Circuit Breaker

A circuit breaker is an automatic stop mechanism on stock exchanges. When a stock index — that is, a basket of many stocks that measures the state of the overall market — falls by a defined percentage within a short period of time, trading is automatically halted. The halt usually lasts between 15 minutes and the rest of the trading day. Trading can then resume, provided the situation has not deteriorated further. The name comes from electrical engineering: a physical circuit breaker interrupts the flow of electricity when the voltage becomes dangerously high — the stock market circuit breaker works on the same basic principle, only for flows of money instead of electricity.

Circuit breakers as a safeguard against market panic

On stock exchanges, negative movements can reinforce themselves. A falling price triggers automatic sell orders, which push the price down further, triggering even more selling — a vicious circle. Without intervention, this can lead to a crash within minutes that would hardly be justified on fundamental grounds.

The circuit breaker breaks this mechanism with a simple measure: it switches the system off briefly. During the pause, traders, algorithms, and institutional investors — that is, large funds and banks that manage enormous sums of money — can reassess the situation instead of continuing to sell in a herd mentality. This is not an intervention in prices, but merely a forced pause for thought.

Whether circuit breakers really help is a matter of debate among economists. Critics argue that the pause merely postpones panic instead of preventing it. Proponents, on the other hand, maintain that even brief interruptions can dampen the worst excesses.

Thresholds and levels of a circuit breaker

Circuit breakers typically operate with several levels. On the US stock exchange, where they are best known, there are three thresholds for the S&P 500 index: if it declines by 7 percent in a single day, trading is halted for 15 minutes. At 13 percent, a second pause follows. If the index falls by 20 percent, trading ends for the remainder of the day.

The thresholds are deliberately set so that they are only triggered by extraordinary events. A normal bad trading day with a decline of two or three percent is not enough. This prevents the mechanism from becoming an everyday brake and undermining confidence in the tradability of securities.

In addition to the market-wide circuit breaker, there are also individual trading halts for specific stocks. These kick in when a single stock fluctuates by more than ten percent within five minutes. Both variants exist side by side and complement each other.

Circuit breakers in practice: the Corona crash and Chinese stock markets

The best-known recent use of circuit breakers was in March 2020. As the Corona pandemic gripped the markets, the 7 percent threshold was triggered multiple times within just a few trading days on the New York Stock Exchange — something that had not occurred for years before that. At the time, the pauses were widely regarded by observers as stabilizing.

A counterexample was provided by China in January 2016. Chinese authorities introduced circuit breakers that ended the entire trading day once a decline of just 7 percent was reached. The threshold proved to be too low: traders began selling their stocks even more aggressively before the market closed — out of fear that they would no longer be able to exit their positions afterward. The mechanism amplified the panic instead of curbing it, and was abolished again after just four trading days.

In financial reporting, the term is encountered mainly in times of crisis. When headlines report that trading has been “suspended” or “halted,” a circuit breaker is behind it in most cases. The term also comes up in debates about cryptocurrencies: many crypto trading venues have no circuit breakers, which critics see as a structural risk.