New York ? Investigators looking for causes of the massive August blackout are taking a hard look at relays that are supposed to sense problems on the electrical grid.

The devices vary from simple mechanical switches that turn off shorting transmission lines to complex digital monitors meant to detect more subtle problems — including the sorts of power swings that doused the lights across eight states and parts of Canada.

Experts are still unsure whether digital relays could’ve strangled the blackout in its early stages — or might have fueled its spread by sequestering key parts of the grid that could have absorbed excess flows of electricity.

These days, most sensors guarding the grid are electromechanical relays that may be 40 years old, operating on technology that dates back even further.

In southeastern Michigan, old-style mechanical relays monitoring three power lines on the grid operated by International Transmission Co. failed to trip, even though they experienced imbalances in load — or demand for electrical current — said Richard Schultz, vice president of planning for the Ann Arbor-based company.

Those three lines tied into the Ohio grid operated by FirstEnergy Corp., the Akron-based utility under scrutiny as the potential starting point for the Aug. 14 power failures.

“The protective schemes in the FirstEnergy line were designed to (shut) lines if there were short circuits, not imbalances of load,” Schultz said. “Relays are not installed for imbalances.”

But the electricity load imbalances might have been picked up by the newer type of digital relays that have been installed in the grid for the past decade or so, said Dave Dolezilek, technology director of Schweitzer Engineering Laboratories, a manufacturer of digital relays in Pullman, Wash.

If the computerized relays had sensed load imbalances — like the soaring demand on FirstEnergy’s lines, for instance — they might have slammed those tie lines closed and kept the blackout isolated to northeast Ohio, said Dolezilek and Hoff Stauffer, a transmission analyst with Cambridge Energy Research Associates.

“If you had a digital multipurpose device that was watching for short circuits and other things, it would have said ‘Look at this voltage imbalance. I need to react to that,'” Dolezilek said.

Relays on surrounding grids operated by American Electric Power Co. and PJM Interconnection did order the shutdown of lines that interlocked with FirstEnergy. PJM’s lines were brought down when relays sensed a large amount of current flowing at low voltage, said Ray Dotter, spokesman for the Valley Forge, Pa.-based company.

PJM, the grid operator east of FirstEnergy, said it was examining the relays to ensure that they were working and calibrated properly — or whether they were even able to detect the confluence of strange events on the power grid, Dotter said.

Perhaps “what it detected was something that it wasn’t programmed for,” he said.