Shutdown zaps ComEd's grid plan; Critics fear gap in Midwest's electricity flows.

Byline: BOB TITA

Fallout from the largest blackout in U.S. history threatens to short-circuit Commonwealth Edison Co.'s controversial bid to join a Mid-Atlantic regional power network.

Chicago-based ComEd's plan to join the PJM Interconnection LLC grid by Nov. 1-which federal regulators approved earlier this year-is coming under renewed scrutiny as part of the nationwide review of utility operations and network safeguards following the Aug. 14 blackout that crippled the Northeast and Canada.

For ComEd, membership in PJM would provide a more effici