National Grid faces a civil-court action and penalties following a finding by the state Public Service Commission that it failed to follow rules in shutting off gas to a Water Mill home that later exploded, severely injuring two people.

The PSC has asked National Grid, which operates the natural-gas delivery system on Long Island, to show why the state agency should not commence the court action and penalties following an investigation of the Feb. 11, 2015, explosion.

The state probe found the company “failed to follow regulations regarding discontinuance of gas service to the property.”

Rules require gas companies to shut off service when customers request it. In this case, National Grid workers “simply removed the customer’s name from the account when they called to close the account,” but left gas service on, PSC said.

Two workers renovating the house at 21 Old Country Rd. “struck the natural gas facilities in the basement, causing gas to be released, resulting in the explosion,” the PSC said. Both workers were airlifted to the hospital with serious, but non-life-threatening injuries.

National Grid could be forced to pay $100,000 or more per day for a “continuing violation,” starting with the date the customer requested the shut-off to the date of the explosion. In a 2014 Schenectady explosion, National Grid agreed to pay $500,000 to ratepayers to settle the case, which did not result in injuries.

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In its Water Mill probe, the PSC found “the customer specifically told National Grid that the new owner would not need gas service for the foreseeable future and yet National Grid still failed to lock the service. The explosion occurred two months after that call.”

Any penalty would be paid by National Grid shareholders, not customers, PSC said.

National Grid spokeswoman Wendy Ladd said the utility is taking the issues raised by the PSC “very seriously” and will respond “within the time frame required by the order.”

“We believe we have fully addressed the issues identified as part of our commitment to continuously improve the safety of our gas system,” she said. “We cannot comment further on pending proceedings.”

The commission said the incident was of “particular concern” because it’s the second resulting from a National Grid gas shut-off violation, following the Schenectady incident.

PSC chairman John Rhodes in a statement said the commission had “zero tolerance” for violations of gas-shutoff standards.