Q. The Code prohibits "objectionable currents;" what is this, what are some examples, and how can it be prevented?
A. Section 250.6 of the 2011 NEC addresses objectionable current. First of all, the Code states that to prevent a fire, electric shock, or improper operation of circuit overcurrent devices or electronic equipment, electrical systems and equipment must be installed in a manner that prevents objectionable neutral current from flowing on metal parts [250.6(A)]. The Code further states that temporary currents from abnormal conditions, such as ground faults, aren't to be classified as objectionable current [250.6(C)]. Likewise, currents that introduce noise or data errors in electronic equipment are not considered objectionable currents for the purposes of this section. Circuits that supply electronic equipment must be connected to an equipment grounding conductor [250.6(D)].
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.
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.
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.
Wiring errors. Objectionable neutral current will flow when the neutral conductor from one system is connected to a circuit of a different system (see Figure).
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.
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.
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 explosive gases, vapors, or dust.
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.
Operation of overcurrent devices. When objectionable neutral current travels on metal parts, tripping of electronic overcurrent devices equipped with groundfault protection can occur because some neutral current flows on the circuit equipment grounding conductor instead of the neutral conductor.
