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

The New Code … grounding bonding 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,

The New Code … grounding bonding Section establishes new performance requirements which clarify what grounding and bonding are required to accomplish

The New Code … Section introduces and defines the new terms: Ground Fault Ground Fault Current Path Effective Ground Fault Current Path

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 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 Code … grounding The importance of grounding electrical equipment seems to be well understood bonding The purpose and intent of bonding to create a low impedance ground fault return path seems to be less understood

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

L O A D 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…... A Typical Circuit …

L O A D From the transformer to our service … Path of Current Flow - Normal Operation

L O A D Through the overcurrent device to our load … Path of Current Flow - Normal Operation

L O A D Through the load returning to the service … Path of Current Flow - Normal Operation

And back to the transformer. L O A D Path of Current Flow - Normal Operation

What determines the amount of current that will flow in this circuit? L O A D Path of Current Flow - Normal Operation

resistanceimpedance The total resistance or impedance in the circuit will determine the amount of current that will flow in the circuit. L O A D Path of Current Flow - Normal Operation

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!

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 Overload and Short Circuit Conditions

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

What happens if the hot conductor comes into contact with our metal box? L O A D Ground Fault Condition

L O A D And our friend comes along and touches it? Is he in jeopardy jeopardy? Ground Fault Condition

No … not at all … and why not? L O A D Ground Fault Condition

Because the transformer we’re looking at is not grounded grounded so there is no path through the earth 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 Ground Fault Condition

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

So our friend in this situation is perfectly safe … however … L O A D Ground Fault Condition

What do we know about utility company transformers? L O A D Ground Fault Condition

They’re grounded … and, with this transformer grounded, our friend is in serious jeopardy. L O A D Ground Fault Condition

Because utility transformers are grounded, we need to do something to our equipment to keep our friend from being electrocuted. L O A D Ground Fault Condition

Can we protect our friend by grounding our metal equipment? Let’s take a look. L O A D Ground Fault Condition

Grounding our equipment provides a second path for fault current. L O A D Ground Fault Condition

L O A D The first path is through our friend to earth and back to the transformer. 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

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

The voltage is 120V. We need to know the resistance in this circuit to calculate current. L O A D Ground Fault Condition

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 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 Path Only 12 Amps … Will 12 Amps trip our 15A circuit breaker? Absolutely not!

L O A D So the overcurrent device does not open and we have fried our friend! With Equipment Grounded

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. L O A D 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 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”

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

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 Path The resistance in this path includes 100’ - #2 AL OH Distribution ’ - #4 AL Service Drop ’ - #2 CU Service Entrance ’ - #14 CU Branch Circuit.307 Resistance to the point of fault.357 ohms

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 ohms..3 ohms The Bonding Connection

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

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

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

In Review … The overcurrent device protects our electrical system from overload and short circuit

In Review … The overcurrent device protects our electrical system from a ground fault condition if …..

In Review … 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 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 Critical The safety of our electrical system relies on –The wiring method selected, and –Its proper installation

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