Grounding and Bonding, Part 2
Myths of Grounding and
Bonding
By Tom Baker,
Puget Sound
Electrical
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Grounding is misunderstood because it is
misapplied. In most cases, we are concerned with
Bonding, not Grounding. Even the terminology in
the NEC is misunderstood, for example, the Article
100 definition of Effective Grounding is:
Intentionally connected to earth through a ground
connection or connections of sufficiently low impedance
and having sufficient current-carrying capacity to
prevent the buildup of voltages that may result in undue
hazards to connected equipment or to persons.
Bonding quiz, here are the answers:
1. Electricity always takes the path of least
resistance.
True
False
False: Electricity takes all paths of resistance.
We have been taught that electricity takes the
path of least resistance, however electricity
takes all paths. If you are touching a metal
lighting pole energized with a line to case fault
of 120V, even with a nominal body resistance
of 1,000 ohms, you would have sufficient
current through your body to have a severe
shock or electrocution.
2.
In current flow, electrons leaving a grounded
power supply are trying to get to back to earth
ground.
True
False
False: Electrons are trying to get back to their
source. Grounding the power supply does not
provide a path to the source.
3.
A 15-ampere breaker will ultimately trip if an
energized 120-volt conductor is directly
attached to an isolated ground rod that has a
measured ground resistance of 10 ohms.
True
False
False: Using ohms law, the current through
the ground rod will be 4.8 amperes. The
overcurrent protective device will not open.
Ten ohms is low ground resistance and many
installations will have a much higher
grounding resistance. However, the purpose
of grounding electrical systems to earth has
nothing to do with clearing line to case faults.
4.
A ground rod at a metal light pole with no
equipment ground (green wire) serves to
reduce the touch potential and makes the
installation safer.
True
False
False: This is a myth and misconception. The
voltage falls off so rapidly that there will be a
90 volts difference from the ground rod to earth
at only 3 feet distance. This voltage is more than
enough to severely shock or electrocute
someone.
In Article 406 Receptacles, Cord Connectors, and
Attachment Plugs (Caps), Section 406.3 General
Installation Requirements, Part (B) To Be
Grounded requires that Receptacles and cord
connectors that have grounding contacts shall have
those contacts effectively grounded.
We think that compliance with [Section 406.3 (B)]
could be done by intentionally connecting the
grounding contact of the receptacle to earth, in
compliance with the Article 100 definition of
Effective Grounding. Doing so would result in a
dangerous condition, even thought the receptacle
would be grounded, it would not be bonded, and
the impedance of the earth ground fault return
path would be too high to allow the overcurrent
protection device to clear quickly.
The Article 100 definition of Bonding is:
The permanent joining of metallic parts to form an
electrically conductive path that ensures electrical
continuity and the capacity to conduct safely any
current likely to be imposed.
In the 2002 NEC, a new definition was added for
the Effective Ground-Fault Current Path:
An intentionally constructed, permanent, lowimpedance electrically conductive path designed and
intended to carry current under ground-fault conditions
from the point of a ground fault on a wiring system to
the electrical supply source.
Bonding, not Grounding, ensures that the lowimpedance path is installed to clear line to case
faults. Many cases of shock and electrocution from
energized metal poles and traffic signal junction
boxes are due to a lack of Bonding, not Grounding.
The metal lighting pole or traffic signal junction
box may have been grounded, but the impedance
of the ground fault return path was too high to
allow the overcurrent protective device to quickly
clear the line to case fault.
Myths of Grounding
The last issue had a Myths of Grounding and
5. The grounding electrode for a 480 x 120/208
volt transformer (separately derived service)
is intended to help remove dangerous voltage
(touch potential) that can be imposed on the
metal parts of the electrical system from a lineto-ground fault.
True
False
False: The grounding electrodes for a
separately derived service do not clear line to
case faults. Separately derived services and
electrical systems are grounded for other
reasons.
Continued on page 24
IMSA Journal
Code Corner . . .
6.
Ground rods are not required at a separate building
supplied by a 60 ampere 4 wire feeder (120/240 V -, 2
hots, neutral and ground, with the neutral floated at
the separate building disconnect).
True
False
False: A separate building or structure always requires
a grounding electrode system, with one exception
[250.32 (A)]. The grounding electrode system is installed
for lightning protection; lightning does not diferentiate
if the separate building or structure has a 3 wire, 4 wire
or no feeder.
Myths of Bonding
1.
2.
3.
4.
Ground rods are not required at a separate building
supplied by a 60 ampere 4 wire feeder (120/240 V -, 2
hots, neutral and ground, with the neutral floated at
the separate building disconnect).
True
False
False: Ground rods are installed for lightning protection,
see the preceding question. However, as a bonding
question, and for bonding, a “4 wire feeder” is required
by the NEC if there are parallel paths for neutral current.
If the neutral is “regrounded” at the separate building
or feeder, dangerous neutral current will flow on all
metal parts of the separate building or feeder, possibly
resulting in fires, improper operation of overcurrent
protective devices, and problems with sensitive
electronic equipment. The 1999 NEC essentially
removed the allowance to “reground” the neutral.
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An equipment-grounding conductor provides the low
impedance path necessary for proper operation of a
GFCI.
True
False
False: A GFCI does not require an equipment grounding
conductor (green wire). A GFCI detects the imbalance
between the “hot” (ungrounded) and “neutral”
(ungrounded) conductors. If there is more than a 4-6
mA unbalance, the GFCI trips.
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The bonding location of a metal water pipe (250.104)
must be done within 5 feet of where the pipe enters a
building.
True
False
False: Bonding of a metal underground water pipe is
only required within 5 feet of the entrance to the
building if the water pipe is used for a Section 250.52
grounding electrode. A bonding connection per 250.104
can be made in any accessible location.
Per 250.122, an equipment ground never has to be larger
unless the conductors are increased in size for voltage
drop.
True
False
False: The note to Table 250.122 now reads: Where
necessary to comply with 250.4(A)(5) or 250.4(B)(4), the
equipment grounding conductor shall be sized larger than
given in this table. The 1996 NEC read may be sized larger.
This was changed to make clear that there are
installations that require the equipment grounding
conductor to be increased in size. In a ground fault, if
Page 24
the equipment grounding (bonding) conductor is
undersized, it may burn clear before the fault clears.
Continued from page 22
5.
Reducing washers (donuts) provide the low impedance
path necessary for fault clearing and it is not necessary
to bond around them.
True
False
False: Reducing washers do not provide for electrical
bonding. If the raceway is being used as an equipment
grounding (bonding) conductor per [250.118], then a
bonding bushing must be installed to bond around the
reducing washers.
6.
The ground terminal of an isolated ground receptacle
cannot be bonded to the box at where the receptacle is
mounted.
True
False
False: It is commonly thought that the isolated ground
terminal must be bonded at the service or source of the
separately derived service. The bonding of the isolated
ground receptacle can be done at any point in the circuit,
per [250.146(D)]. Note that isolated ground circuits are
not always effective, and lack of proper maintenance
can render them ineffective.
Bonding, not Grounding
One of the problems with our understanding of grounding
and bonding is that we confuse the terms, and the NEC itself
is confusing on the use of grounding. While the electrical
system must be grounded, we are not sure when we are
grounding and when we are bonding. For the 2005 NEC,
the title of Article 250 will be changed to Grounding and
Bonding, to reflect the importance Bonding has in the
electrical system. The focus in installing Traffic Signal and
Roadway Lighting electrical systems must be in bonding,
to maintain a low impedance fault current return path.
Next Issue:
The series on “Bonding, not Grounding” will continue. Find
out how Section 250.4’s The prescriptive methods contained in
Article 250 shall be followed to comply with the performance
requirements of this section, can be understood by comparing
a cup of coffee to a double tall latte.
IMSA Member Tom Baker is a Master Electrician, and
is certified as an IMSA Level II Traffic Signal and
Roadway Lighting Level I. His business, Puget Sound
Electrical Training, provides classes on the NEC,
Grounding and Bonding, and other electrical subjects.
He is the IMSA representative to the Illumination
Engineering Society. Contact him via email at:
tom@psetraining.com
Italic text excerpted from the 2002 NEC, National Electric
Code® and the NEC® are registered trademarks of the
National Fire Protection Association, Inc., Quincy
Massachusetts.
I M S A
IMSA Journal