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Bonding, Grounding and the NEC . Presented by The National Association of Certified Home Inspectors. www.NACHI.org. The New Code ….

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Bonding, Grounding and the NEC

Presented by The National Association of Certified Home Inspectors

www.NACHI.org

the new code
The New Code …
  • The 1999 and 2002 editions of the NEC* have now clearly defined the separate and vitally important purposes of grounding and bonding in making safe electrical installations

*The National Electrical Code (NEC) is a registered trademark of the National Fire Protection Association, www.nfpa.org.

the new code1
The New Code …
  • Section 250-4 establishes new performance requirements which clarify what grounding and bonding are required to accomplish
the new code2
The New Code …

Section 250-2 introduces and defines the new terms:

  • Ground Fault
  • Ground Fault Current Path
  • Effective Ground Fault Current Path
the new code3
The New Code …
  • These new definitions are in addition to the two important definitions in Article 100 which apply to Section 250-2
the new code4
The New Code …

These definitions are:

  • Grounded - Connected to earth
  • Bonded - The permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct safely any current likely to be imposed
the new code5
The New Code …
  • The importance of grounding electrical equipment seems to be well understood
  • The purpose and intent of bonding to create a low impedance ground fault return path seems to be less understood
the new code6
The New Code …
  • The importance of bonding is best described in the following graphics which review how bonding performs in a typical circuit
a typical circuit
A Typical Circuit …

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  • 100’ of Overhead Distribution Line
  • 25’ of Service Drop
  • 25’ of Service Entrance Conductor
  • 100’ of Branch Circuit Conductors
a typical circuit1
A Typical Circuit …

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Current flows…...

path of current flow normal operation
Path of Current Flow - Normal Operation

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From the transformer to our service …

path of current flow normal operation1
Path of Current Flow - Normal Operation

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Through the overcurrent device to our load …

path of current flow normal operation2
Path of Current Flow - Normal Operation

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Through the load returning to the service …

path of current flow normal operation3
Path of Current Flow - Normal Operation

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And back to the transformer.

path of current flow normal operation4
Path of Current Flow - Normal Operation

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

path of current flow normal operation5
Path of Current Flow - Normal Operation

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The total resistance or impedance in the circuit will determine the amount of current that will flow in the circuit.

things you can count on
Things You Can Count On …
  • Ohm’s 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
Overload and Short Circuit Conditions

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How is our circuit protected against overload and short circuit?

overload and short circuit conditions1
Overload and Short Circuit Conditions

15A Circuit Breaker

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The overcurrent device protects this circuit from both overload and short circuit.

ground fault condition
Ground Fault Condition
  • So let’s 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 condition1
Ground Fault Condition

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

ground fault condition2
Ground Fault Condition

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And our friend comes along and touches it? Is he in jeopardy?

ground fault condition3
Ground Fault Condition

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No … not at all … and why not?

ground fault condition4
Ground Fault Condition

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Because the transformer we’re looking at is notgrounded so there is no path through the earth for current to return to the transformer.

ground fault condition5
Ground Fault Condition

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Yes, that was a “trick” question … sorry about that … but the intent was to make a point.

things you can count on1
Things You Can Count On …
  • No circuit – no current
  • Current does not flow unless there is a continuous 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 condition6
Ground Fault Condition

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So our friend in this situation is perfectly safe … however …

ground fault condition7
Ground Fault Condition

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What do we know about utility company transformers?

ground fault condition8
Ground Fault Condition

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They’re grounded … and, with this transformer grounded, our friend is in serious jeopardy.

ground fault condition9
Ground Fault Condition

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

ground fault condition10
Ground Fault Condition

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Can we protect our friend by grounding our metal equipment? Let’s take a look.

ground fault condition11
Ground Fault Condition

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Grounding our equipment provides a second path for fault current.

ground fault condition12
Ground Fault Condition

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

ground fault condition13
Ground Fault Condition

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The new second path is through our metal equipment to earth and back to the transformer.

fault current path
Fault Current Path
  • We need to open a 15A Circuit Breaker as quickly as possible. This will require a fault current of 60A to 75A (four to five times the rating of the breaker)
  • We can use Ohm’s Law to find out how much current will flow on our new path
ground fault condition14
Ground Fault Condition

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

fault current path1
Fault Current Path
  • 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 path2
Fault Current Path
  • Therefore, using Ohm’s Law:
    • E = I x R
    • Transposed to: I = E / R
    • Where: I (current) = E (voltage) / R (resistance)
    • And so, I = 120 / 10 = 12A
fault current path3
Fault Current Path

Only 12 Amps …

  • Will 12 Amps trip our 15A circuit breaker?

Absolutely not!

with equipment grounded
With Equipment Grounded

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So the overcurrent device does not open and we have fried our friend!

conclusion
Conclusion …
  • Grounding does not protect equipment or personnel from a ground fault!
the bonding connection
The Bonding Connection

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The vital connection left out of our discussion until now is the bonding of metal equipment to the service neutral.

the bonding connection1
The Bonding Connection
  • 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 connection2
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 creating an:
  • “Effective Ground Fault Current Path”
the bonding connection3
The Bonding Connection

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

the bonding connection4
The Bonding Connection

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

fault current path4
Fault Current Path
  • We need to open a 15A circuit breaker as quickly as possible. This will require a fault current of 60A to 75A (four to five 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 path5
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

the bonding connection5
The Bonding Connection

.357 ohms

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.3 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 ohms.

the bonding connection6
The Bonding Connection

.357 ohms

.57 ohms

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.3 ohms

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

fault current path6
Fault Current Path
  • Therefore, using Ohm’s Law:
    • E = I x R
    • Transposed to: I = E / R
    • Where: I (current) = E (voltage) / R (resistance)
    • And so, I = 120 / .714 = 168A
the bonding connection7
The Bonding Connection

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The effective ground fault current path allows 168A of fault current to flow and forces the overcurrent device to open.

the bonding connection8
The Bonding Connection

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This path does not rely on grounding and works even if our system is not grounded.

conclusion1
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
In Review …
  • Grounding is a connection to earth intended to protect our electrical system from lightning and high voltage
in review1
In Review …
  • The overcurrent device protects our electrical system from overload and short circuit
in review2
In Review …
  • The overcurrent device protects our electrical system from a ground fault condition if …..
in review3
In Review …
  • Proper bonding …
  • Has created a permanent, electrically continuous and low impedance path
  • Which allows fault current to return to the neutral at the service
wiring methods are critical
Wiring Methods Are Critical
  • Clearly, an effective ground-fault current path must be created throughout our wiring system
  • This is accomplished through the proper installation of a listed wiring method
wiring methods are critical1
Wiring Methods Are Critical
  • The safety of our electrical system relies on
    • The wiring method selected, and
    • Its proper installation
wiring methods are critical2
Wiring Methods Are Critical
  • A wiring method that has been specifically designed as an equipment grounding conductor, and
  • Engineered to assure a low impedance fault current path, is
  • Clearly the best choice for a safe electrical installation
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