Mike, Remember the bus bench incident that electrocuted a teenage boy in Dade County a couple of years
Well, now we have an energized light pole that killed two more teenagers. Both of these cases, shows
that Dade County allows unqualified persons to maintain county owned equipment with untrained and
unqualified personnel. These are the quotes about this recent electrocution from the Miami Herald:
"There was something wrong with the pole, and the safety systems in place didn't work. The
insulation on part of the wiring inside the pole, which is just like the rubber insulation around
any electrical cord, was burned off or worn off from a section near the bottom of the pole. It's
possible if there was defective wiring - and standing water made contact with it - it could charge
it. The main breaker is seven blocks from the pole, which detects malfunctions within poles in
the area, was not triggered to shut off electricity. We don't know why that didn't happen. Another
factor in the accident may have been the age of the pole."
Mike, give me a break the pole was installed in 1990! Seven people have died in two years in Dade County
due to unqualified people working on county owned property. Having attended your grounding and bonding
class, I found out that I had many misconceptions about "proper grounding".
Why did I learn about these new views and methods? Because, the State of Florida requires Continuing
Education for all electrical contractors, I have to attend a minimum of 14 hours every two years covering
National Electrical Code and Electrical Safety. And what about the unlicensed electrician, contractor
and/or inspector? How does he/she get the education on proper wiring of electrical systems, particularly
grounding and bonding?
This pole incident (killing two kids) will be like the bus bench incident where an unqualified person
was sent to install electrified signs on county owned bus shelters, on county property---without a
county permit. No inspections--no license and NO GROUND!
I am not a street lighting contractor, nor do I intend on doing any street lighting, but I don't think
Dade County is getting the professional workmanship that it is being charged for. Every one of these
deaths is caused by improper grounding of the frame of the equipment back to the source voltage, not
some useless ground rod!
Some attorney will have the answers and the county insurance will pay, but the sad thing is the county
will not learn from these mistakes!
Mike Holt’s Comment:
Charlie, your right is what you say. It is a common practice for government
agencies to use unqualified persons to do electrical installations without an inspection. To compound
the problem many parts of the United States do not even require the installer to be licensed,
and many states do not required continuing education for those that are licensed.
I hope that one day, people will learn that electricity is very dangerous and the installer, contractor
and inspector must be licensed, and continuing education is a must, not an option. If not, we’ll
continue to kill people with improper and unsafe electrical systems.
Because of the important of this subject, I have posted a graphic on the dangers of
a metal pole that is not properly connected to a low impedance path to the earth at http://mikeholt.com.
Also, the following is an article to be published in EC&M magazine in the next couple of months.
Equipment Grounding Not What You Might Expect!
The concept of equipment grounding is misunderstood by most in the electrical industry
and I’m sure it’s not what you might think. To better understand the concept of “equipment grounding”
we need review the following information contained in the National Electrical Code:
Ground. A conducting connection, whether intentional or accidental, between equipment and the earth,
or to some conducting body that serves in place of the earth [Article 100].
Grounding Conductor. A conductor used to connect equipment to a grounding electrode or electrodes (earth)
Grounding of Electrical Equipment. Conductive materials enclosing electrical conductors or equipment,
or forming part of such equipment, shall be connected to earth so as to limit the voltage to
ground on these materials [Section 250-2(b)].
The above seems simple enough. Connect metal parts of an electrical system to the
earth (grounded) to limit the voltage-to-ground on the metal parts. But where did the voltage come
from and how is the voltage limited?
Metal parts of the electrical system are grounded to the earth to limit the voltage-to-ground (prevent
the destruction of electrical components, as well as electric shock that can occur) from superimposed
voltage from lightning and voltage transients [250-2(b)]. In addition, earth grounding of metal parts
helps in preventing the build-up of static charges on equipment and material.
Author's Comment: Failure to properly ground
communications systems [800-40(b), 810-12(f), 820-40(b) and 830-40(b)] has led to $500 million
dollars of property or equipment damage annually due to lightning, surges, according to insurance
The resistance of the ground determines how effective high-voltage surges can be dissipated into the
earth. The impedance of the earth ground is dependent on the resistance of the electrodes, the termination
resistance, contact resistance of the electrodes to the adjacent earth, and the resistance of the
body of earth surrounding the electrodes (soil resistivity). Most of the earth resistance comes from
the resistivity of the soil, which is greatly impacted by the soils moisture and salt content. The
National Electrical Code DOES NOT required the ground resistance of the grounding electrode to be
measured unless a single ground rod is to be used. When a ground rod is used for equipment grounding,
the resistance can be more than 25 ohms if two ground rods are used.To protect communication systems,
instruments, and sensitive electronic equipment from high-voltage transients, the resistance of the
grounding electrode should be as low as practical. Many installations require the ground resistance
to be below 10 ohms, sometimes as little as 3 ohms or less, and on some rare occasions, below 1 ohm!To
achieve and maintain a low resistive ground, special grounding configurations, design, equipment and
measuring devices must be used.CAUTION: Failure to properly ground the metal parts of the electrical
system to the earth can result in electric shock, fires and the destruction of expensive electronic
equipment from lightning, line surges or other high-voltage transients.
In accordance with the NEC, metal parts of the electrical system are grounded by electrically connecting
the building or structure disconnecting means to an appropriate grounding electrode as identified
in Sections 250-50 or 250-52 [250-24(a) and 250-32(b)(1)].
DANGER: Contrary to the believe of many in the electrical
industry, grounding metal parts of an electrical system to the earth DOES NOT assist in removing dangerous
voltage from line-to-ground faults by opening the circuit overcurrent protection device for the systems
that operate at less than 600 volts!
Additionally, the following is to be published in Power Quality Magazine in Early 2001.
The National Safety Council estimates that approximately 300 people in the United States die each year
as a result of an electric shock from low voltage systems (120 or 277 volt circuits). People become
injured and death occurs when voltage pushes electrons through the human body, particularly through
the heart. Death can occur in less than 1 second, if the touch potential is as little as 50 volts
and the current flow through the body is over 50 milliamperes.
To protect against electric shock, dangerous voltage on metal parts of the electrical system and the
building from a line-to-ground fault must be quickly removed by opening the circuit’s overcurrent
protection device (trip the breaker or blow the fuse). Since death can occur in less than 1 second
from ventricular fibrillation, it is critical that the overcurrent protection device open quickly.
The time it takes for an overcurrent protection device to open (clear the phase-to-ground fault and
remove dangerous voltage) is inversely proportional to the magnitude of the fault current. This means
that the higher the ground-fault current, the less time it will take for the overcurrent device to
open and clear the fault.
Example: A 120 volts, 20 ampere circuit breaker will clear a 120 ampere line-to-ground fault
in approximately 1/100th of a second (impedance of the fault path is 1 ohm, I = E/Z, I
= 120 amperes). But it will take 30 seconds to clear a 40 ampere line-to-ground fault (impedance is
3 ohms, I = E/Z, I = 40 amperes). If the impedance of the fault path was 6 ohms, then only 20 amperes
of fault current would flow (I = E/Z, I = 120 volts/6 ohms) and the circuit breaker would never open
to remove dangerous voltage on the metal parts.
REMINDER: Dangerous voltage from line-to-ground faults cannot be removed by grounding the metal
parts to the earth! This is because the earth is a poor conductor whose resistivity is around one
billion times that of copper [IEEE Std. 142 Section 2.2.8] and it will not permit sufficient fault
current to flow back to the power supply to open the circuit overcurrent protection device from a
line-to-ground fault [250-2(d) and 250-54], see 8-16-99 [ Word
] [ PDF ]
As we can see, the impedance of the fault current path plays a critical and vital
role in removing dangerous voltage from metal parts by opening the circuit overcurrent protection
device in less than 1 second. To open the circuit overcurrent protection device in less than 1 second,
the fault current path must have sufficient low impedance to allow the line-to-ground fault current
to rise to a value of at least 5 times (some books recommend 2x and others 10x) the rating of the
overcurrent protection device.
A low impedance fault current path is created by bonding the metal parts of the
electrical system together [250-90] and to the power supply system grounded (neutral) conductor [250-2(d)]
in accordance with Section 250-24 for service equipment and Section 250-30(b) for separately derived
Author’s Comment: The NEC defines
Bonding as “The permanent joining of metallic parts to form an electrically conductive path that will
ensure electrical continuity and the capacity to conduct safely any current likely to be imposed” [Article
100 and 250-90].
The requirement for creating and maintaining the low impedance fault current path are contained in
the following NEC rules.
Section 250-96(a) General Requirements. All metal parts that serve as the low impedance fault
current path (such as raceways, equipment, and enclosures) must be effectively bonded together to
assure electrical continuity [300-10] and nonconductive coatings such as paint, enamel, tarnish, etc.,
on contact points and surfaces must be removed [250-12]. The standard practice of driving a locknut
tight is considered sufficient in removing paint and other nonconductive finishes to assure proper
CAUTION: For the overcurrent protection device to operate properly (clear the fault), the low impedance
ground fault path must have sufficient capacity to conduct safety any fault current likely to be imposed
on them [110-10 and 250-122]. Reducing washers (donuts) are not listed for this purpose; therefore,
bonding jumpers must be installed around this high impedance path.
Section 250-118 Low Impedance Fault Path Consists of:
(2) Rigid Metal Conduit
(3) Intermediate Metal Conduit
(4) Electrical Metallic Tubing
(6) Flexible Metal Conduit [350-14]**
(8) Liquidtight Flexible Metal Conduit ***
(9) Armored Cable
(10) Mineral Insulated Cable
(11) Metal Clad Cable
(12) Cable Trays
(14) Other metal raceways
* When the low impedance fault current path consists of a conductor, it can be solid, stranded, bare,
covered, or insulated and it is sized in accordance with Section 250-122. In addition, the conductor
must be installed within the same raceway, cable, or trench with the other circuit conductors to insure
a low impedance fault current path [300-3(b), 300-5(i), 300-20(a)].
Example: A 100 ampere disconnect is located 200 feet from the electrical system has a feeder with
No. 3 THHN for the ungrounded conductors and a No. 8 for the low impedance fault current path [250-122].
If a line-to-ground fault occurs at the disconnecting means, the fault current would rise to a value of
approximately 583 amperes (I = E/Z, I = 120 volts/0.206 ohms). Result: The 100 ampere circuit protection
device should open within 1 second, thereby removing dangerous voltage from the metal parts of the electrical
Author’s Comment: When selecting the low impedance
path, consideration must be give to the circuit length. Longer runs might require a larger conductor
to carry fault current and metal raceways might require a conductor in parallel to provide the
low impedance path.
** In non-hazardous locations, if the ground return path of the flex does not exceed 6 feet, and the
circuit conductors are protected by overcurrent protection devices rated 20 ampere or less.
*** In non-hazardous locations, if the ground return path of the liquidtight does not exceed 6 feet
for 3/8 inch and ½ inch liquidtight contains conductors protected by a protection device rated 20
ampere or less, or ¾ inch through 1¼ inch liquidtight contains conductors protected by a 60 ampere
or less protection device.
Section 250-102 Bonding Jumpers. Where bonding jumpers are necessary to maintain the low impedance
ground fault path, they shall comply with the following:
(a) Material. Be of copper or aluminum, or other corrosive-resistant material.
(b) Attachment. Terminate by exothermic welding, listed pressure connectors, listed clamps, or other
listed means and be accessible [250-8].
(d) Size. Sized to Table 250-122 to the largest overcurrent protection device for any circuit contained
in the raceways.
(e) Flexible Raceways. Bonding jumper can be installed outside the raceway if routed with the raceway
and it does not exceed 6 ft in length, Fig. 12-13.