The Federal Energy Regulatory Commission (FERC) has rejected a proposal to improve electric grid security, sending the proposal back to the North American Electric Reliability Corp. for revision. The group must strengthen its rules about how utilities should assess and protect power substations and systems, the ruling says.

According to a Wall Street Journal article, the proposal would have required companies to develop security plans for any substation that could create enough instability to cause cascading blackouts if it were disabled, which could apply to only a few hundred of the roughly 55,000 highest-voltage electrical substations in the U.S.

A separate internal analysis by FERC experts concluded that the U.S. could suffer coast-to-coast power outages if as few as nine major substations were knocked out at once. Because transformers at these key locations are custom-made, recovering from such an incident could be time-consuming. “It often takes a year or more to build, transport and install a transformer at a substation because the units can weigh hundreds of tons and are hand-built for their locations,” the WSJ article reports.