Member Needs Help - Grounding For Lightning

What's the Best Ground?

Researching several sources there are several different approaches advocated for deep vertical and shallow horizontal grounding electroded layouts.  There are two strong schools of layout the deep to very deep approach and the horizontal short radials to very long radial.  The emergence of conductive concrete has resulted in a growth in the use of both long horizontal trench and of deep vertical rod approaches.   If we have any reservations about dry conductive concrete then we could advocate the use of soil material but we do know that water content is key or tapping into conductive fractures and small areas is significant.

In rock there are some indications of almost 200 meter vertical rod and conductive concrete installations we have considered the 30 meter approach and it is possible that the cost and time becomes prohibited if the depth extends to the bedrock or is all in rock.  From similar advocates there is a different view that the best layout for a telecoms tower are radials coming horizontal off each leg with conductive concrete filling.

What is not clear is if there are variants for the horizontal radials to have down conductors by the tower also to which the horizontal is connected and/or the horizontal is connected at the far end also to a vertical ground rod.  The use of horizontal with verticals in not addressed in the literature and the use of extra horizontals across the radials along the length is not addressed.  

We have specific problems to address and it would be useful to know what the horizontal with the vertical methods, verticals only or horizontal only benefits and disbenfits are.

There are several arrangements that could be compared   A  _____  Tower   _______ Horizontal                                                                                     B              Tower                                                                                                |                |                                                                                              |                |        Deep vertical grounds                                                                                            C ______    Tower   ________ Horizontals and end placed deep verticals                                                                                    |                                        |                                                                                    |                                        |                                                                                                                             D _________ Tower ________                                                                                                   |          |                Horizontals and near tower verticals                                                                                                   |          |                                                                                  E _________  Tower _________ Horizontals with near and end verticals                                                                                  |                |          |               |                                                                                  |                |          |               |    

Does a combination of ground electrodes and tapes in particular directions or in one direction attached in single of multiple elements to each leg of a 30 meter tower achieve the prime requirements below.  Are either horizontal and or vertical electrodes equally as effective or ineffective to:

1.  Reduce the peak ground potential (voltage) within the tower base area to ensure that equipment and cables within 50m or 20m or 10m or 5m or 2m  of the boundary of base station and tower facility does not see a voltage relative to remote of earth of 300v.  If the ground potential rises above this value it is assumed it will malfunction or be damaged.   (What is recommended to fit within the base station tower footprint and what is recommended to be in and outside the footprint) .

2. Dissipate the ground potential voltages across and into the footprint and surrounding area and volume of soil-earth most efficiently to ensure the ground potential levels within the surrounding corridor of the footprint are minimized per meter around the footprint of the basestation.  This should be done for corridors around the footprint of 50m, 20m, 10m, 5m and 2m.  Cables can be in surface ducts along the side of a basestation and equipment may be in the radii of 50m, 20, 10m and 5m of the basestation.  Again the objectives are to reduce the ground potential rise level at these radii.

What is the different ground electrode and tape layout to achieve either of these two requirements.

The base station footprints being considered are 3m by 9m  or 5m by 10m or 7m by 20m.  The ground electrodes can be horizontal and-or vertical in the footprint.  If the ground electrodes are not all within the footprint it is only possible to come from the narrow sides of the facilities for 20 meters if there is no equipment either side except if the ground potential rise is less than 300v then the ground electrodes can come outside the footprint if there is equipment also in the area.

A typical layout could be:
                                Can use up to 10 meters from this 9m side
                                          _____9m___________                                          |Tower    Basestation   |  can use up to 20 meters either short side                                                                            3m |___________________|    _______________________Trough_2m to 5m from the footprint boundary_________ Surface ducts with cables

What role does the system ground resistance value play.  There is one school of thought that says a range of 1 ohm to 50 ohms is not relevant and does not alter the ground potential rise and another which says it does?

The range of soil resistivity in ohms meters to be 10 ohms meters to 100 ohm meters,  100 ohm meters to 300 ohm meters, 300 ohm meters to 1000 ohm meters and 1000 to 3000 ohm meters.

Where the number of  bags 25Kg of conductive cement become more than 20 per tower base station or the horizontals become greater than 20m and 20 cm wide and the number become greater than 10 radial and the length of verticals become more than 30 meters and the number exceed 20 verticals then the site is not groundable and will be moved or the equipment around the site will be moved.

Any suggestions as to determining the horizontal and vertical layout, lengths and depths and numbers and the effectiveness in reducing the ground potential for a 1,000,000 volt 100,000 Amp 80 Coulomb strike will be appreciated.   Mike Holt's Comment: I know nothing about this, so if you do, please respond to sandy@chrisandsandy.freeserve.co.uk and cc me at Mike@MikeHolt.com.

 

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