I read this story decades ago (probably between 1975 and 1990). I remember it only vaguely. If I recall correctly it involved humans exploring the galaxy with an alien contact policy reminiscent of Star Trek's Prime Directive.
In the story, aliens on one of these explored planets somehow acquired a piece of human equipment, or perhaps plans for the equipment, and attempted to make their own copy. They duplicated it with great care, complete with the cord that plugged into the wall. However, they knew nothing about electricity, so the socket in the wall wasn't wired to anything. They couldn't figure out why their machine didn't work.
The machine might have been a weapon, but again I'm not sure. I'm pretty certain it was large, as in a fixed installation, or at least too big for someone to carry.
I think it was a short story, but it's possible it was part of a novel. It was told from a human's point of view. Probably third person, but I can't remember for sure. If I recall correctly, the human was engaged by the aliens to help them figure out why the machine didn't work.
Can anyone identify this story for me? Please?