Relation between earthing grid resistance and touch/step voltage.

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hisham1986

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Dear all ,

Assume we have a site where we have 13.8KV/380/220 V transformer (neutral point grounded) feeding generic loads in a distant building which in turn (the building itslef) has its own earthing grid installed around its perimeter.To cut it short the adopted grounding system is TNC-S, but the designer has added an earthing grid at the installation (around the building) and has connected bonding conductors from that grid to all grounding bus bars in all the panels inside the building , as we know already protection against indirect contact in TNC-S system is either provided by tripping of over current protection device ( Circuit breaker) within a acertain allowable time ususally 0.4 sec for a 230V system voltage or the use of differential protection device (residual current circuit breaker) , the designer is claiming that he has added earthing grid around the building itself in order to reduce the touch/step voltage that arises during an insulation fault mainly Line to Ground Fault , would any one please answer this question thoroughly:

How does having an earthing grid buried around the building reduces the touch/ step voltage during an insulation fault and how does contact/step voltage decreases when the earthing grid resistance decreases as well?
 
Unless you build an equipotential plane, there is very little reduction in step potential. The ground ring only raises the voltage for a small area above the ring.
 
Welcome to the forum. One reason many of us can't answer your question thoroughly is we don't fully understand the TNC-S and other designations for grounding methods. That system is not used in North America and this forum is for the NEC which has a different grounding philosophy than the many alternatives presented in the IEC standards.

What is meant by E=I*R is if the ground grid reduces the R, the E for step or touch is reduced for the same fault current.
 
Bob, I thought TNC-S is what we used. Combined conductor for neutral and grounding on the distribution and separate conductors for the neutral and grounding at the building.
 
The only way to really reduce the step and touch potentials is to clear the fault. Additional connections to the earth do not do the job.
 
hisham1986 said:
Dear all ,

Assume we have a site where we have 13.8KV/380/220 V transformer (neutral point grounded) feeding generic loads in a distant building which in turn (the building itslef) has its own earthing grid installed around its perimeter.To cut it short the adopted grounding system is TNC-S, but the designer has added an earthing grid at the installation (around the building) and has connected bonding conductors from that grid to all grounding bus bars in all the panels inside the building , as we know already protection against indirect contact in TNC-S system is either provided by tripping of over current protection device ( Circuit breaker) within a acertain allowable time ususally 0.4 sec for a 230V system voltage or the use of differential protection device (residual current circuit breaker) , the designer is claiming that he has added earthing grid around the building itself in order to reduce the touch/step voltage that arises during an insulation fault mainly Line to Ground Fault , would any one please answer this question thoroughly:

How does having an earthing grid buried around the building reduces the touch/ step voltage during an insulation fault and how does contact/step voltage decreases when the earthing grid resistance decreases as well?
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To understand this will take the simple understanding of why a bird can sit on an energized power line that may be many thousands of volts in reference to Earth, to put it in a simple terms as long as a person standing on this grid and can not make a complete electrical path to another conductor (including remote Earth) that would provide a path back to the source of the voltage, then this person is protected from a shock hazard if for some reason the grounding and the protection grids were to become energized and did not open an over current protection device.

The design of a equal potential grid around any source of medium to high voltage substations or transformers is very important that the edges of this grid must slowly go deeper into the ground as sort of a taper so that at the edges of the grid do not present a high voltage gradient at the edges of these grids, otherwise if a person were to step between the edge of the grid and un protected Earth there would be a high voltage gradient that could be an electrocution hazard.

Almost all of our substations here in the US have this kind of grid through out the substation.

I hope this helps in your understanding.
 
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