Earthing Systems Design Steps – Part Four


In Article " Earthing Systems Design steps – Part One ", I indicated the following points:




Earthing Systems Design Steps

A grounding system design process has (3) main steps:

  1. Data Collection,
  2. Data Analysis,
  3. Grounding Design Calculations.





In the above Article and Article Earthing Systems Design steps – Part Two ", I explained the first step: Data Collection.

Also, in Article " Earthing Systems Design steps – Part Three ", I explained the second step: Data Analysis.


Today I will explain the third step of earthing system design which is Grounding Design Calculations as follows.



You can preview the following Articles for more info:









Third Step: Earthing System Design Calculations

Under this title, I will differentiate between two types of buildings as follows:

  1. Substations buildings,
  2. Other Non-Substation Buildings which can be called – in this course – General buildings.


The design process of a substation grounding system requires many steps as established by the IEEE Standard 80, and it is more complicated than that of the general building (Non-substation buildings). However, the design process of a substation grounding system is not under the scope of this course.







What we are going to design for grounding system in any building?

Grounding system in any building can be broken down into several subdivisions:

  1. The building exterior grounds,
  2. The electrical service grounding,
  3. The building interior bonding,
  4. Equipment grounding and bonding,
  5. Lightning protection.








First: The Building Exterior Grounds

Generally, the building exterior grounds consists of:

  1. The building’s perimeter grounds,
  2. Fence grounds,
  3. Other grounds.








1- The Building’s Perimeter Grounds

The building’s perimeter grounds consist of the following components:

  1. A copper conductor that is directly buried in the earth and installed around the perimeter of the building.  The steel building columns are bonded to this conductor to complete the grounding system.
  2. The columns around the perimeter of the building are excellent grounding electrodes and provide a good path into the earth for any fault currents that may be imposed on the system.


Note:

In some cases, an electrical design requires ground rods to be installed in addition to the perimeter ground ring. The use of ground rods helps to minimize the effects of dry or frozen soil on the overall impedance of the perimeter ground system. This is because the ground rods can reach deeper into the earth where the soil moisture content may be higher or the soil may not have frozen.

Design Recommendations For Building’s Perimeter Grounds:

  • The electrical designer, based on the size and usage of the building, will determine whether every column or just some of the columns are bonded. However, at least one column every 50 feet shall be connected to the above described ground ring. (see Fig. 1)




Fig. 1


  • The size of the ground ring will depend upon the size of the electrical service but is seldom less than 1/0 AWG copper.
  • It is recommended that the ground ring and ground rods be copper or copperbonded steel and installed at least 24 inch from the foundation footer and 18 inch outside the roof drip line. This location will allow for the greatest use of the water coming off of the roof to maintain a good soil moisture content.
  • “Triad” ground rod arrangements (rods placed in a triangular configuration) are sometimes specified, usually at the corners of the building or structure. Figure 2 shows possible conductor/ground rod configurations. Triad arrangements are not recommended unless the spacing between the ground rods is equal to or greater than the individual ground rod length.




Fig. 2


  • Three rods in a straight line spaced at least equal to the length of the individual ground rods are more efficient and result in a lower overall system impedance.
  • When the required resistance is not achieved using the usual grounding layouts, a prefabricated wire mesh can be added to lower the overall grounding impedance (see Fig. 3). Prefabricated wire mesh products are existing in sizes ranging from No. 6 to No. 12 AWG solid conductors.



Fig. 3

  • Another method which can be used to lower the grounding system impedance is ground enhancement materials. These materials can be added around ground rods or other conductors to enhance system performance. (see Fig. 4)





Fig. 4








2- Fence Grounds

  • The National Electrical Safety Code (NESC) recommends that where fences are required to be grounded, such grounding shall be designed to limit touch, step and transferred voltages in accordance with industry practice.
  • The NESC requires that the grounding connection be made either to the grounding system of the enclosed equipment or to a separate ground. In addition, the NESC in Section 92E, lists six separate requirements for fences:


1- Where gates are installed, the fence shall be grounded at each side of the gate or similar opening (see Fig. 5).


Fig. 5


2- If a conducting gate is used, a buried bonding jumper must be installed across the opening (see Fig. 5).

3- Where gates are installed, they shall be bonded to the fence, grounding conductor or other bonding jumper (see Fig. 6).



Fig. 6


4- If the fence posts consist of a conducting material, the grounding conductor must be connected to the fence posts with a suitable connecting means (see Fig. 6).

5- If the fence contains sections of barbed wire, the barbed wire must also be bonded to the fence, grounding conductor or other bonding jumper (see Fig. 6).

6- If the fence posts consist of a nonconducting material, a bonding connection shall be made to the fence mesh strands and barbed wire strands at each grounding conductor point (see Fig. 6).

  • Any fence around a substation on the property should be grounded and tied into the substation ground system. If a facility fence meets the substation fence, it is recommended to isolate the two fences to prevent any fault in the substation from being transferred throughout the facility using the fence as the conductor (see Fig. 7).


Fig. 7









3- Other Grounds

Other grounds that are located on the outside of the building that should be considered are:

  1. Handhole, manhole and pull box covers,
  2. Metal poles,
  3. Lighting fixture standards,
  4. Rails.



A- Handhole, Manhole And Pull Box Covers

  • Handhole, manhole and pull box covers, if conductive, should be bonded to the grounding system using a flexible grounding conductor (see Fig. 8).


Fig. 8


  • The NEC Section 370-40 (d) requires that a means be provided in each metal box for the connection of an equipment grounding conductor. Metal covers for pull boxes, junction boxes or conduit bodies shall also be grounded if they are exposed and likely to become energized.



B- Metal Poles

  • The NEC in Section 410-15 (b) Exception, permits metal poles, less than 20 feet (6.4 m) in height to be installed without handholes if the pole is provided with a hinged base. Both parts of the hinged base are required to be bonded to ensure the required low impedance connection. (see Fig. 8)



C- Lighting Fixture Standards

Lighting standards in parking lots and other areas where the public may contact them should be grounded (see Fig. 8). Keep in mind that the earth cannot serve as the sole equipment grounding conductor. Light standards which are grounded by the use of a separate ground rod must also be grounded with an equipment grounding conductor to ensure that the overcurrent protective device will operate.


D- Rails

Rails or sidings into hazardous locations such as grain storage facilities, ammunition dumps, etc., should also be properly bonded and grounded (see Fig. 8). Designers and installers must not forget that distant lightning strikes can travel through the rails for many miles.








Installation Recommendation For Building Exterior Grounds

  • Installers of these perimeter ground systems need to provide a “water stop” for each grounding conductor that passes through a foundation wall. This is especially important when the grounding conductor passes through the foundation wall at a point that is below the water table. The water stop ensures that moisture will not enter the building by following the conductor strands and seeping into the building. A CADWELD Type SS (splice) in the un-spliced conductor and imbedded into the concrete wall provides the required water stop (see Fig. 9).


Fig. 9

  • When “inspection wells” are required to expose points from which to measure system resistance, several methods are available. Inspection wells are usually placed over a ground rod.
  • If the grounding conductors do not have to be disconnected from the rod, the conductors can be welded to the rod, and a plastic pipe, a clay pipe, or a commercial box. (see Fig. 9)., can be placed over the rod. The plastic pipe also works well when an existing connection must be repeatedly checked, since it can be custom made in the field to be installed over an existing connection. If the conductors must be removed from the rod to enable resistance measurements to be made, either a bolted connector or lug may be used (see Fig. 9).
  • Grounding conductors shall be protected against physical damage wherever they are accessible (see Fig. 10).


Fig. 10

  • Grounding conductors installed as separate conductors in metal raceways always must be bonded at both ends to ensure that current flow is not choked off by the inductive element of the circuit.







In the next Article, I will continue explaining Other Building’s Earthing System Divisions. Please, keep following.





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