PORTLAND – Maine Supreme Court justices questioned Thursday whether utilities regulators shirked their responsibility for ensuring the safety of “smart meters” when they allowed customers to opt out of having them without giving assurances to customers who had them installed.

Opponents of Central Maine Power Co.’s wireless-enabled electricity meters say the devices emit radio frequency radiation that risks health and are an invasion of privacy because of the detailed data they collect.

The Maine Public Utilities Commission allows customers to opt out of having the meters if they pay $12 a month. Opponents say customers shouldn’t have to pay more if they don’t want the devices.

CMP contends that the 600,000-plus smart meters that have been installed are safe, cut energy use and enable utilities to quickly pinpoint problems during power outages.

Bruce McGlauflin, a lawyer for opponents of smart meters, said eliminating the $12-a-month-fee would alleviate some of the opponents’ concerns, but not other concerns including the safety problems.

On Thursday, skeptical justices questioned a lawyer for the PUC about why the agency failed to meet its statutory obligation of addressing the safety of smart meters.

Jordan McColman, lawyer for the PUC, responded that other agencies with greater expertise had found no credible studies that demonstrated harm, and that the panel felt the opt-out option addressed lingering concerns for customers.

The justices repeatedly pressed him about why the commission didn’t put that in writing.

Federal stimulus dollars funded roughly half the $200 million cost of the smart meter project. All but a couple thousand of them have been installed in CMP’s coverage area, and the rest are expected to be installed by next month, said company spokesman John Carroll.