Grounding presentation

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,


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


• Under an overload condition,


• PROTECTION FROM OVERLOAD IS PROVIDED BY• THE OVERCURRENT DEVICE• Note that current is only flowing on the conductors that we installed to

carry current


• Under a short circuit condition,


• PROTECTION AGAINST SHORT CIRCUIT IS PROVIDED BY • THE OVERCURRENT DEVICE

• Again, current is only flowing on the conductors we installed to carry current


• Under a ground fault condition,


• 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


Current flows…...

L O A D


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


Through the Load returning to the Service


And back to the transformer

L O A D


What determines the amount of current that will flow in this circuit?

L O A D


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


What happens if the hot conductor comes into contact with our metal box?

L O A D

L O A D


And our friend comes along and touches it?

IS HE IN JEOPARDY?


NO NOT AT ALL

AND WHY NOT?

L O A D


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


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


So our friend in this situation is perfectly safe

HOWEVER.....

L O A D


What do we know about utility company transformers?

L O A D


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.


Because utility transformers are grounded, we need to do something to our equipment to keep our friend from being electrocuted

L O A D


Can we protect our friend by grounding our metal equipment? Lets take a look.

L O A D


Grounding our equipment provides a second path for fault current

L O A D

L O A D


The first is through our friend to earth and back to the transformer

L O A D


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


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



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


These bonding connections let us use the neutral as a return path for fault current

L O A D


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


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


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 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


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


PROTECTS OUR ELECTRICAL SYSTEM

FROM OVERLOAD AND SHORT CIRCUIT

IN REVIEW


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.


(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.


(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.


(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)



(3) Bonding of Electrically Conductive Materials and Other Equipment

(4) Path for Fault Current



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



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.


(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.


(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.

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Grounding presentation