Welcome to a NECA Webinar based on the National Electrical Code. This seminar is the second in a series of electrical grounding and bonding Webinars

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Randolf Fox
5 years ago
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1 Welcome to a NECA Webinar based on the National Electrical Code. This seminar is the second in a series of electrical grounding and bonding Webinars presented by NECA. This Webinar topic is system grounding. Introduction of speaker and subject. 1

2 Presenters should use this slide to provide and overview of what will be covered in the presentation. This program covers the general requirements for grounded systems as well as helping students understand when systems are required to be grounded, permitted to be grounded, and not permitted to be grounded. A review of ground detector requirements is provided. 2

3 Presenters should review the fundamental reasons systems are grounded. Section 250.4(A)(1) provides general performance criteria for both grounded systems and ungrounded systems. Emphasize that grounded systems are those that have a direct and intentional reference to ground (the earth). 3

4 This slide shows three different system configurations, all of which include a conductor that is intentionally grounded. This slide provides presenters with an opportunity to explain the between a neutral point and a neutral conductor. These defined terms were new to the 2008 NEC. 4

5 System grounding requirements are provided in NEC Sections , , and This slide just provides users with a brief review of the requirements for grounding systems. It is important to make the point about system grounding by choice and explain that there are systems not permitted to be grounded.

6 Presenters should use this graphic to describe the difference between grounded systems and those that are ungrounded. A grounded wye system is shown on the left and an ungrounded delta is shown on the right. 6

7 The basic requirements for system grounding are found in Section of the NEC. The NEC typically sets the requirements for system grounding. For some systems there is no choice, they have to be grounded. For others, the system is permitted to operate ungrounded or it can be grounded. Make the point about grounded systems having to follow all requirements for grounded systems, whether grounded because of a requirement or grounded by choice. 7

8 This slide provides a summary of Code rules that require system grounding. The appropriate section of the NEC is included for attendees. Impedance grounded systems is beyond the scope of this presentation, but the presenter should mention that highimpedance grounded neutral systems are grounded systems and must follow the rules in

9 The presenter should review the requirements for grounding systems of less than 50 volts. The slide provides a brief review of the requirements in NEC (A). 9

10 The presenter should review the requirements for grounding systems of less than 50 volts. The slide provides a brief review of the requirements in NEC (A). This slide includes a graphic view of the requirements for grounding systems less than 50 volts. 10

11 The presenter should review the requirements in (B). This section provides the rules for systems of 50 to 1000 volts that are required to be grounded. Make the point about systems where the neutral is used as a circuit conductor such as a 3 phase, 4 wire, wyeconnected system rated at 480Y/277 volts. Where the neutral of this particular system is not used as a circuit conductor, the system is not required to be grounded. 11

12 Presenters should explain the requirement in (B)(1). If a system can be grounded and the phase to ground voltage exceeds 150, the system is permitted to be grounded, but not required to be grounded. An example would be a 480 volt delta system. If one phase is grounded, then the phase to ground voltage on the ungrounded phases would be 480, thus the system is not required to be grounded. 12

13 Section of the NEC indicates which conductor of various systems is required to be the grounded conductor. This slide provides a graphic illustration of 3 grounded systems and shows which conductor of each system should be grounded. 13

14 The requirements in (1) are shown graphically on this slide. Indicate the common system voltages and also emphasize that these grounded conductors are not neutrals. 14

15 This slide is a graphic illustration of (2). Presenters should emphasize that this grounded conductor is a neutral of this system. This is a system typically found supplying single family dwelling occupancies. 15

17 This slide shows a typical system that could be operated grounded as shown, or ungrounded. In the slide, the system is shown as an end grounded system (cornergrounded system). Presenters should make the point about the rules for grounded conductors applying to grounded phase conductors as well as grounded neutral conductors. 17

19 This slide provides the basic requirements for system grounding where the system is of 1000 volts and over. This requirement applies where the system supplies portable or mobile equipment. The rule indicates how the system should be grounded. 19

20 Presenters should review the requirements of Section (D) and indicate that if a system is required to be grounded, then the rules in (SA) are applicable. 20

21 21

22 Section (D) includes clear guidance about separately derived system grounding requirements when the transfer equipment provides a switching action in the grounded conductor. 22

23 When there is no switching action in the grounded conductor through the transfer switch, the generator has to be grounded by applying rules for equipment grounding rather than system grounding. The system is grounded with the transfer switch in the standby position or the normal position. 23

24 Systems permitted to be grounded are those covered in (A). In these cases, the system grounding is by choice rather than as a requirement. Usually the choice is made to operate a system ungrounded to increase continuity of service. Ina first phase to ground fault condition, the ground detection system provides annunciation, but allows the system to remain operational. If a second phase to ground fault were to develop before the first one is cleared up by qualified persons, then there would be a phase to phase short circuit condition in addition to a phase to ground fault condition. It is best to react to the first phase to ground fault indication in timely fashion. 24

25 This slide is a continuation of the provisions in (A)(3) and (4). 25

28 Presenters should review a couple of important rules that apply to grounded conductors. First, grounded conductors have to be identified in accordance with Grounded conductors are also not permitted to have overcurrent devices installed in series with them based on the general restrictions in Note that there are two exceptions to this rule. 28

29 Presenters should make the point that while most neutral conductors are grounded, not all grounded conductors are neutrals. This slide shows neutrals as grounded conductors. 29

30 Presenters should make the point that while most neutral conductors are grounded, not all grounded conductors are neutrals. This slide shows phase conductors as grounded conductors. 30

31 This slide provides the requirements for ground detectors on ungrounded systems. Note that the Code does not yet indicate where the ground detection system should be installed. The NEC rule is found in Section (B). 31

32 This slide shows a simple version of a ground detection system installed on an ungrounded delta system. Note that not all of the components of the ground detection equipment are shown. The lights operate because of a difference of potential across the lamp to ground (through an appropriately sized resistor). Ground detection equipment is available for this purpose. 32

33 This slide shows a simple version of a ground detection system installed on an ungrounded delta system. Note that not all of the components of the ground detection equipment are shown. The lights operate because of a difference of potential across the lamp to ground (through an appropriately sized resistor). Ground detection equipment is available for this purpose. This slide shows a phase to ground fault condition on one phase and the ground indicator lamp goes out because the potential across the lamp is 0. 33

34 This slide provides a review of the various systems and circuits that are not permitted to be grounded based Section of the NEC. 34

35 This slide serves as the summary of the presentation and a review of the learning objectives. The NEC provides rules that require systems to be grounded. There are systems addressed in the Code that are not required to be grounded, but are permitted to be grounded. There are systems that are not permitted to be grounded. Where grounding is a requirement or by choice, all applicable requirements in Article 250 apply. Ungrounded systems are generally required to have ground detection installed on them.

36 Take the opportunity to thank attendees for their participation and welcome them to join in the next NECA Webinar on Grounding Electrical Services. This slide provides the transition to a short question and answer period following the Webinar.

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