categories separate when discussing market integrity.

Halts change execution risk rather than eliminate it. Stop orders resting below the market do not execute during a pause; they become live again at the reopening, and a stop that converts to a market order can fill well away from the trigger price if the auction clears at a gap. Limit orders survive but may sit unfilled while information continues to arrive. Positions carried on margin remain exposed, since maintenance requirements are assessed on marked values that will reflect the post-halt price. For ETFs, a pause can suspend the arbitrage that keeps price near net asset value, widening premiums and discounts until creation and redemption activity resumes.

Designs differ across markets. Several Asian exchanges impose daily percentage limits on individual issues, so trading continues at the limit price without a formal stoppage. Many European venues replace halts with short volatility auctions. Futures markets use daily price limits and, in equity index contracts, limit rules that interact with cash market circuit breakers overnight.

Level I questions on circuit breakers test classification and reasoning rather than memorised tables. Be able to state the purpose of a halt as an interruption that allows order flow to be re-aggregated and information to be absorbed, not as a mechanism that alters fundamental value or supports prices. Distinguish market-wide triggers keyed to a broad index from security-level bands keyed to a rolling reference price, and separate both from news-driven regulatory halts and from futures price limits.

Be prepared to reason through second-order effects that appear in market structure and liquidity questions: the magnet effect, in which prices are drawn toward a known threshold as participants race to trade ahead of a pause; the migration of order flow to venues or instruments that remain open; and the treatment of resting orders across a reopening auction. Recognising that a halt redistributes rather than removes volatility is the analytical point that most items reward.