Here's a brief review of the stock circuit breakers

http://www.cnbc.com/2015/08/24/heres-a-brief-review-of-the-stock-circuit-breakers.html

In 2012, the Securities and Exchange Commission revised the system-wide circuit breakers that would halt the broad market under times of severe stress.

Under those rules, trading halts occur when the S&P 500 decreases 7 percent (Level 1), 13 percent (Level 2), and 20 percent (Level 3) from its previous close.