The record-setting heat wave over the weekend may have contributed to the explosion and fire at a Los Angeles Department of Water and Power facility in Northridge that left thousands of people in the San Fernando Valley without electricity, the utility said Monday.

DWP officials said the explosion was caused by the failure of a 40- to 45-year-old transformer at the utility’s Receiving Station J at Parthenia Avenue and Wilbur Street, and estimated it would cost at least $5 million to replace the transformer and repair the damage.

The facility where the malfunction occurred is used by DWP to turn high voltage power into lower voltage power to distribute to customers.

The event and its response plunged much of the Valley into darkness Saturday night, affecting street lights and traffic signals, causing businesses to close early and leaving thousands of residents to grapple with the unusually oppressive heat.

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What prompted the malfunction in the high-voltage transformer has yet to be determined, but DWP General Manager David H. Wright said in a phone interview Monday that the utility suspects the record-high temperatures, which had the system working at peak capacity, played a role in the transformer’s failure.

“Saturday was the second-hottest day in history; that’s the time when our system is the most stressed,” Wright noted. “We don’t know why this happened, but obviously, yes, we really do think the heat played a factor.”

Immediately after Saturday’s explosion, the DWP turned off the power at the receiving station temporarily, leaving as many as 140,000 customers without power at one point, for the safety of first responders and utility crews, officials said. Several transformers at the facility are now carrying the load. A temporary transformer will be put in place in the coming days, and a new transformer will be built over a couple of months and then energized.

“With the temporary transformer and the others that are there, it should be able to function to handle all of our load,” Wright said.

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The last time a receiving station went down was 12 years ago in Toluca Lake, Wright said. That failure was due to a cut communications line.

The average life span of these transformers is about 50 years. But there was no indication there were any problems with the one that failed Saturday, Wright said.

“We go through and do an analysis and prioritize which ones are ready for replacement based on location, power factor, dissolved gas test, the age, and a number of things,” he said. “This didn’t indicate that we thought this was going to be a problem.”

Andrew Kendall, the DWP’s senior assistant general manager of power, sat in on the same phone interview. Kendall said transformers are inspected on a cycle, typically within every two years, but he could not say when the failed apparatus at the Northridge station was last analyzed.

DWP has a “proactive” infrastructure replacement plan that has so far replaced about 50 transformers in the city, Kendall said. About 20 are left to be replaced as part of the program. Typically, they replace at least two or more a year, the officials added.

Wright said the money to repair the damage to the facility will come from the utility’s infrastructure replacement plan that is funded by ratepayers. That cost is already built into the utility’s new rates, he said.

“We apologize to any customers that were inconvenienced because of the outage,” Wright said. “It was a highly unusual item, but it did occur.”

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The outage came as the first of a series of gradual utility rate hikes went into effect this past year. DWP officials had said these increases are aimed at upgrading the utility’s aging infrastructure and improving the reliability of its electrical service. The last time the DWP raised electricity rates was 2009, while water rates were last raised in 2012.

While DWP officials say ratepayers on average experience less than one outage a year, lately there have been some relatively widespread outages due to failures.

On Saturday, DWP crews also were called to work on a smaller outage in Hollywood caused by a failed underground cable that left about 788 customers without power for at least six hours.

And on Friday, power went out for about 12 hours in Fairfax Village after an underground electrical vault exploded.

Last month, nearly 2,000 customers in Sylmar went without power for about 90 minutes after a series of explosions at a DWP facility on Glenoaks Boulevard. Firefighters were also called to the scene to douse the flames from that flare-up.

There were explosions last year too, including a failure at a substation that left 9,000 customers in Sherman Oaks and Studio City without power for about 20 hours.

Los Angeles City Councilman Mitchell Englander, whose district includes several San Fernando Valley areas affected by Saturday’s Northridge station outage, said it was “more than an inconvenience, it was an emergency situation that posed a danger to communities throughout the San Fernando Valley.”

He said what is needed now is “a hard look at the root causes of the fire and resulting outage and taking steps to ensure that we can prevent such a situation from developing in the future.”

Meanwhile, the DWP is urging customers to conserve energy partly due to the role the high demand may have played in the Northridge transformer explosion.

“Give your appliances the afternoon off, set air conditioners at 78 degrees,” Wright said.

Even as residents began to put Saturday’s power failure behind them, as many as 112 DWP customers in Canoga Park on Monday evening were dealing with yet another outage that knocked out electricity at the corner of Canoga Avenue and Nordhoff Street.

Generators that were powering DWP transformers, which supply electricity to a mobile home park and nearby streetlights, caught fire before being quickly put out, DWP and Fire Department officials said.

Joseph Ramallo, a DWP spokesman, said this smaller outage is “not in any way related” to the Northridge outage.

“This is a local outage on our distribution side,” he said. “Not transmission, substation or receiving station related.”

However, the Los Angeles Fire Department said in a press release that the generators were brought in to “power transformers likely affected by the outage in Northridge.”

The recent series of failures and explosions recall the mass power outages that occurred amid another heat wave in the summer of 2006. Those outages were blamed on the hot weather, and as many as 300 transformers failed.

During one hot stretch that year, from July 21-26, temperatures reached 119 degrees in Woodland Hills. During that spell, power was cut off for 79,000 or 6 percent of DWP customers.

Other utilities were also affected that week, with nearly a quarter of customers losing power from Southern California Edison, an investor-owned utility that provides power to areas outside of the city, losing power.

Editor’s note: This article was updated on July 11 to note that DWP typically replaces at least two of these transformers a year. (The official had previously stated it was usually one a year.)