Trading curb

A trading curb, sometimes referred to as a circuit breaker is a financial regulatory instrument that is in place to prevent from occurring. Since their inception, circuit breakers have been modified to prevent both speculative gains and dramatic losses within a small time frame. As a result of being triggered, circuit breakers either stop trading for a small amount of time or close trading early in order to allow accurate information to flow amongst market makers and for institutional traders to assess their positions and make rational decisions.

On the (NYSE), one type of trading curb is referred to as a "circuit breaker". These limits were put in place after Black Monday in 1987 in order to reduce market volatility and massive panic sell-offs, giving traders time to reconsider their transactions. The regulatory filing that makes circuit breakers mandatory on United States stock exchanges is Securities and Exchange Commission Rule 80B. It is there that the specifics of circuit breakers are elaborated and the various price limits are outlined for investors to see.

The most recently updated amendment of rule 80B went into effect on April 8, 2013, and has three tiers of thresholds that have different protocols for halting trading and closing the markets.

At the start of each day, the NYSE sets three circuit breaker levels at levels of 7% (Level 1), 13% (Level 2) and 20% (Level 3). These thresholds are the percentage drops in value that the S&P 500 Index would have to suffer in order for a trading halt to occur. Base price levels for which these thresholds will be applied are calculated daily based on the preceding trading day’s closing value of the S&P 500. Depending on the point drop that happens and the time of day when it happens, different actions occur automatically: Level 1 and Level 2 declines result in a 15-minute trading halt unless they occur after 3:25pm, when no trading halts apply. A Level 3 decline results in trading being suspended for the remainder of the day.

Circuit breakers are also in effect on the Chicago Mercantile Exchange (CME) and all subsidiary exchanges where the same thresholds that the NYSE has are applied to equity index futures trading. However, there is a CME specific price limit that prevents 5% increases and decreases in price during after hours trading. Base prices for which the percentage thresholds are applied are derived from the weighted average price on the future during the preceding trading day's last thirty seconds of trading. Price limits for equity index and foreign exchange futures are posted on the CME website at the close of each trading session.

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