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Can you ignore the effects of Step & Touch Voltage hazards on concrete pads because of the rebar mesh and the low resistance?

Jagadish asks us:

In a solid grounding system is it appropriate to exclude evaluation for GPD or GPR, touch and step potentials just because the resistance is gowing to be lower than 1 ohm by calculation using IEEE 142 method using the (large mass of concrete and rebar in the slab) ufer grounding. How do evaluate galvanic corrosion, anticorrosive coating for rebar, bonding of disimilar metals inside the concrete?

Hi Jagadish,

Thank you for your question regarding GPD and GPR evaluations for solidly grounded systems on concrete slabs, it is our pleasure to help.

Solidly grounded systems are subject to Ground Potential Rise (GPR) events, and therefore Ground Potential Difference (GPD) events as well.  Ultimately, GPR events are based on ohms law which is simply the fault current times the resistance-to-ground of the ground system.  So, even if your rebar system is 1-ohm, what would be the GPR for a 50,000 amp fault? It is of course, 50,000 volts!  Is this good or bad?

This is where GPD comes in.  A plain steel rebar system may have some big differences in potential from one end of the grid to the other side.  If your concrete encased rebar grid has a resistance from one corner of the building to the other side opposite coroner of 1-ohm, you will see 50,000 volts form across the building. Yes? Keep in mind, it is not uncommon to see the resistance-to-ground of a buildings foundation to be 1-ohm, however the corner-to-corner resistance can be several ohms.  When this happens, you can actually form some serious differences in potential horizontally.  This is where time domain and leakage current calculations come into play, but that is another topic.  Also, this is why you see so many copper ground rings installed around buildings.

A complete answer to this topic would really take some time, however the short answer is, “No”.  You may not exclude the effects of a GPR event simply because you assume a low resistance rebar system.  It just does not work that way.  In fact, in certain cases grounding engineers will actually add resistance or remove grounding in certain areas to avoid GPR issues that can result.

One last point, you cannot simply assume that any rebar system is less than 1-ohm.  Every day, we deal with massive concrete encase rebar grids that measure in excess of 100-ohms!  It all depends on the soil the concrete is sitting on.  And of course, if your concrete slab has a vapor barrier or if the rebar is coated in plastic, you cannot use the slab as an electrode at all.

You should also remember that copper is 12 to 17 times more conductive than steel, and is at least 250 times less magnetic.  When dealing with AC currents, the magnetism of the steel is very important to consider as the magnetic fields that form in the steel take time to form and collapse, this keeps the fault currents in your system longer, much longer.  Amperage is time based (one coulomb past a given point in one second), so the difference between clearing a fault in 6 cycles, versus 120 cycles, can be a matter of life or death.  Copper paths are the best way to keep magnetism down and the time domain of the fault to a minimum.

Hope this helps.  Please feel free to contact us again in the future should your have any further questions.

Best regards,

The Engineering Team at E&S Grounding Solutions

 

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