What is the term "Counterpoise" as referenced relating to Lightning Protection in NFPA 780 1

[power2engineer]

thread238-215783

I have recently joined a government (DoD) engineering consulting firm (as Elec engineering role) who is involved in designing new and upgrade of fuel facilities handling petroleum fuels with outdoor canopy roof structures Flat roof, Gable roof etc calling for Lightning Protection system upgrade or installation per NFPA 780 "Chapter 7- Protection of Structures containing Flammable Vapors, Flammable Gases or liquids that can give off Flammable Vapors". These are relatively small structures from 40-100 Ft long X 15- 60 Ft Wide. I frequently get questions from Lightning protection contractors if the Lightning protection system components on roof of these structures such as Air Terminals and lightning protection conductors runs between these air terminals to down-conductors to be tied to 3/4" X 10 Ft ground rods (minimum at 2 points) require a ground ring (OR ground loop) around the perimeter of the roof of such structures. The contractors refers to these ground loop or ground ring as "Counterpoise" and DoD standards show them as 4/0 bare copper conductors. Some contractors refers to NFPA 780 Chapters 8 "Protection of structures housing explosive materials" 8.4.1 reads "Ground ring conductor shall be required for all lightning protection systems on structures containing explosives, with all down conductors, structural steel, ground rods, and other grounding systems connected to the ground ring conductor." EXCEP A ground ring electrode shall not be required for structures with areas of 500 Sq.Ft or less or those that can be protected by a single mast or air terminal.

Questions 1) Is the COUNTERPOISE by definition the ground ring or ground loop conductor? 2) Chapter 7 that applies to FLAMMABLE VAPORS does not mention this requirement of ground ring but only the Chapter 8 that applies to EXPLOSIVES calls out this but DoD standard drawings do show the ground loop (ring) using bare 4/0 conductors. I never get any straight answer for interpretation of this requirement from DoD engineering staff.

Can my learned colleague's help clarify my confusion here? thanks

 

[jghrist]

Counterpoise is a system of buried ground wires run underneath a power line that has overhead ground wires. I don't think that the use of the term "counterpoise" to refer to a perimeter ground or "ground loop" is correct.

 

[mparenteau]

I've heard it called a counterpoise. Re:

http://lightningsafety.com/nlsi_lhm/lpts.html.

Mike

 

[jghrist]

I've heard it called a counterpoise. Re:

http://lightningsafety.com/nlsi_lhm/lpts.html.

That reference is not to a perimeter ground or ground loop, but to multiple radial wires extending radially out from a central ground or ground loop to help dissipate lightning surges.

 

[FacEngrPE]

A discussion on the subject on Mike Holt's Forum includes this.

https://forums.mikeholt.com/threads/counterpoise.28990/[/URL]]The function of the counterpoise is to lower the transmission in areas where the impedance needs to be lowered. The reduction of impedance will reduce the insulator flashover due to lightning strikes.