Circuit Breaker — Trading Wiki

What Circuit Breaker means

Automatic trading halts triggered when a market index falls beyond preset thresholds (7%, 13%, 20%). Designed to prevent panic-driven crashes.

In depth

Circuit breakers are regulatory mechanisms that automatically halt trading across an entire exchange or in individual securities when prices decline by predetermined percentages within a single trading session. Implemented to prevent panic-driven cascading sell-offs, circuit breakers provide a 'cooling off' period that allows market participants to digest information, reassess positions, and restore orderly market functioning. In the United States, market-wide circuit breakers were first introduced after the stock market crash of October 19, 1987 (Black Monday), when the Dow Jones fell 22.6% in a single day.

The current system, updated in 2013, uses the S&P 500 as the reference index with three threshold levels: Level 1 (7% decline) triggers a 15-minute trading halt if triggered before 3:25 PM ET. Level 2 (13% decline) triggers another 15-minute halt if triggered before 3:25 PM ET. Level 3 (20% decline) halts trading for the remainder of the day regardless of when it's triggered. These percentages are calculated from the previous day's closing price. Level 1 and Level 2 halts can each only be triggered once per day. Individual stock circuit breakers, known as Limit Up-Limit Down (LULD), prevent trades from occurring outside of specified price bands that are recalculated every 5 minutes.

The bands are typically 5-10% from the reference price for large-cap stocks and wider for smaller or more volatile issues. Cryptocurrency markets notably lack circuit breakers, which is one reason why crypto experiences more extreme flash crashes and volatility events. Some crypto exchanges have implemented their own versions — Binance has a 'cooling-off period' feature and BitMEX has historically paused trading during extreme events — but there is no industry-wide standard. The absence of circuit breakers in 24/7 crypto markets means that liquidation cascades can run unchecked during low-liquidity periods.

Key points

• US market-wide halts at S&P 500 declines of 7%, 13%, and 20%
• Individual stock halts (LULD) prevent trades outside price bands
• Crypto markets lack circuit breakers — contributing to extreme volatility

Practical tip

When a Level 1 circuit breaker triggers in equities, the 15-minute halt creates a predictable event. Historically, the market tends to test the pre-halt low within the first few minutes of resumption. Place limit buy orders 1-2% below the halt trigger level to capitalize on the post-halt volatility spike.

Why it matters when you are learning

Circuit breakers are your safety net in regulated markets. Understanding when they trigger helps you stay calm during crashes and prepare for post-halt opportunities.

Practising Circuit Breaker on the simulator

The fastest way to understand Circuit Breaker is to use it once. Place a small simulated order that involves it, watch exactly how the fill and the portfolio line respond, and repeat it on a second instrument so you can tell what is general and what is specific to one market. Educational simulation only — not financial advice.