April 30 (Bloomberg) — Fixed mortgage rates fell for a third consecutive week, matching the record low reached earlier this month, driven by Federal Reserve Chairman Ben S. Bernanke’s plan to buy home-loan securities to bolster the housing market.

The average rate of a 30-year mortgage dropped to 4.78 percent, the lowest in records dating to 1970, McLean, Virginia- based Freddie Mac said today in a statement. The rate hit that level the week of April 2. This week’s 15-year fixed rate was unchanged at 4.48 percent.

Mortgage rates at or near four-decade lows are raising optimism that lower home-loan costs may spur home buying. Bernanke is seeking to reduce rates by purchasing securities from Fannie Mae, Freddie Mac and Ginnie Mae, freeing up money from those institutions so they in turn can bolster liquidity for home lenders.

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