Ecmweb Com Sites Ecmweb com Files Uploads 2013 11 110713 Code Q Aobjectionablecurrentweb
Ecmweb Com Sites Ecmweb com Files Uploads 2013 11 110713 Code Q Aobjectionablecurrentweb
Ecmweb Com Sites Ecmweb com Files Uploads 2013 11 110713 Code Q Aobjectionablecurrentweb
Ecmweb Com Sites Ecmweb com Files Uploads 2013 11 110713 Code Q Aobjectionablecurrentweb
Ecmweb Com Sites Ecmweb com Files Uploads 2013 11 110713 Code Q Aobjectionablecurrentweb

Code Q&A: Understanding Objectionable Current

Nov. 7, 2013
Your most pressing National Electrical Code (NEC) questions answered

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.

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.

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