14.) The improper bonding and grounding of transformers.
Section 250-30 (a) (1) requires that a bonding jumper, installed in accordance with Sections 250-87 (a)-(d) and sized for the derived phase conductors, be used to connect the equipment grounding conductors of the derived system to the grounded conductor. Section 250-30 (a) (2) requires that a grounding electrode conductor, sized in accordance with Section 250-66 for the derived phase conductors, be used to connect the grounded conductor of the derived system to the grounding electrode as specified. In most cases both the bonding jumper and grounding electrode conductor connection can be made at any point on the separately derived system from the source to the first system disconnecting means or overcurrent device; or it can be made at the source of a separately derived system that has no disconnecting means or overcurrent devices.
Example: The most common application is a dry type transformer used to step down the voltage from 480 to 120/208. In this example the transformer is feeding a panelboard located adjacent to the transformer. The secondary conductors run from the transformer to the panelboard in a metallic raceway. The bonding strap installed by the manufacturer of the transformer usually connects the grounded conductor terminal (XO) to the frame of the transformer. If the bonding screw, that connects the grounded conductor of the panelboard (neutral) to the panelboard enclosure, is in place a parallel path over the metallic raceway is created. The grounded conductor must be isolated ( floated) from the equipment grounds either at the transformer or the panelboard in order to eliminate the possibility of a parallel path.
The 1999 NEC attempts to address the parallel path issue by requiring the grounding electrode conductor connection and the bonding jumper connection to be in the same location. The NEC Handbook has great commentary and illustrations that address this issue.