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How do you protect a steel-pipe electrode from unwanted stray currents?

Hi S. Y. Go,

Thank you for your excellent question regarding the grounding specifications for your project in the UAE.  The specifications you sent regarding protecting pipe electrodes from stray currents are asking for four (4) things:

1. They are requesting two (2) separate grounding systems

2. They are detailing the required grounding electrodes

3. They are specifying a required resistance-to-ground for the electrodes

4. They are requiring a an unspecified impedance between the grounding systems to reduce interference.  This is often called a “Clean Ground”.

To point #1, they have requested that the two (2) separate grounding systems be installed for the building/structure/compound in question.  The first is called “Electrical Earth” and the other is called “Instrument Earth”.  This is very common to have these two systems, the one designed to handle the normal electrical grounding requirements, and the other to eliminate unwanted stray currents and harmonics from the chassis of sensitive instruments.

To point #2, they have specified continuous galvanized pipes of specific lengths, diameters and depths, for use as the electrodes.  It appears that the specification is asking that the pipes be driven into the earth vertically to a depth of at least 3 meters.

To point #3, the specification is asking that the resistance-to-ground (or impedance-to-ground?) of the individual electrodes (from point #2) be less than one-ohm (1-ohm).  It is important to understand that this is in conflict with point #2.  Let us explain.  The resistance-to-ground (RTG) of an electrode becomes exponentially more difficult the closer to zero you desire the resistance to be; zero of course being impossible.  A 5-ohm or less grounding system is considered difficult.  One-ohm or less grounding systems typically require extensive copper grids and must be designed by a qualified engineering firm.  The idea that a single 3.5m electrode could achieve 1-ohm is nearly impossible; unless your facility happens to be sitting on some really conductive soil, such as moist clay with a high metal content.  Obviously, the more resistive the soil, the more difficult it is to achieve a low resistance ground.  We really have no idea what the soil resistivity for the UAE is, but we can imagine that it is not good.  Without soil-resistivity data and proper computer models, it is impossible to know how long and how many electrodes would be required at your site to achieve a one-ohm or less ground system.  It is not unusual to see grounding systems in poor soil conditions to extend over two-kilometers in length!  Here is a link with some more information:

http://www.esgroundingsolutions.com/about-electrical-grounding/how-to-do-electrical-grounding-system-design.php

To point #4, often with “Instrument Earth” the goal is to remove the unwanted electrical noise associated with “Electrical Earth”.  “Electrical Earth” is commonly tied to building steel and many harmonic-generating devices that produce undesirable interference.  If the grounding electrode system for “Instrument Earth” is too close to the electrode for “Electrical Earth” the unwanted stray currents can come right back into the system.  While your specification doesn’t list it, there are electronic companies that request one-million ohms (1,000,000 ohms) resistance between the two grounding systems.  Please keep in mind that these grounding systems must be bonded to each other at some point in the system; how they are bonded together is the key to providing proper grounding.  Also, providing a “Clean Ground” does not simply involve the grounding electrode, it involves how the ground system is routed and isolated from the instrument needing the special grounding, all the way back to the grounding electrode.  In almost every case, the routing and the way the conduit is isolated for the grounding conductors is far more critical to providing a “Clean Ground” than the physical placement of the “Instrument Earth” electrode.  Additionally it is important to note, that while customers often ask for separate grounding systems, they often do not need two separate systems; they need one properly designed system.

It is clear you have an expensive and challenging grounding requirement that will need to be designed and engineered very carefully.  Have you considered the impact that the lightning protection system may have on these grounding systems?  What about any on-site electrical substations?  How are you handling the step & Touch voltage hazards?  These other grounding systems (the lightning protection ground and substation grounds) can really change the dynamic of your final grounding design and should be taken into account.

We recommend that you consult a firm specializing in electrical grounding design, one that has the ability to conduct computer simulations and provide 3d modeling of your grounding systems. Particularly if you have a compound with many buildings and interconnected instrumentation wiring.  We are of course happy to help, but if not us, please consult someone with the proper expertise and computer software.

We hope you find this information helpful, and good luck!

The Engineering Team at E&S Grounding Solutions

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