Circuit Breaker
Circuit breakers are automatic trading halts that stop the market when prices fall too far, too fast. In US equities, a 7% intraday drop in the S&P 500 from the prior close halts all trading for 15 minutes (Level 1), a 13% drop triggers a second 15-minute halt (Level 2), and a 20% drop closes the market for the day (Level 3).
Individual stocks have their own mechanism, limit up-limit down, which pauses a single name for five minutes when it moves outside a rolling price band.
The math
The mechanism costs nothing; reacting badly to it does. Picture an investor holding an $80,000 portfolio when a Level 1 halt hits.
During the 15-minute pause, fear does its work, and the moment trading resumes the investor market-sells everything into the reopening auction, where crowded sell orders fill about 3% below the pre-halt prices: $2,400 surrendered to the queue.
| Amount | |
|---|---|
| Portfolio at the halt | $80,000 |
| Reopening fill vs pre-halt prices | 3% lower |
| Surrendered to the queue | $2,400 |
Markets that fall 7% in a day have historically recovered on timelines of months to a few years; the $2,400 paid for acting in the worst 15 minutes never comes back.
The trap
Halts amplify the urge they exist to interrupt. A frozen screen feels like a trapdoor, and the reopening attracts a rush of market orders from holders who spent the pause imagining zero.
Those orders collide with wide spreads and thin depth, producing exactly the disorderly fills the halt was meant to prevent. The breaker pauses the market; it cannot pause the panic of any individual participant.
The move
A stock picker’s plan for circuit breakers should be written on a calm day and contain mostly the word nothing. A market-wide halt changes no company’s cash flows, so the only positions worth reviewing are ones that were oversized or leveraged before the drop, a sizing failure the halt merely exposed.
The prepared investor’s actual reflex is the opposite one: a 7% day puts the watchlist on sale, and shopping beats selling at those prices.
Reference: SEC investor.gov