PORTLAND — The Maine supreme court is hearing arguments over the safety of so-called smart meters installed by Central Maine Power.

Opponents who contend smart meters cause symptoms including headache, fatigue and sleep loss are challenging a December conclusion by the Maine Public Utilities Commission that 600,000 smart meters installed in homes and businesses pose no credible health threat.

They say the PUC disregarded its mandate to ensure “safe, reasonable and adequate services.”

CMP contends the smart meters are safe.

This is the second time smart meter opponents have gone to the state supreme court. The court ruled three years ago that regulators failed to adequately address safety concerns, leading to another review by regulators.