zinc grounding electrodes

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lexkyphil

lexkyphil

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Lexington, Kentucky
Gentlemen:

I have a client who builds natural gas compressor stations. For a long time, they have been using a 2/0 bare copper buried around the perimeter of the site, bonded to fencing, vessels, building frames, platforms, etc., with 5/8" x 8' ground rods on 100' intervals as a grounding system. We also bond the rebar in all slabs on grade. It works well, but they are concerned about their cathodic protection vs. the copper (a more noble metal than steel pipe) accelerating the deterioration of pipelines thru the station. They now want to insulate the wire with HMWPE, similar to XLP or XHHW insulation and place zinc electrodes instead. The zinc electrodes are 5' long, about 2" square, enclosed in a cotton bag filled with a mixture of bentonite, gypsum and some sodium salts. My question is, if you have experience with this type of grounding design, how well did it work and are there any special precautions or gotchas I need to be aware of?
 
Sounds more like a zinc anode for a sacrificial cathodic protection system than a grounding system. If corrosion of steel structures is a concern, this could be used in addition to the grounding system, but they are two different things. You could use galvanized steel ground wire and rods in conjunction with the cathodic protection.
 
I have no experience with pipeline protection. My understanding (based on watercraft) is that zinc is used as a sacrificial anode; in other words, the zinc is supposed to corrode in order to protect other system components.

A sacrificial electrode would not be suitable as an NEC required grounding electrode.

I see no problem bonding _additional_ zinc electrodes to a grounding electrode system which includes sufficient NEC acceptable electrodes.

Does the NEC apply to the facilities in question?

-Jon
 
zinc grounding electrodes

To answer your questions, yes-the facility is subject to the NEC. I am using copper #2 to bond to extensive cast-in-slab rebars that greatly exceed the 20' minimum limit in NEC 250 for that type of approved electrode. I guess I'd like to know if the zinc anodes I described are of any "real" value for conducting fault current and for establishing a low impedance ground path for the system overall. They are not the only electrode we have-just one other way to improve overall system performance. Any help you could give would be appreciated. We do have a cathodic protection system at the site but I am not using what we are doing to augment that in any way.

Thanks in advance for your help.

Phil in Kentucky
 
I do not believe that such electrodes will in any significant way improve the grounding of the electrical system.

No practical form of _grounding_ provides any significant benefit for low voltage fault currents; for low voltage systems protection is provided by _bonding_, the continuous metallic connection back to the source. Electrical connection to soil will carry far less fault current than is required to trip a breaker in all but the most extreme cases. So if you have good grounding, improving that grounding a bit will make no difference.

Grounding can provide fault clearance for high voltage systems, where the trip current is lower and the voltage pushing current through the grounding electrode is higher.

The zinc electrodes that you describe would likely not _hurt_ your current grounding electrode system, and might provide long term physical protection of any ferrous grounding electrodes. If this is the case, then over the long haul the zinc electrodes would help prevent the degradation of your current system. However I am only guessing that this is a possibility, not stating that it will happen.

-Jon
 
I am about to install a cathodic protection system (Mg anodes, Zn electrodes)on our steam and condensate lines on our site.

I am a little confused by what you are doing. Since you already have a separate CP (is that the official abbreviation?) system to protect the system components, is the intention of this to protect the grounding system from corrosion as well?
 
lexkyphil said:
....... It works well, but they are concerned about their cathodic protection vs. the copper (a more noble metal than steel pipe) accelerating the deterioration of pipelines thru the station. They now want to insulate the wire with HMWPE, similar to XLP or XHHW insulation and place zinc electrodes instead. .....
Click to expand...

I'm sorry, but I'm confused. Insulate the wire from what? Place the zinc anodes where and connect them to what?
 
zinc grounding electrodes

noonan said:
I am about to install a cathodic protection system (Mg anodes, Zn electrodes)on our steam and condensate lines on our site.

I am a little confused by what you are doing. Since you already have a separate CP (is that the official abbreviation?) system to protect the system components, is the intention of this to protect the grounding system from corrosion as well?
Click to expand...

Thanks for your question, noonan. The separate CP is to protect gas pipelines. The client is concerned that the exposed copper used in 2/0 bare electrodes and clad rods might accelerate the degredation of their steel piping. I don't think that is a seriously valid concern because there is a lot of distance between the systems. After all the good comments I have gotten on this, for which I am thankful, I am considering dropping the zinc electrodes. I think I will retain the insulated copper and use stainless steel electrodes. We already have a good ground with bonding to rebar in large concrete slabs on grade, so the quality of the ground was never a question.

Any thoughts?

Phil in Kentucky
 
Oh, I think that I misread a bit.


This is so beyond my area of professional knowledge that it isn't even funny. But this is the internet and I am free to speculate :)

1) The amount of grounding required by the NEC is quite a bit less than provided by your current system. If you eliminated the ground ring entirely along with all of the ground rods along the ring, it would probably make no difference as far as the NEC is concerned.

2) The amount of corrosion caused by the copper grounding electrode is probably minimal. Copper and steel are just not that far apart electrochemically.

3) The massive grounding system may be serving some purpose other than that required by the NEC...or it may have been someone's 'good idea' without any basis in real function

4) The ground ring itself probably does more for whatever sort of grounding is being used than the rods;

-Jon
 
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