Electrical
Shock
An estimated 58 people lose their life each week as
a result of electric shock.
In an electrical system, the grounding and
bonding system is the primary protection against electrical
shock hazards. It provides a low resistance path to ground to
protect against electrical faults. The effective ground-fault
current path ensures facilitation of overcurrent device
operation under ground-fault conditions. The earth is not to
be considered as an effective ground-fault current path [see
250.4(A)(5)]. Using proper grounding and bonding techniques,
testing and maintaining a good electrical ground and
installing protection devices are the best ways to protect
people and equipment from electrical shock.
Proper
Grounding Techniques
Maintaining a good quality equipment grounding
system starts with wiring the circuits correctly. The NEC requires
that the removal of any device cannot interrupt the grounding
path, in accordance with 250.148(B). Receptacle manufacturers
have responded by supplying receptacles with only a single
grounding connection. This would prohibit electricians from
wiring the device in series with the grounding circuit.
Figure
1. Receptacles with a single grounding connection
Photo
1. Proper grounding
Pigtail
Connections
A common method of ensuring that the equipment
grounding connection remains intact is through the use of a
pigtail connection. The Code term for this
"pigtail" is equipment bonding jumper which
is defined in Article 100. To make a pigtail connection, take
both ground wires and join them with a 6-inch wire of the same
color that has been stripped on either end. Hold all three
tightly and bind them together with a wire connector. Be sure
to use the right size connector for the size and number of
wires.
Photo
2. Pigtail connector
Special connectors are available to make
this job easier. With one, a bare copper wire is inserted
through a hole at the top of the connector. All the wires are
then bound together, by twisting the connector until tight.
Pre-made pigtails are becoming popular
because of the timesaving involved. For example, some
connectors now combine a twist-on wire connector with a
pre-crimped pigtail. The ultra-flexible, six-inch lead
provides hassle-free positioning in a junction box, and the
grounding pigtails come with a pre-crimped fork connection for
quick and easy installation of the device.
Photo
3. Pigtail connector (lead to device is shortened for
the photo).
Bonding
the Junction Box to the Grounding Conductor
In many wiring circuits, more than one equipment
grounding conductor enters an outlet box. According to NEC 250.148, where more than one equipment grounding conductor
enters a box, all such conductors shall be spliced or joined
within the box or to the box.
The only exception is for isolated
receptacles, covered in Section 250.146(D) where isolated
receptacles are required for the reduction of electrical noise
(electromagnetic interference).
For metal junction boxes, the grounding
conductors from each device also needs to be connected to the
box with a listed grounding device, or a grounding screw, that
is not used for any other purpose.
Bonding
the Receptacle Grounding Terminal to the Junction Box
A device may have to be bonded to the junction box
with a jumper. According to NEC 250.146, an equipment
bonding jumper shall be used to connect the grounding terminal
of a grounding-type receptacle to a grounded box unless
grounded as in 250.146(A) through (D).
(A) Where the box is mounted on the
surface, direct metal-to-metal contact between the device yoke
and the box or contact device that complies with 250.146(B)
shall be permitted to ground the receptacle to the box. At
least one of the insulating washers shall be removed from
receptacles that do not have a contact yoke or device that
complies with 250.146(B) to ensure direct metal-to-metal
contact. This provision does not apply to cover-mounted
receptacles unless the box and cover combination have been
listed as providing an acceptable ground continuity between
the box and receptacle.
(B) Contact devices or yokes designed and
listed as self-grounding. shall be permitted in conjunction
with the supporting screws to establish the grounding circuit
between the device yoke and flush-type boxes.
(C) Floor boxes designed for and listed as
providing satisfactory ground continuity between the box and
the device.
(D) Where required for the reduction of
electrical noise (electromagnetic interference) on the
grounding circuit, a receptacle in which the grounding
terminal is purposely insulated from the receptacle mounting
means shall be permitted. The receptacle grounding terminal
shall be grounded by an insulated equipment grounding
conductor run with the circuit conductors. This grounding
conductor shall be permitted to pass through one or more
panelboards without connection to the panelboard grounding
terminal as permitted in 408.40, Exception, so as to terminate
within the same building or structure directly at an equipment
grounding terminal of the applicable derived system or
service.
The receptacle grounding terminal is
connected to an insulated equipment grounding conductor that
is run with the circuit conductors and is permitted to pass
through one or more sub-panels without connection to the
panelboard grounding terminal bar as permitted in Section
408.40 Exception.
Note that the use of an isolated equipment
grounding conductor does not relieve the requirement for
grounding the raceway system and junction box.
Maintaining
an Effective Grounding Path
A good electrical grounding system involves more
than following a few NEC requirements; it must also be
an effective grounding system. The path to ground is the
system grounded conductor and equipment connection to the
earth and serves as a path for stray current. If electricity
follows the path of least resistance, then the grounding
circuit (path) must have a lower resistance than an individual
to protect them. The rule of thumb for protecting people is to
maintain a ground impedance of less than one ohm. Note the Code has no set value for this resistance other than for the
maximum resistance values indicated for rod, pipe, or plate
electrodes, which is 25 ohms.
False
Grounds
The grounded (often a neutral) conductor can
generally be connected to the ground only at the service
disconnecting means neutral bus [see 250.24(A)(5) and
250.142(B)]. The main bonding jumper at the service connects
the grounded conductor and equipment grounding conductor
together at that point. The main bonding jumper serves as a
vital link in the ground-fault current path from the service
disconnect to the source windings (usually a utility
transformer). . Sometimes through error or ignorance, the
grounded (neutral) conductor and equipment grounding conductor
are connected together on the load side of the service
disconnecting means which violates the general requirements of
250.24(A)(5). This is often referred to as a false or bootleg
ground and can create unwanted or objectionable current in the
grounding circuit. If the grounded conductor and equipment
grounding conductors are connected anywhere else in the
building, all grounded metal can become part of the grounded
(neutral) conductor return circuit for unbalanced neutral
current that can create various voltage potentials on
electronic equipment. When using common receptacle testers,
this condition typically shows up as normally wired.
Earth
Ground
The path to ground extends beyond the main panel to
the earth ground system known as the grounding electrode
system as covered by Section 250.50. The earth ground could be
a single ground rod, multiple ground rods, a mat or a grid
system, or various other conducting elements that establish a
connection to the earth. The Code requires that if
present, all items listed in 250.52(A)(1) through (6) be
bonded together to form the grounding electrode system. There
is one exception for concrete-encased electrodes but this
applies only to footings of existing buildings or structures.
Section 250.56 addresses ground resistance by indicating that
if the grounding electrode (rod, pipe, or plate type) does not
have a resistance to ground of 25 ohms or less, an additional
electrode of any of the types listed in 250.52(A)(2) through
(7) must be added and installed at least 1.8 m (6 ft) from the
first electrode. The grounding electrode system can be tested
with an earth resistance tester, or a ground resistance clamp
meter.
While testing the resistance of the ground
electrode of the rod, pipe, or plate types after installation
will satisfy NEC requirements in 250.56, it is not
always enough to ensure protection of personnel or electronic
equipment.
The resistance of the ground electrode is
heavily dependant on the amount of soil resistivity. Because
soil resistivity relates to moisture and temperature, the
resistance of the grounding system will vary throughout the
different seasons of the year. To ensure an effective
grounding electrode system, include the ground electrode or
earth ground as part of your standard testing procedures in
your facility. A ground resistance clamp meter enables
electricians to measure the resistance of the ground electrode
in a fraction of the time required using the traditional
three-point fall of potential test.
Ground-Fault
Circuit Interrupters
The Code requires the installation of
ground-fault circuit interrupters (GFCIs) in residential
dwellings to protect against shock. Receptacles in bathrooms,
garages, outdoors, crawl spaces, unfinished basements,
kitchens, near wet bar sinks, utility sinks and laundry sinks
require protection. All 125-volt 15- and 20-ampere receptacles
at boathouses are required to be GFCI as well as any
branch-circuit outlet for a dwelling unit boat hoist (see
210.8(A) for additional information). The Code also
requires GFCI protection for many installations in other than
dwelling units. [See 210.8(B) for a more complete list of
those areas where this ground-fault circuit interrupter
protection is required].
A GFCI receptacle is a device with a
built-in circuit to detect leakage current to ground on the
load side of the device. When the GFCI detects leakage current
in the 4–6 milliampere range, it will interrupt power to the
load side of the device, preventing a hazardous ground-fault
condition. [See the definition of ground-fault circuit
interrupter (GFCI) Class A GFCI device in Article 100 for
additional information].
These devices should be tested regularly,
because they rely on mechanical connections that can degrade
over time. According to a recent study performed by the
Leviton Institute, on average 15 percent of GFCIs were
inoperative when tested. "Voltage surges from lightning,
utility switching and other sources all take their toll on the
devices, which is why Underwriters Laboratories (UL) requires
that GFCIs be tested monthly."
Equipment
Failure
When sensitive electronic equipment fails, the
initial reaction is to throw our hands up and blame it on poor
power quality. This makes the problem seem unmanageable and
out of our control. Most of these problems are actually under
our control, because 80 percent of all power quality problems
are found in the electrical distribution and grounding and
bonding system.
In addition to preventing the possibility
of fire, a good low-impedance electrical grounding and bonding
system will serve to protect electronic equipment. A
high-resistance connection, like a loose wire, will cause the
voltage to fluctuate, or drop, when a large load is applied.
If the voltage drops low enough, it can cause electronic
equipment to lock up, reset or shut down completely. Grounding
is another concern for electronic equipment. While ground
impedance of one ohm or less may protect people from electric
shock, it may not be adequate protection for electronic
equipment. IEEE recommends a ground impedance to be less than
0.25 ohms for proper protection.
Isolated
Grounds and Dedicated Circuits
In some cases, it is easier to isolate sensitive
electronic equipment than to re-wire an entire circuit. This
can be done by running an isolated ground for the equipment in
question, or by running a new dedicated circuit. The Code does not currently include the term, dedicated circuit;
however, the term individual branch circuit is
defined; and such a circuit is often installed for
sensitive electronic equipment. Individual branch circuits can
also include isolated grounding conductors installed to meet
the provisions of 250.146(D).
An isolated ground protects the equipment
from other equipment on the same grounding circuit. Electronic
equipment can create electrical noise on the grounding
circuit, which can interfere with the operation of other
equipment on the circuit. It is important to note that an
isolated ground will not protect equipment from harmonic
distortion running through a shared neutral conductor of
typical multiwire branch circuits.
In some cases, running a dedicated circuit
(individual branch circuit) is necessary to completely isolate
a piece of equipment in order to ensure protection.
Article 285 provides the rules and covers
the use of transient voltage surge suppressors. These devices
protect power, telephone and cable lines from transient
voltage. Transients are short high amplitude pulses caused by
a release of energy onto the electrical system. These pulses
of energy can be caused by internal sources, such as a
capacitor releasing energy into the system, or external
sources like lighting.
Conclusion
The hidden dangers associated with branch-circuit
wiring are very serious, but fortunately the precautions are
straightforward. We can protect ourselves and equipment by
using certified devices and testing equipment from reputable
manufacturers and by implementing policies on branch-circuit
testing. These policies should include verifying proper
wiring, testing devices, checking the integrity of the branch
circuit, and measuring the integrity of the grounding system.
Photo
4. Ground resistance clamp meter
Installers should always check all devices
immediately after installation to verify proper wiring and
test devices. It is generally not the responsibility of the
electrical inspector to test installations upon completion.
The installing contractor is generally responsible for this
type of testing. Receptacles should be checked to avoid common
wiring errors, such as reversed polarity or an open neutral.
Checking the voltage level with a voltage tester quickly
verifies that the receptacle has been correctly wired for
either 120 or 220VAC. Checking continuity across a switch
verifies that it working correctly. A variety of testers are
available on the market to test these devices quickly and
accurately.
Test electrical circuits under load to test
the integrity of the branch circuit. The voltage drop test can
identify high resistance connections, which can lead to fires,
breakdown in insulation, and poor efficiency of the electrical
system, which can contribute to erratic equipment operation.
Test the integrity of the grounding system,
which includes not only the equipment grounding conductors,
but also the ground rod or grounding electrode system. A low-
impedance path on both of these systems is essential to
protect against electrical shock. The effective ground fault
current path ensures that overcurrent devices will operate
under ground fault conditions. See 250.4(A)(5).
In summary branch-circuit testing is an
important part of wiring any circuit. It verifies that devices
have been wired up correctly and allows you to protect
yourself against the hidden defects in an electrical system.
Jim Gregorec is group manager, Test
& Measurement Business Unit, Ideal Industries, Inc.
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