WASHINGTON (AP) _ Federal regulators loosened restrictions on when trading must be halted on the nation's stock exchanges when prices go into a tailspin. The new curbs will take effect on April 15.

The Securities and Exchange Commission late Thursday 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 current curbs halted trading when the Dow industrials fall 350 points, which is now equal to about 4 percent, and 550 points, or about 7 percent.

The changes were proposed 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 SEC said 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. The agency said 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.''