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

2. 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, www.nfpa.org. www.nfpa.org

3. The New Code … grounding bonding Section 250-4 establishes new performance requirements which clarify what grounding and bonding are required to accomplish

4. The New Code … Section 250-2 introduces and defines the new terms: Ground Fault Ground Fault Current Path Effective Ground Fault Current Path

5. The New Code … These new definitions are in addition to the two important definitions in Article 100 which apply to Section 250-2

6. 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

7. 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

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

9. 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

10. L O A D Current flows…... A Typical Circuit …

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

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

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

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

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

16. 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

17. 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!

18. How is our circuit protected against overload and short circuit? L O A D Overload and Short Circuit Conditions

19. The overcurrent device protects this circuit from both overload and short circuit. 15A Circuit Breaker L O A D Overload and Short Circuit Conditions

20. 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

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

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

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

24. 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

25. Yes, that was a “trick” question … sorry about that … but the intent was to make a point. L O A D Ground Fault Condition

26. 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

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

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

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

30. 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

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

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

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

34. L O A D The new second path is through our metal equipment to earth and back to the transformer. Ground Fault Condition

35. 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

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

37. 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

38. 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

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

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

41. Conclusion … Grounding does not protect equipment or personnel from a ground fault!

42. 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

43. 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

44. 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”

45. These bonding connections let us use the neutral as a return path for fault current. L O A D The Bonding Connection

46. Bonding provides a third path for fault current to return to the source of supply. L O A D The Bonding Connection

47. 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

48. 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

49. 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

50. 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

51. 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

52. 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

53. This path does not rely on grounding and works even if our system is not grounded. L O A D The Bonding Connection

54. 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

55. In Review … Grounding Grounding is a connection to earth intended to protect our electrical system from lightning and high voltage

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

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

58. 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

59. 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

60. Wiring Methods Are Critical The safety of our electrical system relies on –The wiring method selected, and –Its proper installation

61. 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