Educational only, not financial advice. Market rules and thresholds can change — verify current specifics with the exchanges or your broker.

There are two circuit-breaker systems: market-wide ones halt the entire market on a severe S&P 500 drop, and single-stock ones (LULD) pause just one security on a fast move. They solve different problems.

Market-wide circuit breakers

These are the 7% / 13% / 20% S&P thresholds that halt all trading to prevent a cascading crash. They’re about systemic panic — the whole market falling together — and they’re very rare. When one fires, SPY and everything else stops.

Single-stock circuit breakers (LULD)

Limit Up-Limit Down pauses an individual security for ~5 minutes when its price moves too far too fast outside a set band. It’s about preventing erroneous or disorderly moves in one name — far more common, and usually seen in volatile single stocks rather than a mega-liquid ETF like SPY (though SPY can theoretically trigger it in extreme moves).

Market-wide breakers stop the whole market on a systemic crash; LULD stops one stock on a single disorderly move. Different scope, different trigger.

What it means for a scalper

For SPY 0DTE, the market-wide breakers are the relevant tail risk (a crash day). LULD halts you’ll mostly encounter if you trade volatile individual stocks. Either way, a halt freezes your ability to exit — know both exist. More on the mechanics in what is a trading halt.