OBJECTIONABLE CURRENT
Objectionable neutral current occurs because of improper neutral-to-case connections or wiring errors that violate 250.142(B).
Improper Neutral-to-Case Connection [250.142]
Panelboards. Objectionable neutral current will flow when the neutral conductor is connected to the metal case of a panelboard that’s not used as service equipment.
Figure 250-6 (Click on image to enlarge)
Separately Derived Systems. Objectionable neutral current will flow on conductive metal parts and conductors if the neutral conductor is connected to the circuit equipment grounding conductor on the load side of the system bonding jumper for a separately derived system.
Figure 250-7 (Click on image to enlarge)
Figure 250-8 (Click on image to enlarge)
Disconnects. Objectionable neutral current will flow when the neutral conductor is connected to the metal case of a disconnecting means that’s not part of the service equipment.
Figure 250-9 (Click on image to enlarge)
Wiring Errors. Objectionable neutral current will flow when the neutral conductor from one system is connected to a circuit of a different system.
Figure 250-10 (Click on image to enlarge)
Objectionable neutral current will flow on metal parts when the circuit equipment grounding conductor is used as a neutral conductor such as where:
- A 230V time-clock motor is replaced with a 115V time-clock motor, and the circuit equipment grounding conductor is used for neutral return current.
- A 115V water filter is wired to a 240V well-pump motor circuit, and the circuit equipment grounding conductor is used for neutral return current.
- The circuit equipment grounding conductor is used for neutral return current.
Figure 250-11 (Click on image to enlarge)
Figure 250-12 (Click on image to enlarge)
DANGERS OF OBJECTIONABLE CURRENT
Objectionable neutral current on metal parts can cause electric shock, fires, and improper operation of electronic equipment and overcurrent devices such as GFPs, GFCIs, and AFCIs.
Shock Hazard. When objectionable neutral current flows on metal parts, electric shock, and even death can occur from the elevated voltage on those metal parts
Figure 250-13 (Click on image to enlarge)
Figure 250-14 (Click on image to enlarge)
Fire Hazard. When objectionable neutral current flows on metal parts, a fire can ignite adjacent combustible material. Heat is generated whenever current flows, particularly over high-resistance parts. In addition, arcing at loose connections is especially dangerous in areas containing easily ignitible and explo-sive gases, vapors, or dust.
Figure 250-15 (Click on image to enlarge)
Improper Operation of Electronic Equipment. Objectionable neutral current flowing on metal parts of electrical equipment and building parts can cause electromagnetic fields which negatively affect the performance of electronic devices, particularly medical equipment. For more information, visit www.MikeHolt.com, click on the “Technical Link,” and then on “Power Quality.”
Figure 250-16 (Click on image to enlarge)
When a system is properly grounded and bonded, the voltage of all metal parts to the earth and to each other will be zero. When objectionable neutral current travels on metal parts because of the improper bonding of the neutral to metal parts in violation of the NEC, a difference of potential will exist between all metal parts. This situation can cause some electronic equipment to operate improperly.
Figure 250-17 (Click on image to enlarge)
Figure 250-18 (Click on image to enlarge)
Operation of Overcurrent Devices. When objectionable neutral current travels on metal parts, tripping of electronic overcurrent devices equipped with ground-fault protection can occur because some neutral current flows on the circuit equipment grounding conductor instead of the neutral conductor. |
