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How do you get safe Step and Touch Potentials when using waterproof surface materials?

Original Question:

Project_description: Designing Earthing/Grounding in dolomite area.

There is a cut/fill. Fill material is G6/G7. It is proposed to surface ground with concrete interlocking bricks to prevent water ingress to the prepared terrace. The usual practice is to used crushed stones.

Question: How do I get the safe and touch potential with the concrete sacrificing the resistivity offered by crusher stones. And  what alternative surface materials with waterproofing and good resistivity are available in the market for use?

Hi Sipho,

Thank you for your question regarding surface materials and Step and Touch Voltage hazards, it is our pleasure to help.

Doing a proper Step and Touch Voltage Hazard analysis is a very complex task.  Not only must you consider the soil resistivity model for your site, but you must also design a copper grid that protects personnel in all weather conditions, including rain.  Rain can be a problem as it can damage foundations and prepared terraces.  Proper selection of surface materials will obviously be a key factor in providing personnel safety.

In substations, the large transformers contain many hundreds of gallons of flammable oil.  Concern for fire in case of a leak generally prevents the use of any surface materials other than crushed rock.  Concrete and asphalt are typically banned in these cases as oil fires will spread across water-proof surfaces.  It is much easier to contain an oil fire when it doesn’t spread, thus the crushed rock requirement.  But this can be a problem for rain as it will certainly seep through the rock.  Often these transformers are placed on top of large concrete ‘swimming pools’ filled with crushed rock.  The volume of the pools are designed to accommodate the volume of the oil in the transformer.  When rain occurs, pumps activate to remove the rain water via drains installed at the bottom of the pools.

If a substation is your scenario, you may find that something along the lines of what is described above may be your best option.  If not, you will need to come up with some other solutions.

So how do you provide proper Step & Touch Voltage Hazard protection?  It is a combination of removing electrical energy from the grid (electrodes), balancing electrical energy (grids), bringing electrical energy closer to people and equipment in certain cases (bonding), and taking it away in other cases (insulation).   Frankly, it can get very complex and is a topic that is more than we could ever really discuss in a simple blog.  Here are a few links you should review:

http://www.esgroundingsolutions.com/about-electrical-grounding/what-is-step-and-touch-potential-and-resistance-to-ground.php

http://www.esgroundingsolutions.com/blog/category/step-touch-voltage-hazards

http://www.esgroundingsolutions.com/our-reading-of-earth-resistivity-is-coming-avarage-2800-ohm-meter-we-are-designing-1133-kv-co-generation-substation-what-are-the-next-steps-required-for-designing-earth-mat-for-70-x-33-meter-area

Now, if your site does not have an oil-fire hazard, rebar in concrete is a great way to eliminate touch voltages.  This may sound odd at first, but there are two (2) ways to reduce the Touch Voltage Hazard: 1) Is to place insulation between the hands and the feet in order to prevent current flow. 2) To reduce the difference in potential between the hands and feet so that current wont flow to begin with.  Think of the metal plates that are installed at substations for people to stand on when throwing switches.  These plates insure that the feet and hands are at the same potential, just like the birds on the wire.  No difference in potential = no current flow.

Keep in mind that both of the above methods will benefit from having an electrode system that is designed to dissipate electrical energy into the earth and away from personnel.  But that is another aspect of proper design that would need a full discussion.

The problem is that you must design these systems accurately or else you place people and equipment at risk.  The only real way to do that is via computer modeling and fault simulation software.  We recommend and use the CDEGS grounding software package; we would of course be glad to help you with the design.  We will only need some soil resistivity data and electrical fault characteristics from you and we can handle the rest of the process via email.  We look forward to working with you.

Best regards,

The Engineering Team at E&S Grounding Solutions

 

Photo credit: E&S Grounding Solutions

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