Minimizing the Choke Effect

摘要

RECENTLY, I WAS ASKED TO EXPLAIN THE REASON for the ferrous metal raceway bonding requirements in 250.64(E). Although the National Electrical Code never uses the term "choke effect," many in the business understand its meaning, and the Code rules that it applies when an AC grounding electrode conductor is installed in a ferrous metal raceway. Here is a closer look at why the bonding requirements exist for these installations. Grounding electrode conductors must be protected if subject to physical damage. In addition to concerns about physical damage from external causes, grounding electrode conductors can be affected by magnetic fields. This is a not a concern for grounding electrode conductors that are not installed in ferrous (magnetic) metal raceways or enclosures, such as installations in aluminum conduit or rigid nonmetallic conduit. However, if a grounding electrode conductor is installed in a ferrous metal raceway or enclosure, it must be electrically continuous from the point of attachment to the cabinet or equipment to the grounding electrode and must be securely fastened to the ground clamp or fitting. Ferrous metal raceways or enclosures are those with iron or steel content such as rigid metal conduit, intermediate metal conduit or electrical metallic tubing. These ferrous conduits and tubing have a magnetic property that reacts to rising and falling magnetic fields present in AC systems. The amount of current in a grounding electrode conductor can be relatively high for the duration of a ground-fault event.