Running raceway overhead means you can drop things on whatever or whoever is below. For example, Sam was running raceway inside a fairly tight room that served as a “data center” for a regional bank. While assembling a bracket onto the support channel, he dropped a nut. It fell into a vent in the cabinet below, bounced off something, and came to rest on the cabinet floor. Sam was lucky. The nut could have caused an arch flash between two conductors. After that close call, the electricians placed plywood and tarps over cabinets. Sam now owns a magnetic nut clip for his wrenches, too.
Let’s review another situation that could have turned out much worse than it did.
David was running raceway over a production line that the supervisor didn’t want interrupted. He accidentally dropped a nut into a non-enclosed, fan-cooled motor. The nut bounced off the fan and hit Jim’s leg. What if Jim had been working on that motor and been struck in the eye? What if, instead of the nut, David dropped the much heavier wrench he used to turn that nut? David should have barricaded the area first.