Nasdaq OMX Group Inc. (NDAQ) and NYSE Euronext (NYX) are tweaking the way new single-stock trading halts are triggered, following a series of false alarms.

Stock markets operated by the two exchange groups will now wait to halt trading until three transactions occur at or beyond the current levels prompting a temporary freeze in buying and selling, according to notices issued by the companies.

The so-called circuit breakers for individual stocks were first implemented in June, part of regulators' and exchanges' response to the market volatility of the May 6 "flash crash," which saw some share prices plunge as low as one cent in a matter of minutes.

Regulators are on the verge of releasing a long-awaited report analyzing the factors behind the May 6 session's rapid swings, expected at any time within the next days.

Under the new circuit breakers, if a stock price jumps or falls by 10% within a five-minute period, trading activity is temporarily suspended to give investors time to reassess the price moves.

The current pilot program, set to expire Dec. 10, covers component stocks of the Russell 1000 index as well as 344 exchange-traded funds.

The recently announced tweaks to those rules will require the threshold to be breeched three times within the rolling five-minute period, as a means to prevent a potentially erroneous trade from shutting down the market in a particular stock.

The changes apply to NYSE Euronext's electronic Arca market, as well as the Nasdaq Stock Market and Nasdaq OMX BX.

Several of the trading halts triggered so far have been caused by such accidental trades. Earlier this week, a typo on a sell order priced shares of Progress Energy (PGN) at $4.57 instead of $44.57, automatically setting off that issue's circuit breaker. Trading soon resumed with the stock trading at $44.55.

The notice from Nasdaq OMX said that the exchange still may invoke a circuit breaker if a single trade goes off at or within the best price available nationally, should that price range beyond the 10% threshold.

-By Jacob Bunge, Dow Jones Newswires; 312-750-4117; jacob.bunge@dowjones.com

(Kristina Peterson contributed to this article.)
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