I'm troubled by a phenomenon I've noticed where I work. My office sits above the MCC for a large food processing operation. Power and control wiring pass through my office. The wire is in steel conduits, running vertical up the walls and the conduits are supported by stand offs. The conduit runs up to my office ceiling and then routes into the ceiling space above the production area, from where feeds are dropped to the various equipment. All told, there's about 25' of straight vertical run for each conduit.
When motors are turned on, I hear a snap. Usually, the snap is louder when starting a loaded motor. We use soft starts (phase angle) to start our 100 HP grinders and when those start, I hear a buzz, rather than a snap, that gradually increases in volume. As you might have guessed, the rate of increase of the buzz volume is directly proportional (actually equally) to the soft start ramp time.
I know that the conductors inside the conduit are moving due to the magnetic field generated by the sudden motor starting current as a starter pulls in. Also, I figure the soft start buzzing is due to the increasing, non-sinusoidal current as the soft start firing angle reduces. In both cases, the abrupt leading edge of the start voltage that results in non-linear current is causing some magnetic field in the conductors that cause them to move. Once the 3 phase current is balanced and sinusoidal, there's no noise from the conductors even if the running load changes dramatically.
This has been going on for 12 years. My concern is that those wires banging in the conduit will eventually tear through their insulation and short to the conduit. Maybe they'll get cut by a burr in the conduit or in a few more years wear out the insulation. Some of these motors start 100 times a day. I've had local inspectors witness this a few times and they all said there's nothing in the Ontario Electrical code that requires spacers or some type of shock absorption in vertical runs of conductors over some distance.
How can this be? It seems like it's an accidental waiting to happen. Anyone's district have rules about this sort of thing?