(Bloomberg) -- Houston’s citywide water outage stemmed from the failure of two electrical transformers at a key treatment complex that prevented officials from turning on backup generators. 

The outage that shuttered schools, delayed surgeries and forced some restaurants and other businesses to close their doors in the fourth-largest US city on Monday may stretch into a second day, given that initial results of safety tests won’t be known until 3 a.m. local time.

Water pressure in the city’s distribution network fell below levels required to ensure it’s safe to drink on Sunday after a transformer blew at a major water-treatment complex, Mayor Sylvester Turner said during a media briefing. When a backup transformer also blinked off, the city was unable to feed generator power into the plant. Turner described the backup glitch as “a very unique and unfortunate situation.” 

Within a half-hour of the transformer failures, pressure readings at 16 monitoring stations dropped below 20 pounds per square inch -- the minimum level required under state regulations to ensure the water is free of harmful bacteria and other contaminants, according to the city’s timeline.

Transformers are crucial gear in electrical systems because they modulate the voltage to make it safe for commercial, industrial and residential uses. 

When state environmental regulators assessed the data, they said a boil-water order had to be issued, Turner said. He added he had hoped such an order wouldn’t be needed because the low-pressure readings only lasted for about two minutes at most of the impacted monitoring stations.

Water samples collected early Monday arrived at the city’s laboratory around 9:45 a.m. local time and require at least 18 hours of incubation before the first results can be ascertained, officials said.

The power failure at Houston’s East Water Purification Plant was limited to isolated parts of the complex and an investigation is underway, Houston Public Works spokesperson Erin Jones said during an interview. 

High-rise office towers told tenants that fire-suppression and air-conditioning systems wouldn’t be impacted by the boil order. The water outages spread beyond Houston’s city limits, prompting school closures in suburban districts that rely on municipal supplies. 

Texas Governor Greg Abbott dispatched environmental-testing teams to assist Houston in restoring the potable water system. 

After consultations with Turner, “we’re currently working to fulfill the city’s request for help with rapid turnaround of water sample results,” Abbott said in a statement on Sunday night. “We urge those that the boil water notice affects to continue heeding the guidance of local officials and take adequate precautions when boiling and using water.”

--With assistance from Tal Barak Harif.

©2022 Bloomberg L.P.