Here's how much the stock market must plunge before it gets shut down

Stocks have erased their gains for the year in a volatile week of trading.
The S&P 500 fell by over 4% during the steepest drop of this week's selloff on Monday.
The Securities and Exchange Commission's so-called circuit breakers trigger if the index falls by 7%, 13%, or 20% from the previous session's closing price.The stock market is getting rocked this week.
The Dow Jones industrial average on Monday had its biggest intraday fall in history on a point basis and was down more than 1,500 po