Stock exchange operators are seeking to extend a pilot program of so-called "circuit breakers" for volatile securities until Aug. 11, according to documents filed with the Securities and Exchange Commission.

The circuit breakers, originated in the weeks following the May 6 "flash crash," briefly halt trading in shares that rise or fall by 10% or more in a five-minute period, guarding against rapid price swings.

The pilot, which had been set to expire on April 11, could be withdrawn earlier if a new system of price limits is implemented ahead of the new August end-date, according to a filing by Nasdaq OMX Group Inc. (NDAQ). Other U.S. exchange companies are expected to issue similar filings.

Exchanges have been drawing up plans for such a "limit up-limit down" structure, similar to measures used in futures markets, that would replace the circuit breakers and allow some trade to continue while curbing volatility. SEC Chairman Mary Schapiro has voiced interest in the concept, which would reduce the need to cancel erroneous trades.

The current range of circuit breakers covers shares in the Standard & Poor's 500 stock index, the Russell 1000 Index of large-capitalization stocks and some of the most-used exchange-traded funds.

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