2. Chapter 4 Electrical Systems and Protection Chapter 4 Electrical Systems and Protection

3. Unit 14 The Electrical System Unit 14 The Electrical System

4. Knowing how the electricity gets from the generating station to your service entrance gives you a better sense of "the big picture" in your electrical work.

5. 14.1 Current Flow Electrons leaving a power supply are always trying to return to the same power supply; they are not trying to go into the earth.

6. Transformer definition an electrical device by which alternating current of one voltage is changed to another voltage

7. 14.1 Current Flow When ac is applied to the primary of a transformer, it induces voltage in the secondary, which causes electrons to travel through the secondary circuit. Fig 14–1

8. Figure 14-1

9. 14.2 Utility Neutral Current Path Multipoint Neutral Ground. The electric utility grounds the primary and secondary neutral conductor to the earth at multiple locations for the purpose of reducing the ac resistance of the return current path.

10. Grounding Since the neutral point of an electrical supply system is often connected to earth ground, ground and neutral are closely related. Under certain conditions, a conductor used to connect to a system neutral is also used for grounding (earthing) of equipment and structures. Where a neutral conductor is used also to connect equipment enclosures to earth, care must be taken that the neutral conductor never rises to a high voltage with respect to local ground.

11. 14.2 Utility Neutral Current Path Multipoint Neutral Ground. The multipoint grounded neutral is intended to reduce the primary neutral voltage drop, assist in clearing line-to-neutral faults, and reduce elevated voltage caused by line-to-ground faults. Fig 14–2

12. Figure 14-2

13. 14.3 Utility Ground-Fault Current Path Metal parts of the electric utility equipment are bonded to the utility neutral (which is grounded to the earth) to provide a low-impedance path to the power source to assist in clearing a line- to-case fault.

14. 14.3 Utility Ground-Fault Current Path The earth generally has low enough impedance to permit sufficient fault current to return to the source, thereby opening the utility’s circuit protection device. Fig 14–3

15. Figure 14-3

16. 14.4 Premises Neutral Current Path Single Point Neutral Bond. To prevent fires and electric shock, the NEC specifies that neutral current must only flow in the insulated grounded (neutral) conductor. Fig 14–4

17. Figure 14-4

18. 14.5 Premises Ground- Fault Current Path Single Point Neutral Bond. Metal parts of premises wiring must be bonded to a low-impedance path designed so that the circuit protection device will quickly open and clear a ground fault. Fig 14–5

19. Figure 14-5

20. 14.5 Premises Ground- Fault Current Path Earth as Ground-Fault Path. Because of the earth's high resistance to current flow, it cannot be used for the purpose of clearing a line-to-case ground fault. Fig 14–6

21. Figure 14-6

22. 14.6 Utility High-Voltage Transmission Lines The conductors used for high-voltage transmission lines have relatively low resistance, but because of their length, the total resistance can cause significant conductor voltage drop and power losses.

23. 14.7 Conductor Voltage Drop Conductor voltage drop is directly proportional to the length of the conductor. Fig 14–7

24. Figure 14-7

25. 14.8 Conductor Power Loss Conductor power losses are directly proportional to conductor length and the square of the current: P = I 2 R. Fig 14–8

26. Figure 14-8

27. 14.9 Reducing Voltage Drop and Power Loss The most effective way to reduce conductor voltage drop and power loss is to lower the current flowing through the conductors. This is accomplished by increasing the transmission voltage.

28. 14.9 Reducing Voltage Drop and Power Loss A transmission line operating at 138 kV, 3Ø, can transmit one million watts of 3Ø power with a current of just over 4A! 13.2 kV Primary Line = 43.74A 480V Secondary Line = 1,203A

29. 14.10 Generating Plants The electrical system begins at a generating plant where it converts energy to rotate a turbine, which produces 13.8 kV, 3Ø power. Fig 14–9

30. 14.11 Step-Up Substation at Generating Plant A step-up substation located at the generating facilities transforms the 13.8 kV generator output to 69 kV, 500 kV or even higher. Fig 14–9

31. Figure 14-9

32. 14.12 Transmission Line High-voltage transmission lines carry the 69 kV or higher voltage from the generating plants to various step-down substations. Figs 14–9 and 14–10

33. Figure 14-9

34. Figure 14-10

35. 14.12 Transmission Line High-voltage transmission lines are usually connected to transmission lines from other generating plants. This is known as an interconnected system or an electrical power grid.

36. 14.13 Step-Down Substation Step-down substations reduce the voltage from the high-voltage transmission lines to primary distribution voltage of 35 kV, 7.2/14.4 kV, etc. Fig 14–10

37. Figure 14-10

38. 14.14 Primary Distribution Feeders Distribution feeders transfer the primary distribution voltage to distribution transformers mounted on poles or on the ground next to the building being served. Fig 14–11

39. Figure 14-11

40. 14.15 Distribution Transformer Distribution transformers reduce the primary distribution voltage to secondary distribution voltage, such as 120/208V, 120/240V or 277/480V. Fig 14–11

41. Figure 14-11

42. 14.16 Secondary Distribution Line The customer's power is transferred from the utility's distribution transformer to the customer via the overhead service drop or underground service lateral. Fig 14–12

43. Figure 14-12

44. Unit 15 Protection Devices Unit 15 Protection Devices

45. Circuit Protection How does a fuse differ from a circuit breaker? You’ll learn about the interrupting rating and the short-circuit rating—two very different, but often confused subjects. You’ll understand the role of overcurrent protection in clearing ground faults and critical facts about grounding.

46. Part A – Overcurrent Protection Devices

47. 15.1 Overcurrent Protection The purpose of overcurrent protection is to protect conductors and equipment against excessive or dangerous temperatures due to current in excess of the rated ampacity of equipment or conductors. Fig 15–1

48. Figure 15-1

49. 15.2 Clearing Faults To protect against electric shock or to prevent a fire, dangerous overloads, ground faults and short circuits must quickly be removed by opening the circuit's overcurrent protection device.

50. 15.2 Clearing Faults Time – Current Curves. The opening time for a protection device is inversely proportional to the magnitude of the current. The greater the current value, the less time it takes for the protection device to open. Fig 15–2

51. Figure 15-2

52. 15.2 Clearing Faults Remove Dangerous Touch Voltage. To remove dangerous touch voltage on metal parts from a ground fault, the fault-current path must have low impedance to allow the fault current to quickly rise to facilitate the opening of the protection device. Fig 15–3

53. Figure 15-3

54. 15.3 Overcurrent Protection Device Types The most common types of overcurrent protection include fuses and circuit breakers. GFCI and AFCI are not listed as overcurrent protection devices.

55. 15.4 Fuse Construction. A fuse consists of an element electrically connected to the end blades, called the ferrules. The element is enclosed in a tube and surrounded by a nonconductive filler material. Fig 15–4

56. Figure 15-4

57. 15.4 Fuse Overload Protection. As current flows through the element, it generates heat. When a sustained overload occurs, the heat melts a portion of the element, stopping the flow of current. Fig 15–5

58. Figure 15-5

59. 15.4 Fuse Short-Circuit and Ground-Fault Protection. When a short circuit or ground fault occurs, several element segments melt at the same time, removing the load from the source very quickly. Fig 15–6

60. Figure 15-6

61. 15.5 Circuit Breaker Trip Elements Thermal Trip Element. The thermal sensing element causes the device to open when a predetermined calibration temperature is reached. Fig 15–7

62. Figure 15-7

63. 15.5 Circuit Breaker Trip Elements Magnetic Trip Element. The magnetic time-delay circuit breaker operates on the solenoid principle where a movable core held with a spring, in a tube, is moved by the magnetic field of a short circuit or ground fault. Fig 15–7

64. Figure 15–7

65. 15.6 Circuit Breaker Types Inverse-Time. Inverse-time breakers operate on the principle that as the current increases, the time it takes for the devices to open decreases.

66. 15.6 Circuit Breaker Types Adjustable-Trip. Adjustable-trip breakers permit the magnetic trip setting to be adjusted to coordinate the circuit breakers' operation with other protection devices.

67. 15.6 Circuit Breaker Types Instantaneous-Trip. Instantaneous-trip breakers operate on the principle of electromagnetism only and are used for very large motors.

68. 15.7 Available Short- Circuit Current Available short-circuit current is the current in amperes that is available at a given point in the electrical system.

69. 15.7 Available Short- Circuit Current The available short-circuit current is different at each point of the electrical system, it is highest at the utility transformer and lowest at the branch- circuit load.

70. 15.7 Available Short- Circuit Current The available short-circuit current is dependent on the impedance of the circuit, which increases downstream from the utility transformer. Fig 15–8

71. Figure 15-8

72. 15.7 Available Short- Circuit Current Factors that impact the available short- circuit current include transformer voltage, kVA rating and its impedance, as well as the circuit conductor impedance (material, size, and length).

73. 15.8 Interrupting Rating Circuit breakers and fuses are intended to interrupt the circuit, and they shall have an ampere interrupting rating (AIR) sufficient for the available short-circuit current. Fig 15–9

74. Figure 15-9

75. 15.8 Interrupting Rating Unless marked otherwise, circuit breakers have a 5,000 ampere interrupting capacity (AIC) rating and fuses have a 10,000 AIC rating. Fig 15–10

76. Figure 15-10

77. 15.8 Interrupting Rating Danger: If the protection device is not rated to interrupt the current at the available fault values at its listed voltage rating, it could explode while attempting to clear the fault. Fig 15–11

78. Figure 15-11

79. 15.9 Short-Circuit Current Rating Equipment shall have a short-circuit current rating that permits the protection device to clear a short circuit or ground fault without extensive damage to the components of the circuit. Fig 15–12

80. Figure 15-12

81. 15.10 Current-Limiting Protection A thermal-magnetic circuit breaker typically clears fault current in less than 3 to 5 cycles when subjected to a short circuit or ground fault. A current- limiting fuse should clear the fault in less than 1/4 of a cycle. Fig 15–13

82. Figure 15-13

83. Part B – Ground-Fault Circuit Interrupters (GFCI)

84. 15.11 How a GFCI Works A GFCI is designed to protect persons against electric shock. It operates on the principle of monitoring the imbalance of current between the ungrounded and grounded (neutral) conductors. Fig 15-14

85. Figure 15-14

86. 15.11 How a GFCI Works If the difference between the current leaving and returning through the current transformer of the GFCI protection device exceeds 5 mA, the solid-state circuitry de-energizes the circuit. Fig 15–15

87. Figure 15-15

88. 15.12 Neutral-to-Case Detection A GFCI protection device contains an internal monitor that prevents the device from being turned on if there is a neutral-to-case connection downstream of the device, even if there is no load on the circuit. Fig 15–16

89. Figure 15-16

90. 15.13 Line-to-Neutral Shock Hazard Severe electric shock or death can occur if a person touches the ungrounded and the grounded (neutral) conductors at the same time, even if the circuit is GFCI-protected. Fig 15–17

91. Figure 15-17

92. 15.14 GFCI Fails – Circuit Remains Energized Typically, when a GFCI protection device fails, the switching contacts remain closed and the device will continue to provide power without GFCI protection. This is a most dangerous condition!

93. 15.14 GFCI Fails – Circuit Remains Energized According to a NEMA report, 11% of the GFCI breakers protecting indoor receptacles failed, and 20% protecting outdoor receptacles failed.

94. 15.14 GFCI Fails – Circuit Remains Energized The failures of the GFCI sensing circuits were mostly due to damage to the internal control equipment because of voltage transients. Fig 15–18

95. Figure 15-18

96. Part C – Arc-Fault Circuit Interrupter

97. 15.16 Arcing Definition Arc. Arcing is defined as a luminous discharge of electricity across an insulating medium. Electric arcs operate at temperatures between 5,000 and 15,000°F and expel small particles of very hot molten materials. Figure 15-19

98. Figure 15-19

99. 15.17 Series Versus Parallel Arc Unsafe arcing faults can occur in one of two ways, as series arcing faults or as parallel arcing faults. The most dangerous is the parallel arcing fault.

100. 15.17 Series Versus Parallel Arc Series Arc. A series arc can occur when the conductor in series with the load is unintentionally broken. A series arc fault current is load limited.

101. 15.17 Series Versus Parallel Arc Short-Circuit Parallel Arc. The current in a short-circuit type arc is limited by the system impedance and the impedance of the arcing fault itself. Typically, at a receptacle, fault current will be above 75A, but not likely above 450A.

102. 15.17 Series Versus Parallel Arc Ground-Fault Parallel Arc. A ground- fault type parallel arc can only occur when a ground path is present. This type of arcing fault can be quickly cleared by either a GFCI or AFCI circuit protective device.

103. 15.18 AFCI and the NEC To help reduce the hazard of electrical fires from a parallel arcing fault, the NEC requires a listed AFCI protection device to protect branch-circuit wiring in dwelling unit bedrooms. Fig 15–20

104. Figure 15-20

105. 15.19 AFCI and the NEC An AFCI protection device provides protection from an arcing fault by recognizing the characteristics unique to an arcing fault and by functioning to de-energize the circuit when an arc fault is detected. Photo

107. Part D - Ground-Fault Protection of Equipment

108. 15.20 Ground-Fault Protection of Equipment This is a system intended to provide protection of equipment from damaging ground-fault currents by opening all ungrounded conductors of the faulted circuit. This device is not intended to protect persons.