Catenary Lightning Protection - Grounding Electrode 2

[cslater]

I am supporting the construction of a Catenary (overhead wire) lightning protection system over buildings that store explosive materials. The design has a grounding system made up of a buried ground loop (2' deep) with a number of 10' driven ground rods connected at certain intervals.

We've found that the soil at some of the buildings turns to mixed sandstone and dirt at about 30" depth, such that driving ground rods is impossible. We've tried digging with a rock bit, but the mix of rock and dirt is causing that to fail as well.

As I've done research, and asked around, I get the impression that the ground loop should be sufficient without ground rods, and that we can verify that by doing Ohm tests of just the loop. However, I have not been able to get a definitive answer on whether this is the case, and this is outside my expertise.

Any insight that can be provided would be extremely helpful.

 

[rbulsara]

While there are many variables such as how large the system is, how many rods are buried in "good" soil etc. Measures also depend as to how critical the system is.

In absence of site specifics, consider using grounding mat (wire mesh) in lieu of just ring conductor without rods, in the rocky area. Depending on the site conditions, concrete encased rods or even chemical rods can be considered.

If the buildings are new, ufer grounding, conductor buried in the foundation provides the best system grounding electrode. You will need to test the system as well, of course. I would presume you have a electrical consultant on the project. If not, get one or even a grounding system expert.

Rafiq Bulsara

http://www.srengineersct.com

 

[MikeDB]

If your overhead groundwire is effective, the grounds at the locations the overhead groundwire terminates may be more important than at the buildings. Checking the resistance of the grounding system will tell you if you need to add more runs of buried wire or if you need to add a grounding well.

 

[jghrist]

For lightning protection, the inductance of the grounding electrode system becomes important. Long lengths of wire or ground rods may not be effective because of inductance at high frequency. Deep grounding wells may be less effective than relatively short (50' or less) radials. A series of radials installed away from the buildings may be effective in dissipating lightning surges away from the explosive materials.

 

[sjmgd997]

Have you considered chemical treatment of the soil as part of your installation. Companies like Lightning Eliminators and Consultants have chemical ground rods that change soil resistance which will help give you a low impedance path.

Please note that one of the most important principles of lightning protection is to keep items at as close to the SAME POTENTIAL as possible.

This means that you can have a higher impedance path to ground - as long as all of the components are connected in such a fashion as to give you the same potential at each point such that don't have differences in potential that can damage equipment.