The Metcalf substation—south of San Jose, Calif., off U.S. Highway 101—was targeted early Wednesday morning by intruders who cut fencing at the site and stole construction equipment being used for security upgrades.

Alarms sounded in the utility’s central security-dispatch center shortly after 2 a.m., although the break-in wasn’t discovered until a morning shift arrived four or five hours later, PG&E said. The company reported the incident to police around 7:30 a.m. and disclosed the incident to the public eight hours later. The utility said it is seeking the public’s help in identifying the thieves.

The Metcalf substation is a critical piece of Silicon Valley’s grid, flowing power to America’s technology hub.

Gunmen attacked the substation in April of last year, shooting out 17 large transformers. There was no blackout because of adjustments by the grid operator. The facility was out of service for nearly a month.

After last year’s attack by gunmen, PG&E rolled out a $100 million program to harden security at its Metcalf, Calif., substation and was restoring round-the-clock guards. Associated Press

That attack was termed a terrorist act by Jon Wellinghoff, who at the time was chairman of the Federal Energy Regulatory Commission. No arrests have been made in the incident.

“PG&E is typical for the industry and it’s scary,” Mr. Wellinghoff said of this week’s intrusion. He was critical that the utility didn’t respond to the alarm, especially with security guards at the location.

“It’s pathetic, and this is the company that got hit once before,” said Mr. Wellinghoff, who is now a lawyer in private practice.

PG&E said human error appeared to be responsible for the response time and that the utility was still investigating to determine to what security improvements might be needed. PG&E didn’t estimate the value of the stolen items but said they included tools and other equipment.

Santa Clara County Sheriff’s Office investigators spent hours at the site collecting evidence, said Sgt. Kurtis Stenderup, a spokesman for the office.

“I know there are security cameras on scene, but we are still reviewing the footage, so I don’t know if or what they captured,” he said.

The Federal Bureau of Investigation said it didn’t think last year’s attack and this week’s were connected and that the agency isn’t investigating the latest incident.

Power-transmission security is a major concern of government officials since substations are linchpins in the U.S. electric grid. The Wall Street Journal in March reported details of a federal study that found that disabling as few as nine critical power substations could cause cascading blackouts across the country.

Repairing multiple electrical substations would be slow and difficult. Transformers, which control voltages on power lines, are custom built for their locations and it often takes a year or more to build a new unit. Getting a new transformer in place also is a logistical challenge; transformers can weigh 500,000 pounds and few railcars exist that can move them.

During last year’s attack, Metcalf’s perimeter alarms were activated as bullets nicked the fence. Workers at a PG&E security center ignored early warning signs of trouble.

In response to the episode, federal officials are considering requiring more protections for transmission substations.

Over the past year PG&E and other utilities have announced investments to try to bolster defenses.

PG&E rolled out a $100 million program earlier this year to harden security. It also restored round-the-clock guards to the Metcalf substation.

Write to Rebecca Smith at rebecca.smith@wsj.com