NEC Article 250 Sections 250.6 through 250.12
250.6. Objectionable Current Flowing Through The Grounding
Authors Comment: Objectionable neutral current on the
grounding path is often created by improper wiring of the electrical system:
Improper Neutral-to-Ground Connection [250.142]
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].
Authors 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.
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 authors 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
Case Study of Problems Associated with Improperly Grounded
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
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.
Authors 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)].
(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
250.10 Protection of Ground Clamps
250.12 Clean Surface
If you have any comments or suggestions on how I can improve this, please let me know, Mike@MikeHolt.com.
Copyright © 2003 Mike Holt Enterprises,Inc.