250.6. Objectionable Current Flowing Through The Grounding Path
(A) Arrangement to Prevent Objectionable Current. To prevent a fire or electric shock, the grounding of electrical systems, circuit conductors, electrical equipment, and conductive metal parts must be done in a manner that objectionable current will not flow over the effective fault current path.

Author’s Comment: Objectionable neutral current on the grounding path is often created by improper wiring of the electrical system:
An improper neutral-to-ground connection,
Errors in the wiring installation, or
Improperly using the grounding path to carry neutral current.

Improper Neutral-to-Ground Connection [250.142]
Panelboards - Bonding of the neutral terminal to the case of a panelboard on the load side of service equipment will create a parallel path for neutral current which allows neutral current to flow on the neutral conductor as well as the fault current path.

Separately Derived Systems - The neutral-to-ground connection for a separately derived system, such as transformers, generators, or UPS systems to be installed at the separately derived system or at the first disconnect after the separately derived system, but not at both locations [250.30(A)(1)]. If a neutral-to-ground connection is made at both the transformer and at the secondary panelboard, then neutral current will return on the neutral conductor and objectionable (neutral) current will flow through the grounding path.

If the neutral conductor in a transfer switch is not switched, then the neutral from the generator or UPS will be solidly interconnected to a service system neutral. Under this condition the generator or UPS is not considered a separately derived system, and a neutral-to-ground connection must not be made at the generator/UPS or at the generator/UPS disconnect [250.20(D) FPN 1].

Author’s Comment: Objectionable neutral current often flows over electrical systems because the generator is supplied from the manufacture with a neutral-to-ground connection in accordance to UL requirements, and the transfer switch selected does not separate the neutral from the generator from the feeder neutral.

Wiring Errors
Mixing the grounded neutral between systems - The NEC does not prohibit the mixing of different systems in the same raceway or enclosure and as a result, maintenance personnel, as well as others who are not qualified, can accidentally mix up the grounded (neutral) conductors between the systems in violation of 210.4. When this occurs, the fault current path will carry objectionable neutral current, and a dangerous voltage can exist on metal parts of the electrical system, even when it appears that all circuits have been de-energized.

Using the fault current path as a neutral conductor - At times when a 120 V circuit is required at a location where a neutral conductor is not available, an unqualified person without proper training will use the fault current path as the neutral conductor. This could occur when a 240 V time clock motor is replaced with a 120 V motor or when a 120 V water filter is wired into the circuit of an adjacent 240 V well pump motor.

End of author’s comment

DANGER: Objectionable neutral current can cause electrocution and property damage as well as deaths from fires.

Electrocution. The touch potential on the metal parts of an electrical system as well as the building structure can easily be in excess of 30 V to earth when the fault current path is used to carry neutral current or if a person gets in series with this path.

Death from electric shock (electrocution) can occur when the touch potential (voltage between the metal parts of the electrical system and the earth) is above 30 V RMS resulting in as little as 30 milliamperes of current to flow though the body. Alternating current, particularly 60 Hz disrupts the hearts electrical circuitry causing it to go in to ventricular fibrillation, which prevents the blood from circulation through the brain, resulting in death in a matter of minutes.

Fire Hazard. Fire is created when heat rises to a level that is sufficient to cause ignition of adjacent combustible material in an area that is oxygenated. In an electrical system, heat is generated whenever current flows. Improper wiring resulting in neutral current flows through the fault current path can cause the temperature at loose connections to rise to a level that can cause a fire. In addition, arcing at loose connections particularly in dangerous areas containing easily ignitable and explosive gases, vapors, or dust.

End of Danger

Power Quality Case Study
Electromagnetic interference from objectionable neutral current on the fault current path and building structure can disrupt the performance of sensitive electronic equipment, particularly video monitors, electronic microscopes, etc. In addition, there's the unknown health issue as to the effect of electromagnetic fields on the human body.

Case Study of Problems Associated with Improperly Grounded Systems
Mike, how about if I give you a brief overview of a building I just completed that is typical of what I have been finding. The building is a 106,290 sq. ft, five story plus mechanical penthouse, office building. The 120/208 V, 3-phase service is supplied by a 150 kVA, 3-phase 480 V transformer that feeds a 600-amp main distribution panel. The lighting and mechanical equipment is fed from a 277/480 V, 3-phase riser. An additional 112.5 kVA transformer was installed in the mechanical penthouse to provide normal and emergency 120/208 V, 3-phase power to several cubicles on the third floor and a computer room on the fifth floor

This building was experiencing various problems such as overheating and tripping of two 100 A breakers in the main distribution panel; the main transformer was too hot to touch and was operating at a low power factor.

When I inspected the building, I did a 15-minute walk-through with a triaxial Gaussmeter and had readings up to 30 mG in a variety of office and common areas. The actual readings were not as important as the fact that it simply established that there was a problem. During a walk-through, I try to keep the Gaussmeter at least one meter away from anything. My experience has indicated that any reading of one mG or more with one meter of distance is a sign of a problem.

Next, I went to the 480 V, 3-phase Motor Control Center (MCC) in the mechanical penthouse and checked the equipment grounding conductor for current. Each half of the parallel feed (two raceways) to the MCC had three conductors and one equipment grounding conductor. One grounding conductor was carrying twelve amperes and the other eight amperes. These conduit risers were also warm to the touch and had high mG readings as did other piping, handrails, etc. The grounding electrode conductor from the transformer to the water pipe read 1 ampere.

An inspection of the grounding and bonding revealed that neither transformer was grounded to building steel, the neutral to ground bond was missing in the main distribution panel and the system main was only grounded to the main water pipe. There was no bonding between the service equipment (600 main) to the building steel, and there was no bonding between the water pipe to building steel.

An inspection of the eight 120/208 V, 3-phase, 100 A sub-panels and the six 277/480 V, 3-phase, 100 A lighting panels revealed that 25 circuits had an excess of .5 A net current. The highest reading was 14 A. (In commercial work, I generally disregard any reading below .5 A because it can be difficult to find the problem).

The total of the 120/208-volt net currents came to 90 A and there were 20 A from the 277/480-volt lighting side. In other words, there were 90 A of transformer neutral balancing current that did not have a direct path back to the transformer. A total of 110 A was flowing through the building steel.

I found 18 circuits had neutral to ground shorts in receptacles at connectors or wires cut on the ears of add-on boxes. A large copier had a neutral to ground shorted surge suppressor, five lighting fixtures had ballast wires caught under ballasts or ballast covers. The emergency feed transformer in the mechanical penthouse fed a panel next to it, which fed several emergency circuits in cubicles on the third floor. These cubicles also had normal power fed from the third floor panel. All the neutrals were tied together in the cubicles causing net current.

While working with one electrician to clear the net currents, another installed the needed and supplemental grounding and bonding and grounding rods. Initially, before the net currents were cleared, the current on the grounding electrode conductor for the main transformer (to building steel) was 30 A!

The new (and proper) grounding connection for the transformer allowed the transformer to recover some needed neutral current and the transformer cooled down some, but did not cool fully until all the net currents were cleared. The current on the main transformer grounding electrode conductor (after the fixes) is now about 2 A.

Clearing the net currents and correcting the bonding and grounding has eliminated the problems at this property and improved the power factor on the transformer. The reduction of net current helps in extending the life of lamps and ballasts, and the reduction of fires.

Author’s Comment: For more information about Power Quality as it related to grounding, visit http://www.mikeholt.com/Powerquality/Powerquality.htm

End of PQ case study

(B) Stopping Objectionable Current. If multiple neutral-to-ground connections results in an objectionable flow of current, one or more of the following shall be permitted to be made, provided that the fault current path is permanent, electrically continuous, capable of safely carrying the maximum fault likely to be imposed on it, and it has sufficiently low impedance to facilitate the operation of overcurrent devices under fault conditions [250.2(A)(5)].
1. Discontinue the improper neutral-to-ground connections.
2. Change the locations of the neutral-to-ground connections.
3. Interrupt the continuity of the conductor or conductive path interconnecting the grounding connections.
4. Take other suitable remedial action satisfactory to the authority having jurisdiction.

(C) Temporary Currents Not Classified as Objectionable Currents. Temporary fault current on the fault current path from a line-to-case connection, until the circuit overcurrent protection device removes the fault it is not classified as objectionable current.

250.8 Termination of Grounding and Bonding Conductors
Equipment grounding conductors, grounding electrode conductors, and bonding jumpers must be terminated by exothermic welding, listed pressure connectors (set screw, compression), listed clamps, or other listed fitting. Sheet-metal screws shall not be used for the termination of grounding conductors.
Note: If a screw is used for grounding that screw shall not be used for any other purpose.

250.10 Protection of Ground Clamps
Ground clamps and fittings shall be protected from physical damage by being enclosed in metal, wood, or its equivalent. One way to accomplish this is to terminate the ground clamp on a buried electrode as permitted in Sections 250.53(G, 250.68(A) Ex., and 250.70.

250.12 Clean Surface
Nonconductive coatings such as paint, lacquer, and enamel on equipment must be removed from threads and other contact surfaces to ensure good electrical continuity, or the termination fittings must be designed to provide proper electrical continuity [250.53(A) and 250.96(A)].

If you have any comments or suggestions on how I can improve this, please let me know, Mike@MikeHolt.com.