2. Why is this Important?
Grounding continues to be a Mystery
Proper Grounding is vital for an installation
To protect from Fire and Electrocution
Improper Grounding is Commonplace
3. Why is this Important?
It is my considered opinion,
The intent of the Grounding Rules
Will lead to better, safer installations
That a better understanding of
4. 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
5. 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
6. 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
THE “UNGROUNDED” CIRCUIT
To make it work or To make it safe
7. 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
8. 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
9. 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……...
10. 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
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LETS LOOK AT A TYPICAL CIRCUIT
100’ of Overhead Distribution Line,
25’ of Service Drop,
25’ of Service Entrance Conductor,
100’ of Branch Circuit Conductors
16. PATH OF CURRENT FLOW - NORMAL OPERATION
And back to the transformer
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17. PATH OF CURRENT FLOW - NORMAL OPERATION
What determines the amount of
current that will flow in this circuit?
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18. PATH OF CURRENT FLOW - NORMAL OPERATION
The Total RESISTANCE or IMPEDANCE
in the circuit will determine the amount of
current that will flow in the circuit
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19. 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
20. OVERLOAD AND SHORT CIRCUIT CONDITIONS
How is our circuit protected against
overload and short circuit?
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21. OVERLOAD AND SHORT CIRCUIT CONDITIONS
THE OVERCURRENT DEVICE
PROTECTS THIS CIRCUIT FROM BOTH
OVERLOAD AND SHORT CIRCUIT
15A Circuit Breaker
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22. 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
23. 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
27. GROUND FAULT CONDITION
Because the transformer we’re looking at
IS NOT GROUNDED
so there is NO PATH THROUGH EARTH
for current to return to the transformer
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28. GROUND FAULT CONDITION
Yes, that was a “Trick” question
Sorry about that
But the intent was to make a point
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29. 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
33. 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.
34. GROUND FAULT CONDITION
Because utility transformers are grounded,
we need to do something to our equipment
to keep our friend from being electrocuted
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35. GROUND FAULT CONDITION
Can we protect our friend by grounding
our metal equipment? Lets take a look.
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39. 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
40. GROUND FAULT CONDITION
The voltage is 120V. We need to know the
resistance in this circuit to calculate current
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41. 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
42. THEREFORE, USING OHM’S LAW:
FAULT CURRENT PATH
E = I x R and Transposing, I = E / R
I (current) = E(voltage) / R(resistance)
and so, I = 120 / 10 = 12A
46. THE BONDING CONNECTION
The vital connection left out of our
discussion until now is the bonding of
metal equipment to the service neutral
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47. 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
48. 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
49. THE BONDING CONNECTION
These bonding connections let us use
the neutral as a return path for fault current
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51. 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
52. 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
FAULT CURRENT PATH
53. THE BONDING CONNECTION
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.357 ohms
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
.3 ohms
55. 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
56. THE BONDING CONNECTION
The Fault Current Return Path through
the Neutral allows 168A of fault current to
flow and forces the overcurrent device to open
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57. THE BONDING CONNECTION
THIS PATH DOES NOT RELY ON
GROUNDING AND WORKS EVEN IF
OUR SYSTEM IS NOT GROUNDED
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58. 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
59. IN REVIEW
GROUNDING
IS A CONNECTION TO EARTH
INTENDED TO PROTECT OUR
ELECTRICAL SYSTEM FROM
LIGHTNING AND HIGH VOLTAGE
62. IN REVIEW
PROPER BONDING HAS CREATED
AN ELECTRICALLY CONTINOUS,
LOW RESISTANCE PATH
FOR FAULT CURRENT TO RETURN
TO THE NEUTRAL AT THE SERVICE
63. SO WHAT’S THE PROBLEM?
WHY DOES
“GROUNDING”
CONTINUE TO BE
A SUBJECT OF
MYSTERY AND CONFUSION?
64. 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.
65. 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.
66. 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.
67. 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.
68. 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
69. 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
70. 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.
71. 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.
72. 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.
