Bonding, Grounding and the NEC Presented by The National Association of Certified Home Inspectors

Bonding, Grounding Bonding, Grounding and the NECand 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 groundinggrounding and bondingbonding in making safe electrical installations

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

www.nfpa.org.


• Section 250-4 establishes new performance requirements which clarify what groundinggrounding and bondingbonding are required to accomplish


Section 250-2 introduces and defines the new terms:

• Ground Fault

• Ground Fault Current Path

• Effective Ground Fault Current Path


• These new definitions are in addition to the two important definitions in Article 100 which apply to Section 250-2


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 importance of groundinggrounding electrical equipment seems to be well understood

• The purpose and intent of bondingbonding to create a low impedance ground fault return path seems to be less understood


• The importance of bonding is best described in the following graphics which review how bonding performs in a typical circuit

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A Typical Circuit …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…...

A Typical Circuit …A Typical Circuit …

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From the transformer to our service …

Path of Current Flow - Path of Current Flow - Normal OperationNormal Operation

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Through the overcurrent 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 resistanceresistance or impedanceimpedance in the circuit will determine the amount of current that will flow in the circuit.

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Things You Can Count Things You Can Count On …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!

How is our circuit protected against overload and short circuit?

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Overload and Short Overload and Short Circuit ConditionsCircuit Conditions

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

15A Circuit Breaker

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Overload and Short Overload and Short Circuit ConditionsCircuit Conditions

Ground Fault ConditionGround 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

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 jeopardyjeopardy?


No … not at all … and why not?

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Because the transformer we’re looking at is not groundedgrounded so there is no no path through the earthpath through the 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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Things You Can Count Things You Can Count On …On …

• No circuit – no current 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

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

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

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Grounding our equipment provides a second path for fault current.

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The first path 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.


Fault Current PathFault 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

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

– Transposed to: I = E / R

– Where: I (current) = E (voltage) / R (resistance)

– And so, I = 120 / 10 = 12A


Only 12 Amps …Only 12 Amps …• Will 12 Amps trip our 15A circuit

breaker?

Absolutely not!

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

With Equipment With Equipment GroundedGrounded

Conclusion …Conclusion …

•Grounding does not protect equipment or personnel from a ground fault!

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

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The Bonding The Bonding ConnectionConnection


• 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 creating an:

• “Effective Ground Fault Current Path”

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 (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


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 ohms.

.3 ohms


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.357 ohms

.3 ohms

.57 ohms

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



• 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 effective ground fault current path allows 168A of fault current to flow and forces the overcurrent 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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Conclusion …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 …

• GroundingGrounding is a connection to earth intended to protect our electrical system from lightning and high voltage


• The overcurrent deviceovercurrent device protects our electrical system from overload and short circuit


• The overcurrent deviceovercurrent device protects our electrical system from a ground fault condition if …..


• Proper bonding …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 Wiring Methods Are CriticalCritical

• 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


• The safety of our electrical system relies on– The wiring method selected, and

– Its proper installation


• 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

Documents

Bonding, Grounding and the NEC Presented by The National Association of Certified Home Inspectors