G R O U N D I N G
THE SUBJECT
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
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
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
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
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
L O A D
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
L O A D
LETS LOOK AT A TYPICAL CIRCUIT
Current flows…...
L O A D
LETS LOOK AT A TYPICAL CIRCUIT
From the transformer to our Service
L O A D
PATH OF CURRENT FLOW - NORMAL OPERATION
Through the Overcurrent Device to our Load
L O A D
PATH OF CURRENT FLOW - NORMAL OPERATION
Through the Load returning to the Service
PATH OF CURRENT FLOW - NORMAL OPERATION
And back to the transformer
L O A D
PATH OF CURRENT FLOW - NORMAL OPERATION
What determines the amount of current that will flow in this circuit?
L O A D
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
L O A D
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
How is our circuit protected against overload and short circuit?
L O A D
OVERLOAD AND SHORT CIRCUIT CONDITIONS
THE OVERCURRENT DEVICE
PROTECTS THIS CIRCUIT FROM BOTH
OVERLOAD AND SHORT CIRCUIT
15A Circuit Breaker
L O A D
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
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
GROUND FAULT CONDITION
What happens if the hot conductor comes into contact with our metal box?
L O A D
L O A D
GROUND FAULT CONDITION
And our friend comes along and touches it?
IS HE IN JEOPARDY?
GROUND FAULT CONDITION
NO NOT AT ALL
AND WHY NOT?
L O A D
GROUND FAULT CONDITION
Because the transformer we’re looking atIS NOT GROUNDED so there is NO PATH THROUGH EARTH for current to return to the transformer
L O A D
GROUND FAULT CONDITION
Yes, that was a “Trick” question
Sorry about that
But the intent was to make a point
L O A D
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
So our friend in this situation is perfectly safe
HOWEVER.....
L O A D
GROUND FAULT CONDITION
What do we know about utility company transformers?
L O A D
GROUND FAULT CONDITION
THEY’RE GROUNDED
And, with this transformer grounded, our friend is in serious jeopardy
L O A D
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
Because utility transformers are grounded, we need to do something to our equipment to keep our friend from being electrocuted
L O A D
GROUND FAULT CONDITION
Can we protect our friend by grounding our metal equipment? Lets take a look.
L O A D
GROUND FAULT CONDITION
Grounding our equipment provides a second path for fault current
L O A D
L O A D
GROUND FAULT CONDITION
The first is through our friend to earth and back to the transformer
L O A D
GROUND FAULT CONDITION
The new second path is through our metal equipment to earth and back to the transformer
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
GROUND FAULT CONDITION
The voltage is 120V. We need to know the resistance in this circuit to calculate current
L O A D
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:
FAULT CURRENT PATH
E = I x R and Transposing, I = E / R
I (current) = E(voltage) / R(resistance)
and so, I = 120 / 10 = 12A
ONLY
FAULT CURRENT PATH
12 AMPS WILL 12 AMPS TRIP OUR 15A CIRCUIT BREAKER?
ABSOLUTELY NOT
WITH EQUIPMENT GROUNDED
L O A D
So the Overcurrent Device does not open
And we have fried our friend
GROUNDING
CONCLUSION
DOES NOT PROTECT
EQUIPMENT OR PERSONNEL
FROM A GROUND FAULT
THE BONDING CONNECTION
The vital connection left out of our discussion until now is the bonding of metal equipment to the service neutral
L O A D
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
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
These bonding connections let us use the neutral as a return path for fault current
L O A D
THE BONDING CONNECTION
Bonding provides a third path for fault current to return to the source of supply
L O A D
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
FAULT CURRENT PATH
THE BONDING CONNECTION
L O A D
.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
THE BONDING CONNECTION
L O A D
.357 ohms
.3 ohms
.57 ohms
The total resistance in this path created by bonding is .714 ohms
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
The Fault Current Return Path through the Neutral allows 168A of fault current to flow and forces the overcurrent device to open
L O A D
THE BONDING CONNECTION
THIS PATH DOES NOT RELY ON GROUNDING AND WORKS EVEN IF OUR SYSTEM IS NOT GROUNDED
L O A D
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?
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.