(A) Maintaining Effective Ground-Fault Current Path. All metal parts intended to serve as the effective ground-fault current path, such as raceways, cables, equipment, and enclosures must be bonded together to ensure they have the capacity to conduct safely any fault current likely to be imposed on them [110.10, 250.4(A)(5), and Note to Table 250.122].
Author’s Comment: Bonding jumpers are required around reducing washers (donuts) because they do not provide an effective ground-fault current path [250.4(A)(5)]. Figure 250–136
Nonconductive coatings such as paint, lacquer, and enamel on equipment must be removed to ensure an effective ground-fault current path, or the termination fittings must be designed so as to make such removal unnecessary [250.12].
Author’s Comment: The practice of driving a locknut tight with a screwdriver and pliers is considered sufficient in removing paint and other nonconductive finishes to ensure an effective ground-fault current path.
(B) Isolated Ground Circuit. A metal raceway containing circuit conductors for sensitive electronic equipment can be electrically isolated from the sensitive equipment it supplies by a nonmetallic raceway fitting located at the equipment. However, the metal raceway must contain an insulated equipment grounding (bonding) conductor to provide the effective ground-fault current path to the power source in accordance with 250.145(D). Figure 250–137
DANGER: Some digital equipment manufacturers insist that their equipment be electrically isolated from the building or structure’s grounding (earthing) system. This is a dangerous practice, and it violates 250.4(A)(5), which prohibits the earth from being used as an effective ground-fault current path. If the metal enclosures of sensitive electronic equipment are isolated or floated as required by some sensitive equipment manufacturers, dangerous voltage on metal parts can remain from a ground fault. Figure 250–138
Author’s Comment: For more information on how to properly ground sensitive electronic equipment, visit www.MikeHolt.com, go to “Technical,” then to the “Grounding” link.
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