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  • 1. G R O U N D I N G THE SUBJECT

2. Why is this Important? Grounding continues to be a Mystery Proper Grounding is vital for an installation To protect from Fire and Electrocution Improper Grounding is Commonplace 3. Why is this Important? It is my considered opinion, The intent of the Grounding Rules Will lead to better, safer installations That a better understanding of 4. TYPICAL CIRCUIT OPERATION Only four things can happen when a circuit is energized. It can operate normally There can be an overload There can be a short circuit There can be a ground fault 5. HOW DOES GROUNDING FIT IN? As long as the circuit is operating normally, GROUNDING IS NOT NEEDED As long as the circuit is operating normally, GROUNDING IS NOT NEEDED 6. T A circuit consisting of a transformer, 2 - 15A conductors and a light bulb will operate just fine (Check out the barn) Grounding is not needed THE UNGROUNDED CIRCUIT To make it work or To make it safe 7. HOW DOES GROUNDING FIT IN? Under an overload condition, GROUNDING IS NOT NEEDED PROTECTION FROM OVERLOAD IS PROVIDED BY THE OVERCURRENT DEVICE Note that current is only flowing on the conductors that we installed to carry current 8. HOW DOES GROUNDING FIT IN? Under a short circuit condition, GROUNDING IS NOT NEEDED PROTECTION AGAINST SHORT CIRCUIT IS PROVIDED BY THE OVERCURRENT DEVICE Again, current is only flowing on the conductors we installed to carry current 9. HOW DOES GROUNDING FIT IN? Under a ground fault condition, GROUNDING IS NOT NEEDED PROTECTION AGAINST GROUND FAULT IS PROVIDED BY THE OVERCURRENT DEVICE HOWEVER... 10. RETURN PATH REQUIRED THE OVERCURRENT DEVICE CAN ONLY PROTECT AGAINST A GROUND FAULT IF, THE CIRCUIT IS INSTALLED SO THAT ALL METAL PARTS ARE BONDED TOGETHER AND TO THE SERVICE NEUTRAL, WHICH CREATES A LOW RESISTANCE PATH FOR FAULT CURRENT TO RETURN TO THE SOURCE OF SUPPLY 11. 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 12. L O A D LETS LOOK AT A TYPICAL CIRCUIT Current flows... 13. L O A D LETS LOOK AT A TYPICAL CIRCUIT From the transformer to our Service 14. L O A D PATH OF CURRENT FLOW - NORMAL OPERATION Through the Overcurrent Device to our Load 15. L O A D PATH OF CURRENT FLOW - NORMAL OPERATION Through the Load returning to the Service 16. PATH OF CURRENT FLOW - NORMAL OPERATION And back to the transformer L O A D 17. PATH OF CURRENT FLOW - NORMAL OPERATION What determines the amount of current that will flow in this circuit? L O A D 18. PATH OF CURRENT FLOW - NORMAL OPERATION The Total RESISTANCE or IMPEDANCE in the circuit will determine the amount of current that will flow in the circuit L O A D 19. THINGS YOU CAN COUNT ON OHMS 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 20. OVERLOAD AND SHORT CIRCUIT CONDITIONS How is our circuit protected against overload and short circuit? L O A D 21. OVERLOAD AND SHORT CIRCUIT CONDITIONS THE OVERCURRENT DEVICE PROTECTS THIS CIRCUIT FROM BOTH OVERLOAD AND SHORT CIRCUIT 15A Circuit Breaker L O A D 22. SUMMARIZING TO THIS POINT CIRCUIT CONDITION PROTECTION PROVIDED BY: GROUNDING? O/C PROT? NORMAL OPERATION NO NO OVERLOAD CONDITION NO YES SHORT CIRCUIT CONDITION NO YES 23. So lets 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 CONDITION 24. GROUND FAULT CONDITION What happens if the hot conductor comes into contact with our metal box? L O A D 25. L O A D GROUND FAULT CONDITION And our friend comes along and touches it? IS HE IN JEOPARDY? 26. GROUND FAULT CONDITION NO NOT AT ALL AND WHY NOT? L O A D 27. GROUND FAULT CONDITION Because the transformer were looking at IS NOT GROUNDED so there is NO PATH THROUGH EARTH for current to return to the transformer L O A D 28. GROUND FAULT CONDITION Yes, that was a Trick question Sorry about that But the intent was to make a point L O A D 29. THINGS YOU CAN COUNT ON NO CIRCUIT - NO CURRENT CURRENT DOES NOT FLOW UNLESS THERE IS A CONTINOUS 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 30. GROUND FAULT CONDITION So our friend in this situation is perfectly safe HOWEVER..... L O A D 31. GROUND FAULT CONDITION What do we know about utility company transformers? L O A D 32. GROUND FAULT CONDITION THEYRE GROUNDED And, with this transformer grounded, our friend is in serious jeopardy L O A D 33. SO WHY ARE THEY 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. 34. 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 35. GROUND FAULT CONDITION Can we protect our friend by grounding our metal equipment? Lets take a look. L O A D 36. GROUND FAULT CONDITION Grounding our equipment provides a second path for fault current L O A D 37. L O A D GROUND FAULT CONDITION The first is through our friend to earth and back to the transformer 38. L O A D GROUND FAULT CONDITION The new second path is through our metal equipment to earth and back to the transformer 39. 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 Ohms Law to find out how much current will flow on our new path. FAULT CURRENT PATH 40. GROUND FAULT CONDITION The voltage is 120V. We need to know the resistance in this circuit to calculate current L O A D 41. 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 42. THEREFORE, USING OHMS LAW: FAULT CURRENT PATH E = I x R and Transposing, I = E / R I (current) = E(voltage) / R(resistance) and so, I = 120 / 10 = 12A 43. ONLY FAULT CURRENT PATH 12 AMPS WILL 12 AMPS TRIP OUR 15A CIRCUIT BREAKER? ABSOLUTELY NOT 44. WITH EQUIPMENT GROUNDED L O A D So the Overcurrent Device does not open And we have fried our friend 45. GROUNDING CONCLUSION DOES NOT PROTECT EQUIPMENT OR PERSONNEL FROM A GROUND FAULT 46. 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 47. 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 48. 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 49. THE BONDING CONNECTION These bonding connections let us use the neutral as a return path for fault current L O A D 50. THE BONDING CONNECTION Bonding provides a third path for fault current to return to the source of supply L O A D 51. 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 Ohms Law to find out how much current will flow on our new path. FAULT CURRENT PATH 52. 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 FAULT CURRENT PATH 53. 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 54. 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 55. FAULT CURRENT PATH USING OHMS LAW: E = I x R and Transposing, I = E / R I (current) = E(voltage) / R(resistance) and so, I = 120 / .714 = 168A 56. THE BONDING CONNECTION The Fault Current Return Path through the Neutral allows 168A of fault current to flow and forces the overcurrent device to open L O A D 57. THE BONDING CONNECTION THIS PATH DOES NOT RELY ON GROUNDING AND WORKS EVEN IF OUR SYSTEM IS NOT GROUNDED L O A D 58. 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 59. IN REVIEW GROUNDING IS A CONNECTION TO EARTH INTENDED TO PROTECT OUR ELECTRICAL SYSTEM FROM LIGHTNING AND HIGH VOLTAGE 60. IN REVIEW THE OVERCURRENT DEVICE PROTECTS OUR ELECTRICAL SYSTEM FROM OVERLOAD AND SHORT CIRCUIT 61. IN REVIEW THE OVERCURRENT DEVICE PROTECTS OUR ELECTRICAL SYSTEM FROM A GROUND FAULT CONDITION IF. 62. IN REVIEW PROPER BONDING HAS CREATED AN ELECTRICALLY CONTINOUS, LOW RESISTANCE PATH FOR FAULT CURRENT TO RETURN TO THE NEUTRAL AT THE SERVICE 63. SO WHATS THE PROBLEM? WHY DOES GROUNDING CONTINUE TO BE A SUBJECT OF MYSTERY AND CONFUSION? 64. Help From The 2002 NEC New Section 250-4 (A) (2) Grounding of Electrical Equipment Non-current carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connecte