Grounding, bonding, and ground fault currents Why all the confusion?

Grounding, bonding, and ground fault currents

Why all the confusion?

Why learn about Grounding?

Grounding continues to be a Mystery

Proper Grounding is vital for a proper installation

To protect from fire, shock, & even death

Improper Grounding is Commonplace

TYPICAL CIRCUIT OPERATION

It can operate normally

There can be an overload

There can be a short circuit

There can be a ground fault

Only four things can happen when a circuit is energized.

HOW DOES GROUNDING FIT IN? As long as the circuit is operating NORMALLYGROUNDING IS NOT NEEDED

T

A circuit consisting of a transformer, two 14 AWG wires and a light bulb

will operate just fine (check out the barn)

THE “UNGROUNDED” CIRCUIT

To make it workor to make it safe

Grounding is not needed

SO, HOW DOES GROUNDING FIT IN?

Note that current is only flowing on the conductors that we installed to carry the current

Under an overload situation


Protection from the overload is provided by the Over Current Device (circuit breaker)

Under a short circuit situation


Again, protection from the short circuit is provided by the Over Current Device

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

However

Under a ground fault situation


Once again, protection from the ground fault is provided by the Over Current Device

THE OVERCURRENT DEVICE CAN ONLY PROTECT AGAINST A GROUND FAULT IF,

THE CIRCUIT IS INSTALLED SO THAT ALL METAL PARTS ARE BONDED TOGETHER 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

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Current flows…...

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From the transformer to our Service

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THIS WOULD BE THE PATH OF CURRENT FLOW UNDER NORMAL OPERATION

Through the Over Current Device to our Load

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Through the Load returning to the Service

And back to the transformer

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What determines the amount of current that will flow in this

circuit?

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The Total RESISTANCE or IMPEDANCE in the circuit will determine the amount of current

that will flow in the circuit

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THINGS YOU CAN COUNT ON

OHMS LAW WORKSWe 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?

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OVERLOAD AND SHORT CIRCUIT CONDITIONS

THE OVER CURRENT DEVICE

PROTECTS THIS CIRCUIT FROM BOTH

OVERLOAD AND SHORT CIRCUIT

15A Circuit Breaker

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SUMMARY

CIRCUIT CONDITION PROTECTION PROVIDED BY:

Grounding? Cir Breaker

NORMAL OPERATION NO NO

OVERLOAD CONDITION NO YES

SHORT CIRCUIT CONDITION NO YES

GROUND FAULT CONDITION Let’s talk

So lets talk about a Ground Fault Condition

Which certainly sounds like the one condition where Grounding would be important and decide for ourselves

whetherGrounding Provides Protection for

Equipment or Personnel under a Ground Fault Condition

THE GROUND FAULT CONDITION

GROUND FAULT CONDITION

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

box?

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And our friend comes along and touches it?

IS HE IN JEOPARDY?


NO

NOT AT ALL

AND WHY NOT?

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Because the transformer we’re looking atIS NOT GROUNDED

so there is NO PATH THROUGH EARTH for current to return to the transformer

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Yes, that was a “Trick” question

Sorry about that

But the intent was to make a point

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No Circuit - No Current

CURRENT CANNOT TRAVEL THROUGH THE EARTH TO RETURN TO A TRANSFORMER UNLESS THE TRANSFORMER IS GROUNDED

CURRENT DOES NOT FLOW UNLESS THERE IS A CONTINUOUS PATH FROM ONE SIDE OF THE SOURCE OF SUPPLY TO THE OTHER


So our friend in this situation is perfectly safe

HOWEVER.....

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What do we know about utility company transformers?

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THEY’RE GROUNDED

And, with this transformer grounded, our friend

is in serious jeopardy

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Why are Transformers 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 shocked or electrocuted.

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

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Grounding our equipment provides a second path for fault

current

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

transformer


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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.

The voltage is 120V. We need to know the resistance in this circuit to calculate current

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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.

THEREFORE, USING OHM’S LAW:E = I x R and Transposing, I = E /

RI (current) = E(voltage) /

R(resistance)

I = 120 / 10 = 12A

ONLY 12 AMPSWILL 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

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

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Must be connected together by an electrically continuous metal-to-metal contact or by an

equipment grounding conductor


Every piece of conductive metal which is a part of our system or likely to become energized


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

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Bonding provides a third path for fault current to return to the source of supply

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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.

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


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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

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.357 ohms

.3 ohms

.57 ohms

The total resistance in this path created by bonding is .714 ohms


USING OHM’S LAW:

E = I x R and Transposing, I = E / R

I (current) = E(voltage) / R(resistance)

I = 120 / .714 = 168A

The Fault Current Return Path through the Neutral allows 168A of fault current to flow and forces the Over Current device to open

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THIS PATH DOES NOT RELY ON GROUNDING AND WORKS EVEN IF OUR SYSTEM IS NOT GROUNDED

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CONCLUSIONTHE 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

THE OVERCURRENT DEVICEPROTECTS OUR ELECTRICAL SYSTEM

FROM OVERLOAD AND SHORT CIRCUIT

THE OVERCURRENT DEVICEPROTECTS OUR ELECTRICAL SYSTEM

FROM A GROUND FAULT CONDITION

IF…….

PROPER BONDING HAS CREATED

AN ELECTRICALLY CONTINUOUS,

LOW RESISTANCE PATH

FOR FAULT CURRENT TO RETURN

TO THE NEUTRAL AT THE SERVICE

VOCABULARY

WHICH TERMS ARE EASIER TO READ AND UNDERSTAND?

unGROUNDed

GROUNDed

GROUNDing

HOT

NEUTRAL

GROUND

ENOUGH SAID

Grounding, Bonding and the Ground-Fault Current Path are critical elements

of electrical safety

THANK YOU

http://Electrical-Works.com

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Grounding, bonding, and ground fault currents Why all the confusion?

Documents

neededa circuit

short circuit conditionsthe

short circuit conditionshow

d current flows

d ground fault conditionand

d overload

branch circuit conductors

current deviceprotects