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We use a GPR calculator for figuring the 300V point, however the soil type input is vague: input of 1, 2 or 3 (100/20, 100 and 100/1000 ohm-meters respectively). We do not receive this exact info from the power company and we are trying to understand how to figure based on their ground resistivity readings.

Hi Todd,

Thank you for your question regarding GPR Calculators, it is our pleasure to help.

Ground Potential Rise (GPR) calculators are used to help Telco Protection Engineers determine the appropriate distance from a high-voltage facility (substation, tower, transformer) to position the Copper Fiber Junction (CFJ) box.  The CFJ is the critical location where fiber-optic cables transition back in to copper cables.  If the CFJ is located within range of a GPR event, hazardous voltages can form in the copper wires, and damage Telco equipment.  The 300-volt line is a voltage that has been determined to be safe for most Telco systems to handle.  The 300-volt line distance varies for each site, based on soil conditions, X/R factors, fault clearing time and the impedance and shape of the grounding system.  

In our years in business we have run across a number of Ground Potential Rise (GPR) calculators designed to quickly and easily figure out the 300-volt line.  Unfortunately, none of them are very accurate.  Typically these programs have several immediate short comings (there are many more than the list below):

  1. They have limited soil resistivity inputs
  2. They have limits in regards to asymmetrical voltages
  3. They do not account for the physical shape of the grounding system at all, they merely ask for a best-guess at the resistance of the grid

As you pointed out in your email, the soil resistivity assumptions assume that all soil types can be placed into one of three categories.  100 ohmmeters over 20 ohmmeters, 100 ohmmeter uniform soil, or 100 ohmmeters over 1000 ohmmeters.  As you can see they give you two 2-layer choices, or a single 1-layer choice.  There are many problems with this.  First of all, most soil conditions are 3, 4 or 5 layers, and are certainly never a single layer.  Second, the choices available are very unrealistic and do not even provide you with an option of listing depth.  This is probably because in reality, the calculator is converting the 2-layer model into a 1-layer equivalent and then doing its math.  And of course, without actually conducting proper soil resistivity testing, you are only guessing at what the actual soil conditions are in any case, and you are certainly guessing at the grid resistance (60 Hz impedance values which would be far more accurate).

The power company does not have soil resistivity data, you would have to go out and measure it yourself.  Here is a link to how to conduct soil resistivity tests:


In regards to the other GPR calculator short comings listed above, the asymmetrical voltage limitation would be difficult to explain, but needless to say the limitations greatly reduce the calculation requirements.  They also increase the error rate, but that is generally not a concern when you are already guessing at soil structures.

The physical shape of the ground grid can have a huge impact on the 300-volt line.  Long rectangular shaped grounding grids can have a 300-volt line running to the West that is far longer than what the 300-volt line would be running to the North.  Triangular and other irregular shapes can also have differing 300-volt lines based on direction.  The small ground systems found with high-voltage towers can have 300-volt lines that are far longer than similar faults occurring on much larger ground grids. Add to this the issue of adjacent structures, buried pipes, bodies of water, etc., and your 300-volt line can be at many differing distances based on where you are planning to place you Junction Box.

GPR calculators have proliferated mainly due to their simplicity, but that simplicity comes at a cost.  Most GPR calculators can be purchased for less than $99, and often for less than $20.  They are very small programs that take only a few kilobytes of space and provide instantaneous answers.  Programs such as what E&S Grounding Solutions use, can cost as much as $100,000.00 (or more depending on options), and can take many hours of computer processing time to provide a final result.  The problem most engineers have with these simple GPR calculators, is that they almost inevitably provide 300-volt line distances that are far shorter than what will occur in reality, resulting in costly damage to Telco equipment.

If you should have any further questions regarding 300-volt lines, please do not hesitate to contact us directly at 310-318-7151 and 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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