Ambulances are seen at the entrance of an electric substation after an explosion and fire caused a blackout in parts of San Juan, Puerto Rico, on Sunday. (Ricardo Arduengo/AFP/Getty Images)

A breaker exploded at a power station here late Sunday, leaving more than 175,000 households and businesses in northern Puerto Rico without electricity overnight, part of an expected, if unpleasant, aspect of life on the U.S. island in the months after Hurricane Maria nearly wiped out the public power company¡¯s dilapidated system.

Power was restored to a majority of customers in metropolitan San Juan and points east by Monday morning. But the outage again highlighted the strains that are being placed on the Puerto Rico Electric Power Authority, an indebted public agency that has been struggling to turn on the lights ¡ª and keep them on ¡ª since the September catastrophe.

Material shortages, personnel challenges and a contracting scandal have hampered PREPA¡¯s efforts to move quickly in repairing a complex network of transmission and distribution lines. More than 400,000 customers remain without power nearly five months after the devastating hurricane.

PREPA has restored power to nearly 75 percent of the island ¡ª or more than 1.1 million people, said authority spokesman Carlos Monroig. Two substations are still shut down, cutting off electricity to people in rural sectors in the island¡¯s northern region.

The 9 p.m. explosion Sunday at the Monacillo power plant ¡ª about eight miles from the capital city ¡ª was witnessed by residents in nearby Rio Piedras. Flames shot into the sky in what became a dramatic scene for those in nearby high-rise buildings. Residents quickly posted photos and video on social media accounts just as the lights went dark in Old San Juan.

The outage left San Juan¡¯s international airport, the University of Puerto Rico¡¯s flagship campus and the primary medical facility, Centro Medico, without electricity from the grid for more than four hours. All were back online as of Monday morning. Generators installed by the Army Corps of Engineers kicked into gear at the northern Puerto Rico power station, Palo Seco, helping to reestablish service quickly, officials said.

¡°Last night¡¯s power outage was crushing,¡± said Ramon Luis Nieves, a former Puerto Rico senator who introduced bills aimed at overhauling the island¡¯s energy policy. He opposes the governor¡¯s proposal to dissolve the independent energy commission, which regulates the utility. ¡°I felt like it was, emotionally, September 21st again, the day after the storm. Unfortunately, it¡¯s a signal of what we can expect for the rest of the year.¡±

Alayna Wool was working in her small furniture factory in Santurce preparing for a meeting on Monday when the lights went out. She said she felt a sudden pang of panic and left immediately to buy ice for her refrigerators, not knowing how long the outage might last. By the time she and her husband returned home, the electricity had come back.

Her small business has been struggling since the storm, with bills piling up, her clients going bankrupt and orders for her custom furniture dwindling. The power initially came back to her San Juan-area workshop 77 days after the storm, and the uncertainty every day since has made planning impossible.

¡°It¡¯s like everything is unstable,¡± said Wool, who had just made her first large-scale grocery purchase at Costco since the hurricane; she had long feared losing perishable foods in power outages. ¡°You don¡¯t know where you¡¯re standing, ever.¡±

Monroig, the PREPA spokesman, said that no one was hurt in the explosion Sunday and that it is unclear what caused the mechanical failure. He said investigators are considering many factors, including the possibility that the breaker was overloaded. The breaker distributed wattage along a line that fed into two substations, where service remained down late Monday.

¡°We are evaluating to see if this was an effect of the hurricane, a maintenance issue or something else,¡± Monroig said. ¡°The good news is things are returning to normal for most customers. Everyone who had power before the explosion will have it back today.¡±

The explosion and subsequent blackout add fuel to a growing divide between the public utility and its unionized workers, who oppose Gov. Ricardo Rossell¨®¡¯s plan to privatize the organization and sell off its assets in the next 18 months.

Union leader Angel Figueroa Jaramillo, who represents PREPA¡¯s electrical workers, spoke to reporters outside the Monacillo power station as the foam from fire hoses spilled down the street. He said events such as the explosion help the central government justify moving toward a private model, accusing leadership of intentionally slowing down power restoration on the island.

¡°If you create a deeper crisis, people will be more outraged,¡± Figueroa Jaramillo said. ¡°This has nothing to do with the hurricane. It¡¯s been five months without power. We could have made much more progress than this.¡±

That suspicion of the local power authority¡¯s intentions permeates Puerto Rico¡¯s population. It is a constant discussion point on talk radio and drives debates in San Juan¡¯s gathering places.

Although residents express constant frustration about the lack of consistency in the electrical grid, Monroig said it is important to understand that outages are going to be part of rebuilding an old, worn-out system.

The calls for patience, though, haven¡¯t quelled distrust.

Monica Oquendo was sitting outside her secondhand clothing shop on Calle Loiza, a trendy capital-area shopping and dining district, when a woman started yelling down the busy, narrow street: ¡°This is sabotage!¡±

¡°People here think these blackouts are being done on purpose because of the privatization thing,¡± the 31-year-old entrepreneur said. ¡°I don¡¯t know one way or the other, but it happens all the time. It¡¯s just one more thing we have gotten used to.¡±