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We excuting 220KV Switchyard in Gujrat India. I have required test/measurment method for step and touch voltage . Plz given me a method.

Hi Sandeep,

Thank you for your question regarding the measurement of Step & Touch Voltage Hazards at a 220kV substation.  It is our pleasure to help.

Hazardous Step & Touch Voltages can be created a number of ways.  In a high-voltage environment such as a 220kV substation, you will have a steady-state Ground Potential Rise (GPR) that forms on the grounding system, simply from induction currents and the electric fields that are inherent at the site.  While these voltages are certainly hazardous, what we are really concerned with are the voltages that can form during a single line-to-ground fault or during a lightning strike at the substation.  The substations grounding grid should be designed to safely mitigate the hazardous Step & Touch voltages that will form during a fault.  If the grid can handle the fault, it can also handle the steady-state Ground Potential Rise (GPR). 

There have been a number of laboratory-type tests done to measure Step & Touch Voltages during electrical faults at substations, however it is not a test that people commonly perform.  It is simply to unsafe to intentionally generate a line-to-ground fault at a substation.  And even if you were to set up an intentional fault, you could never measure all the possible voltages.

Keep in mind that there is not one single Step Voltage and another Touch Voltage level at a given substation.  The hazardous Step & Touch Voltages change from point to point around the facility.  So the Touch Voltages that will form at the transformer, are different than the Touch Voltages that will form on a gate, or even the corner of the fence.  In fact, the Touch Voltages at different positions around the faulting transformer will vary.  Which means that it is quite possible for a person standing on the south side of the transformer to see Touch Voltages that are greater than a person standing on the North side of the Transformer.  And it is no different for Step Voltages, as they will vary given the distance from any given buried ground wire.

Also you must keep in mind that even if you do have an actual voltage reading at a given spot, you must then understand what the acceptable human threshold for fibrillation current is at your site.  The Step & Touch Voltages that exceed the acceptable Human Fibrillation Current Threshold (HFCT), must be mitigated for human safety.  Accurately calculating the Human Fibrillation Current Threshold (HFCT) generally requires some computer modeling in order to get an accurate result.

Most people conduct a series of computer simulations on their grounding grid in order to identify potential “hot-spots” in the system.  We have found that a combination of computer simulations with a series of field measurements (point-to-point resistance checks, resistance-to-ground measurements (when possible), and soil resistivity measurements) does an excellent job of validating existing substations.  Also, computer modeling is the only known way of checking proposed changes to the ground grid to see if they will in fact reduce hazardous voltages, and not increase them.  It would be very expensive to install new grounding and then test it after the fact just to find out the changes actually made the voltages increase (which can easily happen).

In conclusion, we are not aware of anyone actually measuring Step and Touch Voltages at a substation as it is simply to unsafe to do so.  It is generally only done when scientists are doing some form of  Research (R&D).  Also, it would be impossible to measure every possible Step and Touch Voltage at a substation, as you would need to measure every possible position a person could stand in the substation, and every possible object at every angle a person could possibly touch.  The only known way of checking Step and Touch Voltages for Human Safety, is through accurate computer modeling.

E&S Grounding Solutions is of course more than happy to help you out with proper ground grid design and analysis.  But if not us, please get someone with the proper test equipment, computer systems, and expertise to properly analyze your substation.  Bad grounding can result in life-threatening situations.  Please feel free to contact our Engineering Department and one of our engineers will be happy to speak with you about your project, free of charge.

Best regards,

The Engineering Team at E&S Grounding Solutions

 

Photo credit: E&S Grounding Solutions

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