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THE SUBJECT
GROUNDING
AND THE
NEC
THE SPEAKER
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BOB LUDECKE
Member IAEI
CA Certified Electrician
Lic. Electrical Contractor
CSLB Industry Expert
CSLB Subject Matter Expert
Hands on Electrician Since 1974
SPECIAL THANKS TO
Thomas E. Trainor
Chief Electrical Inspector
City of San Diego, California
Representing IAEI on CMP 7
Member - NFPA
Member - U.L. Electrical Council
ALSO GUILTY OF:
Member of the original NFPA Task Group
on the
Useability of the National Electrical Code
Chaired the sub-committee assigned to review
Article 250 and make recommendations to
improve it’s “useability”
The ‘99 NEC
Completely Reorganized
Exceptions Minimized
And Most Importantly.......
Performance Requirements Added
Prescriptive vs Performance
NEC rules are typically Prescriptive
A Prescriptive Rule tells you what to do
But, what it doesn’t tell you is.....
Why you’re doing it!
Prescriptive vs Performance
A Performance Rule is descriptive
It describes what has to be accomplished
And this description will most generally
Explain the intent of the rule
Why is this Important?
Grounding continues to be a Mystery
Improper Grounding is Commonplace
Proper Grounding is vital for an installation
To protect from Fire and Electrocution
Why is this Important?
It is my considered opinion,
That a better understanding of
The intent of the Grounding Rules
Will lead to better, safer installations
TYPICAL CIRCUIT OPERATION
• Only four things can happen when a circuit
is energized.
• It can operate normally
• There can be an overload
• There can be a short circuit
• There can be a ground fault
HOW DOES GROUNDING FIT IN?
• As long as the circuit is operating normally,
• GROUNDING IS NOT NEEDED
• As long as the circuit is operating normally,
• GROUNDING IS NOT NEEDED
THE “UNGROUNDED” CIRCUIT
T
A circuit consisting of a transformer,
2 - 15A conductors and a light bulb
will operate just fine (Check out the barn)
Grounding is not needed
To make it work or To make it safe
HOW DOES GROUNDING FIT IN?
• Under an overload condition,
• GROUNDING IS NOT NEEDED
• PROTECTION FROM OVERLOAD
IS PROVIDED BY
• THE OVERCURRENT DEVICE
• Note that current is only flowing on the
conductors that we installed to carry current
HOW DOES GROUNDING FIT IN?
• Under a short circuit condition,
• GROUNDING IS NOT NEEDED
• PROTECTION AGAINST SHORT CIRCUIT
IS PROVIDED BY
• THE OVERCURRENT DEVICE
• Again, current is only flowing on the
conductors we installed to carry current
HOW DOES GROUNDING FIT IN?
• Under a ground fault condition,
• GROUNDING IS NOT NEEDED
• PROTECTION AGAINST GROUND FAULT
IS PROVIDED BY
• THE OVERCURRENT DEVICE
• HOWEVER……...
RETURN PATH REQUIRED
• THE OVERCURRENT DEVICE CAN ONLY
PROTECT AGAINST A GROUND FAULT IF,
• THE CIRCUIT IS INSTALLED SO THAT ALL
METAL PARTS ARE BONDED TOGETHER
AND TO THE SERVICE NEUTRAL,
• WHICH CREATES A LOW RESISTANCE
PATH FOR FAULT CURRENT TO RETURN
TO THE SOURCE OF SUPPLY
LETS LOOK AT A TYPICAL CIRCUIT
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100’ of Overhead Distribution Line,
25’ of Service Drop,
25’ of Service Entrance Conductor,
100’ of Branch Circuit Conductors
LETS LOOK AT A TYPICAL CIRCUIT
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Current flows…...
LETS LOOK AT A TYPICAL CIRCUIT
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From the transformer to our Service
PATH OF CURRENT FLOW - NORMAL OPERATION
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Through the Overcurrent Device to our Load
PATH OF CURRENT FLOW - NORMAL OPERATION
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Through the Load returning to the Service
PATH OF CURRENT FLOW - NORMAL OPERATION
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And back to the transformer
PATH OF CURRENT FLOW - NORMAL OPERATION
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What determines the amount of
current that will flow in this circuit?
PATH OF CURRENT FLOW - NORMAL OPERATION
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The Total RESISTANCE or IMPEDANCE
in the circuit will determine the amount of
current that will flow in the circuit
THINGS YOU CAN COUNT ON
• OHMS LAW WORKS
• We can change the code, or
• Hire a different contractor, or
• Use romex instead of EMT, but
• E = I x R still works
OVERLOAD AND SHORT CIRCUIT CONDITIONS
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How is our circuit protected against
overload and short circuit?
OVERLOAD AND SHORT CIRCUIT CONDITIONS
15A Circuit Breaker
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THE OVERCURRENT DEVICE
PROTECTS THIS CIRCUIT FROM BOTH
OVERLOAD AND SHORT CIRCUIT
SUMMARIZING TO THIS POINT
CIRCUIT CONDITION
PROTECTION PROVIDED BY:
GROUNDING? O/C PROT?
NORMAL OPERATION
NO
NO
OVERLOAD CONDITION
NO
YES
SHORT CIRCUIT CONDITION NO
YES
GROUND FAULT CONDITION
So lets talk about a Ground Fault Condition
Which certainly sounds like the one
condition where Grounding would be
important and decide for ourselves whether
Grounding Provides Protection for
Equipment or Personnel under a Ground
Fault Condition
GROUND FAULT CONDITION
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What happens if the hot conductor
comes into contact with our metal box?
GROUND FAULT CONDITION
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And our friend comes along and
touches it?
IS HE IN JEOPARDY?
GROUND FAULT CONDITION
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NO
NOT AT ALL
AND WHY NOT?
GROUND FAULT CONDITION
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Because the transformer we’re looking at
IS NOT GROUNDED
so there is NO PATH THROUGH EARTH
for current to return to the transformer
GROUND FAULT CONDITION
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Yes, that was a “Trick” question
Sorry about that
But the intent was to make a point
THINGS YOU CAN COUNT ON
• NO CIRCUIT - NO CURRENT
• CURRENT DOES NOT FLOW UNLESS THERE IS
A CONTINOUS PATH FROM ONE SIDE OF THE
SOURCE OF SUPPLY TO THE OTHER
• CURRENT CANNOT TRAVEL THROUGH THE
EARTH TO RETURN TO A TRANSFORMER
UNLESS THE TRANSFORMER IS GROUNDED
GROUND FAULT CONDITION
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So our friend in this situation
is perfectly safe
HOWEVER.....
GROUND FAULT CONDITION
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What do we know about utility
company transformers?
GROUND FAULT CONDITION
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THEY’RE GROUNDED
And, with this transformer grounded,
our friend is in serious jeopardy
SO WHY ARE THEY GROUNDED?
• To minimize the damage caused if
lightning strikes their distribution lines, or
• If a 12 KV line drops onto a low voltage
line,
• In addition, grounding the neutral of the
distribution system stabilizes the voltage.
• So, basically for the same reason we
ground services at buildings.
GROUND FAULT CONDITION
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Because utility transformers are grounded,
we need to do something to our equipment
to keep our friend from being electrocuted
GROUND FAULT CONDITION
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Can we protect our friend by grounding
our metal equipment? Lets take a look.
GROUND FAULT CONDITION
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Grounding our equipment provides
a second path for fault current
GROUND FAULT CONDITION
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The first is through our friend to
earth and back to the transformer
GROUND FAULT CONDITION
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The new second path is through our metal
equipment to earth and back to the transformer
FAULT CURRENT PATH
We need to open a 15A Circuit Breaker
as quickly as possible. This will require
a fault current of 60A to 75A.
(4 to 5 times the rating of the breaker)
We can use Ohm’s Law to find out how
much current will flow on our new path.
GROUND FAULT CONDITION
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The voltage is 120V. We need to know the
resistance in this circuit to calculate current
FAULT CURRENT PATH
Assuming a minimum of 5 ohms resistance
through each grounding electrode, we know
there is at least 10 ohms resistance in the
fault path that we created by grounding our
equipment.
FAULT CURRENT PATH
THEREFORE, USING OHM’S LAW:
E = I x R and Transposing, I = E / R
I (current) = E(voltage) / R(resistance)
and so, I = 120 / 10 = 12A
FAULT CURRENT PATH
ONLY 12 AMPS
WILL 12 AMPS TRIP
OUR 15A CIRCUIT BREAKER?
ABSOLUTELY NOT
WITH EQUIPMENT GROUNDED
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So the Overcurrent Device does not open
And we have fried our friend
CONCLUSION
GROUNDING
DOES NOT PROTECT
EQUIPMENT OR PERSONNEL
FROM A GROUND FAULT
THE BONDING CONNECTION
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The vital connection left out of our
discussion until now is the bonding of
metal equipment to the service neutral
THE BONDING CONNECTION
Every piece of conductive metal which is a part
of our system or likely to become energized
Must be connected together by an electrically
continuous metal-to-metal contact or by an
equipment grounding conductor
THE BONDING CONNECTION
These connections create an electrically
continuous, low resistance path from every part
of our system back to the service equipment
At the Service, these connections terminate on
the Neutral Bus
THE BONDING CONNECTION
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These bonding connections let us use
the neutral as a return path for fault current
THE BONDING CONNECTION
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Bonding provides a third path for fault
current to return to the source of supply
FAULT CURRENT PATH
We need to open a 15A Circuit Breaker
as quickly as possible. This will require
a fault current of 60A to 75A.
(4 to 5 times the rating of the breaker)
We can use Ohm’s Law to find out how
much current will flow on our new path.
FAULT CURRENT PATH
The resistance in this path includes
100’ - #2 AL OH Distribution .032
25’ - #4 AL Service Drop
.013
25’ - #2 CU Service Entrance
.005
100’ - #14 CU Branch Circuit
.307
Resistance to the point of fault
.357 ohms
THE BONDING CONNECTION
.357 ohms
.3 ohms
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The resistance from the point of fault
through our metal equipment back to the neutral
is assumed to be the same as the branch circuit wiring
and 100’ of #14 cu has a resistance of .3 ohm
THE BONDING CONNECTION
.357 ohms
.57 ohms
.3 ohms
The total resistance in this path
created by bonding is .714 ohms
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FAULT CURRENT PATH
USING OHM’S LAW:
E = I x R and Transposing, I = E / R
I (current) = E(voltage) / R(resistance)
and so, I = 120 / .714 = 168A
THE BONDING CONNECTION
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The Fault Current Return Path through
the Neutral allows 168A of fault current to
flow and forces the overcurrent device to open
THE BONDING CONNECTION
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THIS PATH DOES NOT RELY ON
GROUNDING AND WORKS EVEN IF
OUR SYSTEM IS NOT GROUNDED
CONCLUSION
THE OVERCURRENT DEVICE PROTECTS
AGAINST GROUND FAULT CONDITIONS
PROVIDED THAT
OUR CIRCUITS HAVE BEEN INSTALLED
SO THAT ALL CONDUCTIVE METALS
ARE BONDED TOGETHER AND TO THE
SERVICE NEUTRAL
IN REVIEW
GROUNDING
IS A CONNECTION TO EARTH
INTENDED TO PROTECT OUR
ELECTRICAL SYSTEM FROM
LIGHTNING AND HIGH VOLTAGE
IN REVIEW
THE OVERCURRENT DEVICE
PROTECTS OUR ELECTRICAL SYSTEM
FROM OVERLOAD AND SHORT CIRCUIT
IN REVIEW
THE OVERCURRENT DEVICE
PROTECTS OUR ELECTRICAL SYSTEM
FROM A GROUND FAULT CONDITION
IF…….
IN REVIEW
PROPER BONDING HAS CREATED
AN ELECTRICALLY CONTINOUS,
LOW RESISTANCE PATH
FOR FAULT CURRENT TO RETURN
TO THE NEUTRAL AT THE SERVICE
SO WHAT’S THE PROBLEM?
WHY DOES
“GROUNDING”
CONTINUE TO BE
A SUBJECT OF
MYSTERY AND CONFUSION?
IN MY OPINION
There are Three Areas where the CODE
has not dealt well with this subject.
THEY ARE:
CONFUSING VOCABULARY
OVER-EMPHASIS ON GROUNDING
ACTUAL MISINFORMATION
VOCABULARY
WHICH TERMS ARE EASIER TO
READ AND UNDERSTAND?
unGROUNDed
HOT
GROUNDed
NEUTRAL
GROUNDing
GROUND
ENOUGH SAID
OVER-EMPHASIS ON GROUNDING
ARTICLE 250 IN THE 1996 NEC IS
DIVIDED INTO TWELVE PARTS
ELEVEN ARE ON GROUNDING
ONLY ONE IS ON BONDING
The Term “Low Impedance Ground Fault
Return Path” is not mentioned at all
ACTUAL MISINFORMATION
IN THE 1990 NEC, THE FINE PRINT
NOTES TOLD US THAT EQUIPMENT
WAS GROUNDED IN ORDER TO
“FACILITATE THE OPERATION OF
OVERCURRENT DEVICES UNDER
FAULT CONDITIONS”
ACTUAL MISINFORMATION
IN THE 1996 NEC, SECTION 250-51
STILL TELLS US THAT THE
“PATH TO GROUND” MUST BE OF
LOW IMPEDANCE IN ORDER TO
“FACILITATE THE OPERATION OF THE
CIRCUIT PROTECTIVE DEVICES”
ACTUAL MISINFORMATION
ARTICLE 100 PROVIDES GOOD
DEFINITIONS OF:
GROUNDING, which is
A CONNECTION TO EARTH, and
BONDING, which is
AN INTERCONNECTION OF PARTS
ACTUAL MISINFORMATION
BUT ARTICLE 250 HAS TENDED TO USE
THE TERM “GROUNDING”
AS IF IT INCLUDED
THE CONNECTION TO EARTH,
THE INTERCONNECTION OF PARTS
and THE FAULT CURRENT PATH
THE 1999 NEC BRINGS US…….
A RESTRUCTURED AND REVISED
ARTICLE 250
Which not only:
Puts requirements in a more logical order,
And reduces the number of exceptions by
changing them to positive text
THE 1999 NEC BRINGS US…….
BUT ALSO ADDS:
PERFORMANCE REQUIREMENTS
FOR GROUNDING and BONDING,
AND THE NEW TERM
“FAULT CURRENT PATH”
THE 1999 NEC BRINGS US…….
AND, THROUGH THESE NEW
PERFORMANCE REQUIREMENTS,
MORE CLEARLY IDENTIFIES WHAT
GROUNDING, BONDING AND
THE FAULT CURRENT PATH
ARE REQUIRED TO ACCOMPLISH
THE 1999 NEC BRINGS US…….
Appendix E which provides cross references
from the “96 to the ‘99 NEC and vice versa
Figure 250-2 which graphically depicts the
relationship of Bonding to the other Parts of
Article 250 to emphasize it’s importance
Part A. General
Part B. Circuit and
system grounding
Part H. Direct-current
systems
Part K. Grounding of
system and circuits of
1kV and over (high voltage)
Part C. Grounding electrode
system and grounding
electrode conductor
Part E. Bonding
Part D. Enclosure,
raceway, and service
cable grounding
Part F. Equipment
grounding and equipment
grounding conductors
Part G. Methods of
equipment grounding
Part J. Instruments,
meters, and relays
Figure 250-2 Grounding
HOWEVER.............
AMENDMENTS TO ARTICLE 250 IN
THE 1999 NEC HAVE BEEN APPROVED
BY CMP-5 AND AT THE NFPA ANNUAL
MEETING IN ANAHEIM
WE CAN BENEFIT FROM THESE
ADOPTED CHANGES WHICH WILL
BE PUBLISHED IN THE 2002 NEC
Help From The 2002 NEC
In a new Section 250-2, the 2002 NEC
adds definitions of technical terms, such as:
Ground Fault
Ground-Fault Current Path
Effective Ground-Fault Current Path
A real help in understanding the intent of
the Grounding and Bonding requirements
Help From The 2002 NEC
Old Section 250-2 becomes 250-4
Which now better explains the differences
between Grounding and Bonding, and
Separates the requirements for Grounded
and Ungrounded Systems
Another plus for user-friendly code
THE 2002 NEC
New Section 250-2 Definitions.
Ground Fault.
An unintentional, electrically conducting
connection between an ungrounded conductor
of an electrical circuit and the normally noncurrent carrying conductors, metallic
enclosures, metallic raceways, metallic
equipment or earth.
THE 2002 NEC
New Section 250-2 Definitions.
Ground-Fault Current Path.
An electrically conductive path from the point
of a ground fault on a wiring system through
normally non-current carrying conductors,
equipment or the earth to the electrical supply
source.
THE 2002 NEC
New Section 250-2 Definitions.
FPN: Examples of ground-fault current paths could
consist of any combination of equipment grounding
conductors, metallic raceways, metallic cable
sheaths, electrical equipment, and any other
electrically conductive material such as metal water
and gas piping, steel framing members, stucco mesh,
metal ducting, reinforcing steel, shields of
communication cables and the earth itself.
THE 2002 NEC
New Section 250-2 Definitions.
Effective Ground-Fault Current Path.
An intentionally constructed, permanent, low
impedance, 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.
THE 2002 NEC
New Section 250-2 Definitions.
FPN: An effective ground-fault current path is created by
effectively bonding together all of the electrically conductive
materials that are likely to be energized by the wiring
system. Effective bonding is accomplished through the use
of equipment grounding conductors, bonding jumpers or
bonding conductors, approved metallic raceways, connectors
and couplings, approved metallic sheathed cable and cable
fittings, and other approved devices. A ground fault path is
effective when it will safely carry the maximum ground fault
current likely to be imposed on it.
THE 2002 NEC
New Section 250-2 Definitions.
An effective ground-fault current path is
created by effectively bonding together all of
the electrically conductive materials that are
likely to be energized by the wiring system.
THE 2002 NEC
New Section 250-2 Definitions.
Effective bonding is accomplished through
the use of equipment grounding conductors,
bonding jumpers or bonding conductors,
approved metallic raceways, connectors and
couplings, approved metallic sheathed cable
and cable fittings, and other approved devices.
THE 2002 NEC
New Section 250-2 Definitions.
A ground fault path is effective when it will
safely carry the maximum ground fault current
likely to be imposed on it.
Help From The 2002 NEC
Old Section 250-2 becomes:
250-4. General Requirements for
Grounding and Bonding
which is divided into:
(A) Grounded Systems, and
(B) Ungrounded Systems
Help From The 2002 NEC
New Section 250-4 (A)
(1) Electrical System Grounding
(2) Grounding of Electrical Equipment
(3) Bonding of Electrical Equipment
(4) Bonding of Electrically Conductive
Materials and Other Equipment
(5) Effective Ground Fault Current Path
Help From The 2002 NEC
New Section 250-4 (A)
(1) Electrical System Grounding
Electrical systems that are grounded shall
be connected to earth in a manner that will
limit the voltage imposed by lightning, line
surges, or unintentional contact with higher
voltage lines and that will stabilize the
voltage to earth during normal operation.
Help From The 2002 NEC
New Section 250-4 (A)
(2) Grounding of Electrical Equipment
Non-current carrying 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.
Help From The 2002 NEC
New Section 250-4 (A)
(3) Bonding of Electrical Equipment
Non-current carrying conductive materials
enclosing electrical conductors or equipment,
or forming part of such equipment, shall be
connected together and to the electrical
supply source in a manner that establishes an
effective ground fault current path.
Help From The 2002 NEC
New Section 250-4 (A)
(4) Bonding of Electrical Conductive
Materials and Other Equipment
Electrically conductive materials that are
likely to become energized shall be connected
together and to the electrical supply source in
a manner that establishes an effective ground
fault current path.
Help From The 2002 NEC
New Section 250-4 (A)
(5) Effective Ground Fault Current Path
Electrical equipment and wiring and other electrically
conductive material likely to become energized shall be
installed in a manner that creates a permanent, low
impedance circuit capable of safely carrying the maximum
ground fault current likely to be imposed on it from any
point on the wiring system where a ground fault may occur
to the electrical supply source.
The earth shall not be used as the sole equipment
grounding conductor or fault current path.
Help From The 2002 NEC
New Section 250-4 (B)
(1) Grounding of Electrical Equipment
(2) Bonding of Electrical Equipment
(3) Bonding of Electrically Conductive
Materials and Other Equipment
(4) Path for Fault Current
Help From The 2002 NEC
New Section 250-4 (B)
(1) Grounding of Electrical Equipment
Non-current carrying conductive materials
enclosing electrical conductors or equipment, or
forming part of such equipment, shall be connected
to earth so as to limit the voltage imposed by
lightning, line surges, or unintentional contact with
higher voltage lines and limit the voltage to ground
on these materials
Help From The 2002 NEC
New Section 250-4 (B)
(2) Bonding of Electrical Equipment
Non-current carrying conductive materials
enclosing electrical conductors or equipment, shall
be connected together and to the supply system
grounded equipment in a manner that creates a
permanent, low impedance path for ground fault
current which is capable of safely carrying the
maximum fault current likely to be imposed on it.
Help From The 2002 NEC
New Section 250-4 (B)
(3) Bonding of Electrical Conductive
Materials and Other Equipment
Electrically conductive materials that are likely to
become energized shall be connected together and to the
supply system grounded equipment in a manner that
creates a permanent, low impedance path for ground
fault current which is capable of safely carrying the
maximum fault current likely to be imposed on it.
Help From The 2002 NEC
New Section 250-4 (B)
(4) Path for Fault Current
Electrical equipment, wiring and other electrically
conductive material likely to become energized shall be
installed in a manner that creates a permanent, low
impedance circuit from any point on the wiring system to
the electrical supply source to facilitate the operation of
overcurrent devices should a second fault occur on the
wiring system.
The earth shall not be used as the sole equipment
grounding conductor or fault current path.
IN CLOSING
THE CHANGES TO ARTICLE 250 IN
THE ‘99 & ‘O2 NEC ARE EXTENSIVE
IT HAS BEEN COMPLETELY
RENUMBERED AND REWORDED
MANY EXCEPTIONS HAVE BEEN
REWRITTEN INTO POSITIVE TEXT
IN CLOSING
SOME RULES, SUCH AS 250-32, HAVE
BEEN COMPLETELY CHANGED
OTHERS HAVE BEEN RELOCATED,
REWRITTEN AND/OR REORGANIZED
THESE CHANGES WILL AFFECT
EVERYONE INVOLVED IN THE
ELECTRICAL INDUSTTRY
THE POSITIVE APPROACH
These changes are a blessing in disguise
We all need to get a fresh start on this
subject and clarify our understanding of
why we ground and bond electrical
systems and how grounding and bonding
are related to safe electrical installations
THE POSITIVE APPROACH
The best way to do that is to work through
Article 250 as if it were totally new
Check that terms are used consistently as
they are defined
Check the rules against what you see in the
field
THE POSITIVE APPROACH
Always question the “why” of the rule
Don’t stop until you really understand it
And how to apply it on the job
THE POSITIVE APPROACH
And, when you review Grounding
Use the wording in the 2002 NEC
It’s the closest we’ve ever gotten to
accurately describing this topic
THE POSITIVE APPROACH
Using the new wording in the 2002 NEC
Will make it easier to show contractors what
Grounding and Bonding have to accomplish
And that’s important because
THE POSITIVE APPROACH
Grounding
Bonding
and the
Ground-Fault Current Path
are critical elements of electrical safety
THANK YOU
FOR YOUR TIME AND ATTENTION