We have building a Gas Plant in North Dakota. We are using over 2000 bare steel piles 36inch diameter and 50 ft deep. We want to use it as our primary grounding grid. The CP system is an impressed current system. We are bonding all piles together with insulated copper 4/0 ground wires CADWELDED to each other.
1. Is this acceptable to have impressed current on our Safety Ground system?
2. The instumentation ground will be isolated with copper rods, however, the instrumetn themselves will be mounted to steel structures that will be tied to the CP impressed current protected steel. Is this an issue?
Thank you for your excellent questions regarding grounding using pilings, impressed current and instrumentation grounding, it is our pleasure to help.
It sounds from your email that you have the basics of a good ground design. Utilizing the steel pilings is a great way to provide effective grounding. This grounding system is what is called “building steel” and it does require that there are no vapor barriers between the concrete and the earth. Vapor barriers are typically plastic which can prevent the flow of electricity from leaving the rebar in the steel and entering the earth.
Please be aware that there are two key drawbacks when using building steel. The first one is that building steel is an extremely noisy environment, electrically speaking. Generally, you will want to avoid using it for grounding of your electronics systems. The second issue is that during lightning strikes and/or electrical utility faults at your main transformers (which can be tens of thousands of amps), the steel rebar can heat up causing the concrete to crack. You may wish to consider installing some copper down the length of some key pilings to ensure that an effective path to earth exists. This may also help reduce magnetism and some of the electrical noise issues. Typically we recommend conducting Ground Potential Rise (GPR) and Ground Potential Difference (GPD) studies on these systems, to ensure that the proper amount of copper is being installed. You don’t want to much, or too little copper. You may also want bare copper versus insulated, it’s hard to know without seeing your blue prints.
To your question #1 regarding impressed current, the general answer is yes, it is acceptable to have direct current corrosion rectifiers on the grounding system, given that it is not interfering with your instrumentation ground.
Question #2 is a bit more complicated. We recommend you review the Motorola R56 standard and the ANSI/ITA/EIA-J-STD-607A for reference to grounding of your computer server rooms. Maintaining a separate (yet bonded) instrumentation ground that is isolated from the electrical ground is key. You may also need a dedicated (and clean) 5-ohm or less grounding system just for the servers. Bonding of the raised floor pedestals, AC equipment, fire suppression gear, metallic conduits, server racks, etc., can get very complicated. Let alone when you consider the lightning protection system, gas pipelines, water lines, fire systems, telecommunications, and your electrical substations (which will require human safety and step & touch voltage hazard analysis under 29 CFR 1910.269), all of which need to be bonded back to each other.
Here are a few previous blogs that we think may be of interest to you for your particular project:
E&S Grounding Solutions is happy to help you with these issue. Please feel free to call us at 310-318-7151 and one of our engineers will be happy to discuss your project with you, free of charge.
The Engineering Team at E&S Grounding Solutions
photo credit: http://www.flickr.com/photos/[email protected]/5677717803/sizes/o/in/photostream/