Starting Wednesday, the nation's stock markets won't be forced to close as quickly if stock prices go into a tailspin.

Federal regulators have approved the so-called circuit breakers, which raise the threshold for trading curbs to drops of 10 percent, 20 percent and 30 percent in the Dow Jones industrial average.The changes proposed by the stock exchanges and approved late Thursday by the Securities and Exchange Commission - the most radical revision yet of the mandatory trading pauses - will make it harder to halt stock trading when prices fall sharply.

Many regulators, including SEC Chairman Arthur Levitt Jr., believe trading should be halted very rarely. Levitt has warned that early market closings create uncertainty and potential panic for investors and could especially hurt the more than 60 million mutual-fund shareholders.

The SEC said Friday the new triggers more closely reflect the circuit breakers' original purpose - to be used only during a severe one-day market decline "of historic proportions."

"We think this is extremely significant," Robert Colby, the SEC's deputy director of market regulation, said in a telephone interview. "It puts the circuit breakers back where they were intended to be: not disrupting trading when it's not absolutely necessary."

The current curbs halt trading when the Dow industrials fall 350 points, which is now equal to about 4 percent, and 550 points, or about 6 percent.

The changes were proposed early this year by the New York Stock Exchange, other major exchanges and the National Association of Securities Dealers because of concerns the current triggers are too low and can aggravate market instability.

The market has fallen 20 percent in one day just once and 10 percent twice.

At the current levels, the triggers have only been activated once: during the market meltdown of last Oct. 27. But that sparked howls of protest and a push to raise the limits intensified.

The agency has been receiving public comments on the proposed changes since February, Colby noted. He said the stock exchanges have assured regulators they will be able to implement the new system by next Wednesday, the effective date.

The new trigger levels will be converted into point values at the beginning of each quarter of the year, using the average closing value of the Dow average for the previous month.

In a statement, the SEC laid out the new scenarios for each trigger level of decline in the Dow average:

- For a 10 percent decline, the trading halt will be one hour if triggered before 2 p.m. New York time. At or after 2 p.m. but before 2:30 p.m., the halt will be for a half-hour. At or after 2:30 p.m., the market will not halt at the 10 percent level and will continue trading.

- For a 20 percent decline, the halt will be two hours if triggered before 1 p.m. New York time. At or after 1 p.m. but before 2 p.m., the halt will be for one hour. If the 20 percent level is reached at or after 2 p.m., trading will stop for the remainder of the day.

- For a 30 percent decline anytime, trading will be halted for the rest of the day.