grounding, bonding, and ground fault currents why all the confusion?
TRANSCRIPT
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.
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
Grounding is not needed
Protection from the overload is provided by the Over Current Device (circuit breaker)
Under a short circuit situation
Grounding is not needed
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
Grounding is not needed
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
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
THIS WOULD BE THE PATH OF CURRENT FLOW UNDER NORMAL OPERATION
Through the Over Current Device to our Load
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 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?
L O A
D
OVERLOAD AND SHORT CIRCUIT CONDITIONS
THE OVER CURRENT DEVICE
PROTECTS THIS CIRCUIT FROM BOTH
OVERLOAD AND SHORT CIRCUIT
15A Circuit Breaker
L O A
D
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?
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
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
GROUND FAULT CONDITION
THEY’RE GROUNDED
And, with this transformer grounded, our friend
is in serious jeopardy
L O A
D
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.
GROUND FAULT CONDITION
Because utility transformers are grounded, we need to do something to our equipment to keep our
friend from being shocked or electrocuted.
L O A
D
Can we protect our friend by grounding our metal equipment? Lets take a look.
L O A
D
GROUND FAULT CONDITION
L O A
D
The new second path is through our metal equipment to earth and back to the
transformer
GROUND FAULT CONDITION
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
L O A
D
GROUND FAULT CONDITION
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
WITH EQUIPMENT GROUNDED
L O A
D
So the Overcurrent Device does not open
And we have fried our friend
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
Must be connected together by an electrically continuous metal-to-metal contact or by an
equipment grounding conductor
THE BONDING CONNECTION
Every piece of conductive metal which is a part of our system or likely to become energized
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
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
THE BONDING CONNECTION
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
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
L O A
D
.357 ohms
.3 ohms
.57 ohms
The total resistance in this path created by bonding is .714 ohms
THE BONDING CONNECTION
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
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
THE BONDING CONNECTION
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
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