When Do Market Circuit Breakers Kick In

As harrowing as the U.S. stock market's plunges have been in recent days, they still haven't been enough to trigger the "circuit breaker" mechanisms that result in an automatic timeout in trading.

The Big Board implemented the automatic halts after the stock market crashed in the late 1980s to force traders to take a break from frenzied selling. But even amid the massive losses seen in the market this week, the thresholds have yet to be breached, unlike in many other nations which have temporarily suspended trading multiple times.

On Friday, the stock exchange in Vienna was suspended until midday after stocks tumbled 10 percent at the opening bell, and in Russia representatives of the MICEX and RTS exchanges said they suspended regular trading until further notice under orders from financial regulators.

Indonesian authorities suspended trading indefinitely on the Jakarta Stock Exchange after they had halted trading Wednesday after the index plunged more than 10 percent.

The Dow Jones industrial average would have to fall 1,100 points in a day to trigger the first halt. Based on Thursday's Dow close of 8,579, the threshold number to cause the market to stop on Friday would be 7,479. If that point is reached before 2 p.m., the market will shut down for an hour. If the threshold is breached between 2 p.m. and 2:30 p.m., the halt will last 30 minutes. No trading stops would take place if the plunge occurs after 2:30 p.m.

If the index were to fall 2,200 points before 1 p.m., the market would close for two hours. If such a decline took place between 1 p.m. and 2 p.m., there would be a one-hour pause. The market would close for the day if stocks sank to that level after 2 p.m.

In the event of a 3,350-point decline, the market would close for the day, regardless of the time.

The thresholds are computed at the beginning of each quarter to establish a specific point value for the quarter. The 1,100-point drop represented a 10 percent decline at that time; the 2,200 level, a 20 percent drop and the 3,350 level is a 30 percent drop.

The rules would halt trading on the major securities and futures exchanges in a coordinated cross-market halt if the circuit breaker is enacted.

© 2009 CNBC.com