Twice this week, sharp declines in the U.S. stock market have triggered an unusual protective measure known as a circuit breaker, or a brief pause in trading.
The first happened Monday, when the market started the week with its steepest decline in years following a major decline in the oil market over the weekend, along with mounting fears over the continued spread of the novel coronavirus. At 9:34 a.m., just four minutes after the opening bell, all trading stopped on the New York Stock Exchange, and traders had to wait 15 minutes to start again.
A circuit breaker was triggered again Thursday morning, a day after the World Health Organization declared that COVID-19 is now a pandemic and President Trump announced a 30-day ban on travel from most of Europe. This time, the market was open for five minutes before trading halted at 9:35 a.m.
While a circuit breaker is fairly uncommon, it won't have a huge effect on long-term investors. If anything, this protective measure can ensure that a selling frenzy doesn't occur and potentially limit the severity of a one-day market decline. And they could help to temper some of the market headlines that might lead you to make moves out of fear.
Here's what you need to know about how circuit breakers work and their effect on the market.
Video by Stephen Parkhurst
Much of what goes on in the stock market happens based on traders' perceptions at any given moment. If too many traders sell too rapidly, the sense that the markets are trending downward will spread across the trading floor, triggering more frenzied selling.
A circuit breaker stops that frenzy, giving traders 15 minutes to assess the state of the market. Its goal is to remove some of the pressure to react hastily.
"It is an opportunity for the market and investors to take a breath," says Mark La Spisa, a certified financial planner and the president of Vermillion Financial Advisors in South Barrington, Illinois. "The market plays off momentum, and momentum can be a crazy thing at times. So in times of extremes, it takes a time out."
This sentiment was echoed by Stacey Cunningham, president of the NYSE, in a tweet Monday.
The circuit breaker has three levels to stabilize market declines of varying severity. The current system, which went into effect in 2013, was triggered for the first time Monday morning. They're effective for both the NYSE and Nasdaq.
Here's what it takes to trigger different levels of circuit breakers:
The Securities and Exchange Commission first introduced circuit breakers after the "Black Monday" crash of 1987, when the Dow plummeted nearly 23%, which still stands out as this index's largest single-day percentage decline.
Previous systems were set up to trigger circuit breakers after the Dow dropped by a specific number of points. Today's system is based entirely on percentage drops in the S&P 500. The SEC adopted the current, modified system after the previous one failed to stop the 2010 "flash crash."
Steven Rich, database editor for investigations at The Washington Post, tweeted Monday about the value of that change in ensuring that the circuit breaker goes off at the right time.
The NYSE has closed early only once because of the circuit breaker. That was in 1997, when the Dow dropped 554 points, or 7%, in a single day during the Asian financial crisis.
Once trading resumed at 9:49 a.m. Monday, the S&P 500 stabilized within minutes. Through midafternoon, it remained down 6.6% for the day. While not insignificant, that drop wasn't in danger of triggering a second circuit breaker.
It remains to be seen how the circuit breaker will affect the market on Thursday. Shortly after trading resumed, stocks extended their losses, with the S&P 500 briefly down 8%.
"This is operating as it's supposed to," NYSE's Cunningham told CNBC's Bob Pisani on Monday.
While market turbulence can be nerve-wracking, La Spisa maintains that the usual rules still apply for long-term investors: Avoid panicked selling, and take advantage of cheaper stock prices. Young investors in particular stand to gain from staying the course.
"Looking back, this will be in the rearview mirror by the time they go into retirement," says La Spisa. "Just like people that are retiring today don't even think about 1987."
More from Grow: