NEW YORK (AP) — For the second time this week, stock prices tumbled so sharply at the opening bell a circuit breaker meant to slow down panic trading was triggered on Wall Street, halting all activity for 15 minutes.

The S&P 500 fell 7% shortly after the opening bell, triggering an automatic trading halt. Once trading resumes, if the S&P's decline expands to 13%, trading will once again be suspended for 15 minutes. If the drop eventually reaches 20%, trading will stop for the day.

Before Monday, the circuit breakers instituted after the market crash of 1987 had only been triggered once, back in 1997.