How do circuit breakers work on the London Stock Exchange?

Circuit breakers on the London Stock Exchange are automated mechanisms that halt or slow trading when prices move abnormally fast, helping prevent panics, erroneous trades, and disorderly market conditions from cascading into systemic problems.

The LSE operates two main types of circuit breaker. The static price collar triggers when a stock's price deviates more than a set percentage from its reference price (typically the midpoint of the previous auction's price range). The dynamic price collar is narrower and triggers when the price moves beyond a threshold from the most recent traded price. When either collar is breached, the stock enters an auction call period, usually five minutes, during which the exchange collects orders and establishes a new reference price before reopening continuous trading.

In addition to individual stock circuit breakers, the LSE implements market-wide circuit breakers triggered by large moves in the FTSE 100. These are coordinated with other major exchanges to provide consistent protection during severe market stress, such as the kind seen in March 2020.

For retail investors, the most visible effect of a circuit breaker is a brief halt in a stock's trading, sometimes during periods of fast-moving news — earnings surprises, profit warnings, or regulatory decisions. Limit orders placed before the halt remain in the order book and are included in the subsequent auction; market orders may be partially filled or repriced.

Circuit breakers exist to give all market participants a brief pause to process information and make rational decisions rather than reacting in panic. They do not prevent prices from falling or rising significantly, but they reduce the likelihood of extreme, short-lived dislocations caused by automated trading systems rather than genuine market sentiment.