Originally Published: 27 NOV 22 23:13 ET

Updated: 28 NOV 22 14:21 ET

By Chris Boyette and Rosa Flores, CNN

(CNN) -- Houston officials expect the boil water advisory issued Sunday evening will be lifted by 3 a.m. Tuesday.

All the city's public schools closed Monday after a power outage caused a loss of water pressure at a purification plant, officials said.

The purification plant lost water pressure due to a power outage caused by a failed transformer and failed backup transformer, Mayor Sylvester Turner said at a news conference Monday morning.

Turner said two transformers at the power plant went offline around 10:30 a.m., and another followed shortly after.

When the plants went offline, sensors indicated the PSI, or pounds per square inch, dropped below 20 in 16 of 21 sensors, Turner added.

Carol Haddock, the director of Houston Public Works, said PSI lower than 20 increases the potential for contaminants to be introduced and noted PSI lower than 20 could also affect plants treating and pumping water.

Water for drinking, cooking, washing hands and faces and brushing teeth should be boiled for at least two minutes before using to destroy all potentially harmful bacteria and other microbes, the city said.

Houston's drinking and wastewater utility serves about 2.2 million customers per day, according to the utility's website.

Haddock said there was no indication of contamination and the boil advisory was issued out of an abundance of caution.

Turner said he is calling for an overall review of the water plan systems to prevent future transformer failures. He anticipates the boil advisory will be lifted once sampling is approved by the Texas Commission on Environmental Quality.

The Houston Independent School District said on Twitter it was monitoring the situation and would provide additional updates Monday.

"Earlier today, the water pressure dropped below the Texas Commission on Environmental Quality's required minimum of 20 PSI during a power outage at the East Water Purification Plant," a Sunday news release from the city said.

Water pressure has now been restored.

"We believe the water is safe but based on regulatory requirements when pressure drops below 20 psi we are obligated to issue a boil water notice," the office of Houston Mayor Sylvester Turner said on Twitter Sunday night.

A water sample plan was sent to the Texas Commission on Environmental Quality, according to Erin Jones, a Houston Public Works public information officer. Once the water sample plan is approved, water samples are collected by the city, submitted, and will be held for 18 hours in the commission's lab to see if anything grows on them, Jones said.

If nothing grows on the water samples, the state will give the city the green light to rescind the boil water order. The city has to wait for the state and cannot rescind the boil order independently, according to Jones.

If something does grow on those water samples, then the rescinding of the order will obviously take longer, Jones added.

"We are working closely with the City of Houston and stand ready to review the city's water sample results and offer any technical assistance that may be needed," the Texas Commission on Environmental Quality said in a statement.