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We are in process of doing earthing calculation for a small project with 11/.415kV distribution using ETAP software. The output report of ETAP says ”The calculated surface potential is greater than GPR probably because of line current source assumption”. Please help us to understand about this and how to come out of this issue. We have modelled the Earthing in 2 layers.

Hi Arul,

Thank you for your question, it is our pleasure to assist you.

We contacted ETAP last week and we have yet to get a response from them regarding your question.  We can only assume that it is a limitation factor related to one or more of the following limiting factors in ETAP:

  • Layer thickness limitations in the soil model
  • Is your site really 2-layers?  Or is 3 or more?  (2-layer soil models can generate high-contrast issues that are difficult to resolve)
  • High contrast ratio limitations due to precision issues in the soil model
  • Number of segmentations available for the earthing conductors in the model
  • The number of available terms in the computer program

 

Unfortunately, we are not ETAP experts as we do not use this particular program.  ETAP is not able to properly conduct human safety studies, particularly in regards to Touch Voltages, as it cannot calculate reach distances from specific objects.  ETAP merely calculates an estimated Step & Touch value across the entire compound.  Proper Touch Voltage calculations must occur one-meter from any metallic object.  This requires some very high-end computer modeling that can take high-speed multi-core computers hours to process.

The problem as far as clients are concerned is that substation designs using ETAP generally end up with excess and unnecessary copper conductors that drive up costs.  The problem for engineering firms with ETAP is that the program is simply inaccurate, and meeting the requirements of Federal Laws such as 29 CFR 1910.269, is simply too great a liability risk for any professional engineer to take.

We use and recommend the CDEGS software program from Safe Engineering Services.  Here is a recent blog that you may find of interest on this topic:

http://www.esearthingsolutions.com/is-using-cdegs-for-design-of-the-substation-earthing-system-economically-more-beneficial-compared-to-using-ieee80-formulas-i-mean-can-i-reduce-the-used-copper-and-justify-it-or-not

If you should have any questions or concerns, please do not hesitate to call us at 310-318-7151 California time.

Best regards,

The Engineering Team at E&S Earthing Solutions

 

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