This article was posted 09/30/2005 and is most likely outdated.

Grounding vs Bonding Online Training 15

September 30, 2005

Many of you have completed our Online Continuing Education Courses which include video instruction and know this is a very fast way to obtain your CEU credits from the comfort of the chair you're sitting in. For the rest of you we are sending a weekly series of newsletters featuring an instructional page complete with videos.

To view the videos you will need to download the free Real Player plugin for your browser - if you don't already have it. After installing Real Player, you can view the videos by clicking on one of these icons:

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Below is the full course page which includes these videos:

Grounding vs Bonding

PART II.

Page 5 of 13

Sections

250.24 Grounding and Bonding at Service Equipment Continued

Author’s Comments:
• A ground-fault cannot be cleared to remove dangerous voltage on the metal parts, metal piping, and structural steel if the service disconnecting means enclosure is not bonded to the grounded neutral service conductor. Figure 250–58
• If the grounded neutral service conductor is open, a ground fault cannot be cleared and metal parts will become and remain energized. Figure 250–59

In addition, if the grounded neutral conductor is opened, dangerous voltage will be present on metal parts under normal conditions, providing the potential for electric shock. For example: If the earth’s ground resistance is 25Ω and the load’s resistance is 25Ω, the voltage drop across each of these resistors would be 1/2 of the voltage source. Since the grounded neutral is bonded to the service disconnect, all metal parts will be elevated to 60V above the earth’s potential for a 120/240V system. Figure 250–60

This dangerous condition is of particular concern in buildings containing pools, spas, or hot tubs.

To determine the actual voltage on the metal parts from an open grounded neutral service conductor, you need to do some complex math calculations with a spreadsheet to accommodate the variable conditions. Visit www.NECcode.com and go to the Free Stuff link to download a spreadsheet for this purpose.

(1) Minimum Size Grounded Neutral Conductor. Because the grounded neutral service conductor is required to serve as the effective ground-fault current path, it must be sized so that it can safely carry the maximum fault current likely to be imposed on it [110.10 and 250.4(A)(5)]. This is accomplished by sizing the grounded neutral conductor in accordance with Table 250.66, based on the total area of the largest ungrounded conductor. Figure 250–61

Author’s Comment: In addition, the grounded neutral conductors must have the capacity to carry the maximum unbalanced neutral current in accordance with 220.61.

Question: What is the minimum size grounded neutral service conductor required for a 480V, three-phase service where the ungrounded service conductors are sized at 500 kcmil and the maximum unbalanced load is 100A? Figure 250–62

(a) 3 AWG (b) 2 AWG (c) 1 AWG (d) 1/0 AWG

Answer: (d) 1/0 AWG [Table 250.66]

The unbalanced load requires a 3 AWG grounded neutral service conductor, which is rated 100A at 75ºC in accordance with Table 310.16 [220.61]. However, the grounded neutral service conductor cannot be smaller than 1/0 AWG in accordance with Table 250.66 to ensure that it will accommodate the maximum fault current likely to be imposed on it.

Figure 250–58
(Click on image to enlarge)

Figure 250–59
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Figure 250–60
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Figure 250–61
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Figure 250–62
(Click on image to enlarge)

Grounding versus Bonding Library - DVDs

Grounding and bonding problems are at epidemic levels. Surveys repeatedly show a high percentage of power quality problems are due to poor grounding and bonding. Electrical theory has been applied to this difficult to understand Article, making it easier for students to grasp the concepts of grounding and bonding. Additionally, Mike has color coded the graphics so you can easily differentiate between grounding and bonding. For a limited time, you can order the Entire Grounding versus Bonding Library including the textbook, 2 videos, MP3 Audio CD and the Online Program for this great price. You save over $200!

Product Code: 05GBDVD
ISBN: N/A
MultiMedia: Tape & CDROM & DVD

DVD/CD Return Policy: Unopened sealed DVD's or CD's may be returned within 10 days and we will credit your credit card or issue a refund for the price of the item(s). Opened items may not be returned unless they arrive defective.

Table of Contents

Sample Pages

Sample Graphic

Comments

i have a NJ license, can i do my ce .hours here??

glenn

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