"Weightlessness is a great way to work," Hadfield said as the pair labored in a gravity-free construction zone 245 miles (392 kilometers) above planet Earth.

Added Parazynski: "Another day in the office, Chris."

Coming midway through shuttle Endeavour's weeklong visit to the station, the seven-hour, 40-minute excursion proved to be a bit more painstaking than expected.

Working two stories above the winged orbiter, Parazynski quickly connected primary power, data and video cables that electrically link the 57.7-foot (17.5-meter) robot arm with computer control consoles inside the station's U.S. Destiny lab.

The veteran spacewalker, however, ran into problems trying to hook up a string of cables designed to provide a secondary source of power to the arm.

Initial wiring work failed to produce a flow of back-up electricity, forcing ground engineers to pull together a real-time troubleshooting plan.

That effort called for Parazynski to disconnect and then reconnect various wires hidden behind two separate panels on the lab's cylindrical hull. And it drug on for three hours before redundant power finally started flowing between the arm and two control consoles in the lab.

"Perseverance pays off sometimes," Parazynski said.

"We're all celebrating down here," astronaut Ellen Ochoa told the spacewalkers from NASA's Mission Control Center in Houston, Texas.

Considered the heart of Canada's $900 million contribution to the station project, the robot arm is designed to run on electricity routed to it from massive American solar wings that stretch 240 feet (73 meters) from tip to tip.

That power first flows into the Destiny lab and then out to the arm, which now is anchored to a pin-like metal fixture attached to the hull of the bus-sized science center.

The spacewalking wiring work was required to connect the arm to power systems within the lab. Only the primary cables must be connected to operate the arm. But electricity would have to be routed to the arm through the back-up cables if its main power source failed.

Consequently, the troubleshooting effort became a high priority, and the spacewalkers were happy to hear that they fixed the electrical glitch.

"That's just outstanding news," Parazynski said.

The spacewalking electricians also successfully disconnected a string of similar cables linking the robot arm and its pallet-like launch carrier so that the latter can be stowed Wednesday in the shuttle's cargo bay for a return trip to Earth.

But there were other challenges as well.