An explosion at a South Portland home and gas leak in Cape Elizabeth were two of the most serious safety incidents that led the state’s Public Utilities Commission to issue a $2 million fine to Northern Utilities this week.

The commission approved the fine Monday, with stipulations that Northern Utilities, an energy services company based in Massachusetts, directly invest $1.2 million in infrastructure safety improvements, and pay $800,000 to the commission.

According to a statement by the commission, none of the funds can originate from from utility customers, and the commission will hold a separate proceeding to consider distribution of the $800,000 administrative penalty in the public interest.

The Public Utilities Commission and Northern Utilities have been working for more than a year to address issues raised by a series of safety incidents at Northern Utilities facilities going back two years. Those included, in addition to the South Portland and Cape Elizabeth incidents, an explosion at a home in Portland and an over-pressurization incident that resulted in several fires in Saco.

The South Portland home exploded in October 2007 when gas traveled from a ruptured pipe to the basement of the home through a water or sewer line. The gas then filled the home, and was ignited by a pilot light on a water heater in the basement. No one was injured.

Just days later, a Cape Elizabeth neighborhood was evacuated when gas leaked through a main on Oakhurst Road.

“Northern Utilities’ problems were system-wide and compromised public safety,” Sharon M. Reishus, who chairs the MPUC, said in a statement. “That was not acceptable. The company’s shareholders must now pay for major safety investments and management must improve its practices.”

The commission said it will require Northern Utilities to make upgrades that range from the installation of new valve and pressure relief systems, to new safety protocols for management and frontline staff, to improved documentation and tracking of Northern facilities in Maine.

Also on Monday, the commission approved the purchase of Northern Utilities’ Maine operations by Unitil, a New Hampshire-based company. Unitil has proposed to implement safety recommendations made in a comprehensive review the commission ordered of safety and management practices at Northern Utilities’ Maine operations.