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In a given substation ground grid design using copper cable, if you substitute different cable materials such as copper clad steel cable and the grid resistance increases, can you just change the number of cables and increase the ground rod numbers or depth to achieve the same resistance of the copper constructed ground grid?

Hi Jack,

Thank you for your question regarding design changes and grid resistances at electrical utility substations when using buried steel grounding systems.  It is our pleasure to help.  Also, we are sorry that we could’nt email you this responce as it came back as a bad address.  We hope you find this blog.

This is a very interesting question and the answers are not exactly straight forward, but let’s see what we can do.    There are many different factors when dealing with substation grounding.  Grid resistance is only one of a number of engineering factors that must be considered when making such changes; however none are more important than Human Safety.  Please see Title 29 of the Code of Federal Regulations Part 1910.269 (29 CFR 1910.269) for your legal responsibilities regarding Human Safety in high-voltage environments.  Adjusting the ground grid design of a substation without a proper engineering study could have criminal liabilities.  Please remember that IEEE Std-80 is not law, it is a voluntary standard designed to help you meet your legal obligations (the law) found in 29 CFR 1910.269. 

http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=standards&p_id=9868

But that said, the first thing to note is that copper is at least 12 times more conductive than steel and 250 times less magnetic, a very big difference indeed.  When we design a ground grid for a substation, the reduction of Hazardous Step & Touch Voltages is our primary concern, not the grid resistance.  The physical placement of the conductors in relation to “touchable” metal objects (distance and depth) and how effective those conductors are at handling electrical energy IS our primary concern.  That means that during fault conditions at the substation, the electric fields that will form in the ground grid will be far greater using steel, and as such the clearing time of the fault will be increased as the collapse of the electric fields will add energy back into the system.  This will almost certainly change the Step & Touch Voltage Hazard calculations which must account of the amount of current (in amps) flowing through a human heart and whether or not that current will cause the heart to fibrillate (resulting in death).  Changing materials without re-analyzing the grid design would nullify the design and would violate 29 CFR 1910.269.

Additionally, it should be noted that often what is good for reducing Touch Voltages, is bad for reducing Step Voltages.  When dealing with Touch Voltages, we are generally trying to ensure that the feet and hands are at the same potential, and as such we design the grid with the ground conductors close to the feet.  For Step Voltages, we are trying to get the electrical energy as far away from the person as possible.  These two opposing goals must be carefully “balanced” when designing a grounding grid for Human Safety.  Simply adding more conductors into the grid may actually cause more problems than it solves.

Another issue that steel will cause (as you state in your question) is an increase in resistance across the grid itself.  This is sometimes called the point-to-point resistance, as one would measure it by checking the resistance across the grid from one corner to the opposite diagonal corner.  Typically, in a true Direct Current (DC) resistance measurement, one would only be able to detect a slight increase in steel over copper, as there would certainly be many parallel paths for the test signal to travel on from corner to corner.  However, we must remember that the electrical utility fault will actually be at 60 Hz, which means we are more concerned with impedance, than resistance.  Measuring impedance is no easy task as it involves the injection of known signals and being able to measure the angle of the return with precise timing.  This often requires high-precision test gear that is linked together using fiber-optic wires in order to ensure accurate timing to the micro-second.  Generally speaking this is to cost prohibitive and is rarely ever done.  As such, when we conduct point-to-point DC checks (make sure to subtract test lead resistances at temperature) we generally expect the grid to be under 0.1-ohms.  Resistances under 0.5-ohms are questionable, and anything over 0.5-ohms is considered bad.

In regards to the resistance-to-ground of the grid (see: http://www.esgroundingsolutions.com/about-electrical-grounding/how-to-do-electrical-grounding-system-testing.php ), the value will probably not be changed by a difference in material, as the primary determining factor is the sphere-of-influence (see: http://www.esgroundingsolutions.com/about-electrical-grounding/grounding-electrode-sphere-of-influence.php ).

Another concern regarding steel is corrosion.  While NFPA 70 the National Electrical Code (NEC) doesn’t specifically ban steel wire from being buried directly in the earth, it does require that steel it be encased in concrete when used as an electrode.    Now in the case of an electrical substation, the ground grid itself is not technically considered to be simply an electrode, and as such it is usually considered exempt from National Electrical Code (NEC) rules.  In these cases we refer to IEEE Std-80, which states:

IEEE STd-80 Section 11.2.4 Steel- “Steel may be used for ground grid conductors or rods.  Of course, such a design requires that attention is paid to the corrosion of the steel.  Use of galvanized or corrosion resistant steel, in combination with cathodic protection, it typical for steel grounding system.” 

This means that you must install a cathodic protection system when using steel grounding systems at a substation, and that the type of steel used must be corrosion resistant.  Additionally, the next section of the IEEE-80 standard provides further warnings about steel-ground systems in regards to galvanic cell action and material nobilities, which can rapidly corrode the steel even with cathodic protection.

In conclusion, steel is a poor choice for use in any grounding system, but especially at a substation.  However, if steel is the right choice for your situation, you will need to have a new Step & Touch Voltage Hazard Analysis conducted on your substation to ensure that the ground grid design is sufficient.  You will also need to install cathodic protection systems, and consult a Corrosion Expert in regards to material nobilities and galvanic cell action caused by dissimilar materials (copper-clad steels, lead-based alloys, tin, zinc, etc.) in use at the site.

We hope you find this information useful.  If you should have any further questions or need assistance with a new Step & Touch Voltage Hazard Analysis for your substation, please do not hesitate to call.  We will be glad to discuss your project with you free of charge.

Best regards,

The Engineering Team at E&S Grounding Solutions

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