Traders work the floor of the New York Stock Exchange (NYSE) on March 5, 2020 in New York City. Coronavirus fears have whipsawed markets recently, with the Dow Jones Industrial Average ending today down more than 950 points, or nearly 3.6 percent. (Photo by David Dee Delgado/Getty Images)

David Dee Delgado | Getty Images

With U.S. stock futures looking set to plunge at the open, investors will be watching for key market circuit breakers known as ‘limit down’ which have the ability to limit or halt the market altogether.

Limit down occurred on Sunday evening after S&P 500 futures sank 5%, halting trading entirely and preventing futures from falling any further.

What is ‘limit down’?

According to the New York Stock Exchange, a market trading halt may occur at “three circuit breaker thresholds” on the S&P 500 due to large declines and volatility. The exchange classifies this at three levels based on the preceding session’s close in the S&P 500.

The rules, which apply to regular trading hours only, are as follows:

  • Level 1: If the S&P 500 drops 7%, trading will pause for 15 minutes. (This would occur today if the S&P falls 208 points).
  • Level 2: If the S&P 500 declines 13%, trading will again pause for 15 minutes if the drop occurs on or before 3:25 p.m. ET. There will be no halt the drop happens after that. (This would occur today if the S&P falls 386 points).
  • Level 3: If the S&P 500 falls 20%, trading would halt for the remainder of the day. (This would occur if the S&P falls 594 points).

These circuit breakers have never been triggered in their current form during regular trading hours.

CNBC’s Peter Schacknow contributed to this report.

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