250.6 Objectionable Current.

 

Question: Mike, how do you define the "objectionable current over grounding conductors" referred in NEC 70 Article 250.6(A) and its relationship with the "temporary currents not classified as objectionable currents" referred in NEC 70 Article 250.6(C) and "currents that introduce noise or data errors in electronic equipment not being classified as objectionable currents" referred in NEC 70 Article 250.6(D)? 

Answer: From my textbook Understanding the NEC, Volume 1

 

 

 

 

250.6 Objectionable Current. (A) Preventing Objectionable Current. To prevent a fire, electric shock, or improper operation of circuit-protection devices or sensitive equipment, electrical systems and equipment must be installed in a manner that prevents objectionable current from flowing on conductive materials, electrical equipment, or grounding and bonding paths. Author's Comment: Objectionable current occurs because of improper neutral-to-case bonds and wiring errors. Improper Neutral-to-Case Bond [250.142] Panelboards. Objectionable current will flow on metal parts when the grounded neutral conductor is bonded to the metal case of a panelboard that is not part of service equipment. Figure 250-28 Disconnects. Objectionable current will flow on metal parts when the grounded neutral conductor is bonded to the metal case of a disconnecting means that is not part of service equipment. Figure 250-29 Separately Derived Systems. Objectionable current will flow on metal parts when the grounded neutral conductor is bonded at the transformer as well as to the metal case on the load side of the transformer. Figures 250-30 and 250-31

Figure 250-28 (Click on image to enlarge) Figure 250-29 (Click on image to enlarge) Figure 250-30 (Click on image to enlarge) Figure 250-31 (Click on image to enlarge)

Wiring Errors Objectionable current will flow on metal parts when the grounded neutral conductor from one system is connected to a circuit of a different system. Figure 250-32 Objectionable current will flow on metal parts when the equipment grounding (bonding) conductor is used as a grounded neutral conductor. Example: A 240V time-clock motor is replaced with a 120V time-clock motor and the equipment grounding (bonding) conductor is used to feed one side of the 120V time clock. Another example is a 120V water filter wired to a 240V well-pump motor circuit, with the equipment grounding (bonding) conductor used for the neutral. Figure 250-33 Using the equipment grounding (bonding) conductor for the neutral is also seen in ceiling fan installations where the bare equipment grounding (bonding) conductor is used as a neutral and the white wire is used as the switch leg for the light, or where a receptacle is added to a switch outlet that doesn't have a neutral conductor. Figure 250-34 Author's Comment: Neutral currents always flow on a community metal underground water piping system because the grounded neutral conductor from each service is grounded to the underground metal water pipe. Figure 250-35 Dangers of Objectionable Current Objectionable current on metal parts can cause electric shock, fires, and improper operation of sensitive electronic equipment and circuit-protection devices. Shock Hazard. When objectionable current flows on metal parts, electric shock and even death can occur (ventricular fibrillation) from elevated voltage on the metal parts. Figure 250-36

Figure 250-32 (Click on image to enlarge) Figure 250-33 (Click on image to enlarge) Figure 250-34 (Click on image to enlarge) Figure 250-35 (Click on image to enlarge) Figure 250-36 (Click on image to enlarge)

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. Objec¬tionable 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 Author's Comment: 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. Figure 250-41

Figure 250-37 (Click on image to enlarge) Figure 250-38 (Click on image to enlarge) Figure 250-39 (Click on image to enlarge) Figure 250-40 (Click on image to enlarge) Figure 250-41 (Click on image to enlarge)