Former Pennsylvania Gov. Tom Ridge delivered a staggering warning on Tuesday to regional utility officials gathered in Philadelphia to explore ways to protect the nation's electric grid from attack.

"You've got to be right all the time," Ridge, who also served as secretary of the U.S. Department of Homeland Security, told about 200 experts assembled for a two-day conference at the Sheraton Society Hill. "The terrorists only have to get it right once."

In an interconnected post-9/11 world, he told the managers of the nation's electrical transmission system that they need to be in a permanent state of alert.

"Policymakers are getting nervous about grid vulnerability, and rightly so," said Ridge.

The threats to the electricity network are not limited to computer hackers. Grid operators also face bigger natural disasters, as well as physical attacks from terrorists armed with explosives, guns or electromagnetic bombs.

"We're seeing more frequent threats to the grid and more severe threats to the grid . . . than any time in my career," said Terry Boston, chief executive of PJM Interconnection Inc., the regional grid operator based in Valley Forge that sponsored the conference called "Grid 20/20: Focus on Resilience."

Two acts of sabotage earlier this year got little public attention outside of Calfornia, though industry experts described them as a "dress rehearsal" for a grid assault.

In April, unidentified attackers removed manhole covers and cut underground AT&T fiber optic cables near San Jose, disrupting phone service.

Minutes later, someone fired more than 140 shots into the nearby Metcalf electrical substation owned by Pacific Gas & Electric Co., taking careful aim to disable high-voltage transformers.

Despite a $250,000 reward from AT&T, the attacks are still unsolved.

"We live in an open society," said Cheryl LaFleur, a member of the Federal Energy Regulatory Commissionon Monday. "The grid is not hidden in any sense of the word. It's very, very visible. The threat of any physical attack is real."

As frightening as coordinated physical attacks might be, the threat of an attack by hackers working over the Internet from abroad is potentially much more catastrophic.

"The consequences of a physical attack are marginal at best compared to a cyber attack," Ridge said in an interview following his Tuesday address.

Ridge operates a consultancy with Howard A. Schmidt, the former Obama Administration cybersecurity coordinator, so he has something to gain by heightened investment in safety measures.

He said policymakers need to educate the public that security comes at a price, but the risk of a grid disaster is more costly.

"You want to be pre-emptive or reactive?" he said. "You want better security on the grid, you've got to pay. The consumer's got to pay for it."

215-854-2947 @maykuth