As an example of what lightning can do:
Lightning struck a 180 ft. structure, passed through the metal roof, into an MCC at the top level, blew the mastic (tar) out of one of the mounting screw recesses on a size 4 starter, bridged the gap to one of the load side contacts and traveled to the basement, taking out the motor connected to it. I suppose the direction of travel for the discharge current, was from the basement upwards, based on the "tar" incident.
This structure is constructed of steel "I" beams, with a sheet metal skin.
This incident made me wonder why the designer of this system had chosen to place the motor starter at the top level instead of near the motor and routing the control wiring to the top level. Mark Prairie
|