Copper Ground Grid With Structural Steel Piles

[SVincent240]

I am working on designing a grounding system for new 115/34kV substation. The civil engineers want to use steel piles (50' long, 9" O.D. steel pipes) to support the foundations due to difficult soil conditions. We would like to include the piles as ground rods in our ground grid analysis if possible, but we are worried about corrosion issues. Unless the copper ground gird is electrically isolated from the steel piles, will we get galvanic corrossion and lose integrity of the piles? Does anyone know of any good sources to reference when looking into this issue?

 

[oldagpe]

The copper grid must be tied electrically to the steel. This is safety and lightning protection.

Copper and steel are not that far apart on the galvanic scale.

Detailing can be found in NASA KSC-STD-E-0012E, Facility Grounding ...

The other issue would be if the steel piles are in chemically active soil, such as expansive clays. A DC bias might be suggested to prevent corrosion of the steel piles by the soil.

 

[jghrist]

I'm not a corrosion expert, but it seems to me that if there is a problem, it would occur because there is copper in the ground with the steel. You would need to coat the steel with insulating material to isolate it from the grid. Then, if the coating gets scratched off, it concentrates the corrosion. Does it matter if the steel is bonded to the ground grid? For safety reasons, you will have to ground the steel piles anyway, even if only by connection to grounded steel structures.

 

[SVincent240]

Our thoughts were that since the piles will be incased in concrete (for the top 6") with the tops at a depth of 5-6' below grade, there will not be a safety issue with isolation. The piles will be supporting a mat type foundation that will be beneath the ground grid located at a depth of 18" below grade.

Further investigation will be needed to verify if there are safety requirements as far as needing to have the steel connected to the grid.

We have been told that it would not be feasable to coat the piles since they are going to be driven and the coating will likely be scratched off.

An impressed current cathodic protection system has been suggested, but the costs and maintenance may be an issue.

Thanks for the replies!

 

[oldagpe]

Maybe I can offer a bit more experience here.

Coordinate the dickens out of the foundation effort. The CE's see it as holding up the structure; they have no thought as to the grounding, bonding, and lightning performance.

For example, if the mat foundation is designed with post-tensioned cables, it is likely that no provision has been made for bonding the reinforcement from poles to mat to structure frame.

Those steel pipes are an electrode subsystem. And they are likely the lowest impedance path to earth.

 

[7anoter4]

I agree with jghrist. From IEEE 665-1995 "IEEE Guide for Generating Station Grounding" Anexe B1 Corrosion:
"Copper, which is the most widely used material for grounding in the United States, could cause serious dissimilar
metal corrosion to steel pipes or other steel structures. If steel conductors or rods are used for
grounding, as is the practice in many countries in the world, consideration should be given to protect this
material from corrosion due to various corrosion mechanisms that are normally found and easily attack steel
(when buried underground or submerged in an electrolyte).
Some of the most common corrosion prevention measures that are used to minimize corrosion of grounding
material and or steel piping are
a) Use of tin coated bare copper cable as a grounding conductor to minimize the potential difference
between the steel structures and the ground cable
b) Use of cathodic protection for steel grounding cables and/or piping
c) Use of insulated copper ground cable in areas near pipes
d) Electrical isolation of piping from other plant structures and the grounding system"

 

[7anoter4]

See also :
thread238-215141
There are some U.S. manufacturers of lead covered grounding cable as for instance this one:

http://www.amercable.com/doc/catalogs/industrial/37-409.pdf

 

[SVincent240]

Thanks for all the replies, any input is appreciated. Also, if anyone is interested, I presented the questions to the guys at E&S grounding solutions and got a good response:

http://www.esgroundingsolutions.com...ilize-the-piles-as-ground-rods-if-possible-bu