250.6 Objectionable Current Continued
Fire Hazard. When objectionable current flows on metal parts, a fire could occur because of elevated temperature, which can ignite adjacent combustible material. Heat is generated whenever current flows, particularly over high-resistive parts. In addition, arcing at loose connections is especially dangerous in areas containing easily ignitible and explosive gases, vapors, or dust. Figure 250.37
Improper Operation of Sensitive Electronic Equipment. ObjecAČtionable current flowing on metal parts of electrical equipment and building parts can cause disruptive as well as annoying electromagnetic fields which can negatively affect the performance of sensitive electronic devices, particularly video monitors and medical equipment. For more information, visit www.MikeHolt.com, click on the Technical link, then on Power Quality. Figure 250.38
In addition, when objectionable current travels on metal parts, a difference of potential will exist between all metal parts, which can cause some sensitive electronic equipment to operate improperly (this is sometimes called a ground loop).
Improper Operation of Circuit-Protection Devices. When objectionable current travels on the metal parts of electrical equipment, nuisance tripping of electronic protection devices equipped with ground-fault protection can occur because some neutral current flows on the equipment grounding (bonding) conductor instead of the grounded neutral conductor.
(C) Temporary Currents Not Classified as Objectionable Currents. Temporary fault current on the effective ground-fault current path isn't classified as objectionable current. Figure 250.39
(D) Electromagnetic Interference (Electrical Noise). Currents that cause noise or data errors in electronic equipment aren't considered objectionable currents. Figure 250.40
Some sensitive electronic equipment manufacturers require isolation between the metal parts of their equipment and the electrical system, yet they require their equipment to be connected to an independent ground (like a ground rod[s]). This practice violates 250.4(A)(5) and is very dangerous because the earth doesn't provide the low-impedance fault-current path necessary to clear a ground fault.
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