Plant Grounding - Step and Touch Voltage 1

[NickParker]

Why is plant grounding not based on step and touch voltage calculations? Why just substations(IEEE-80)?

 

[DBL-E]

First off, if you have a PV site larger than 5 MW, then the plant should be designed to meet IEEE touch/step voltage criteria per the NESC.

The IEEE 80 guide provides the calculations for touch/step limit as well as the background on the concepts of grounding. The IEEE 80 guide provides practical guidance for applying the concepts to a substation environment. The grounding concepts are more generally applicable so you see other standards reference IEEE 80. For example, IEEE 2778 and 2760 provide more practical guidance on designs for PV and Wind generation, but both point to IEEE-80 for touch/step voltage criteria.

As the Wind and PV industries were are growing, there have been new firms needing guidance for applying the grounding concepts for their particular project. There are many in the industry that misunderstand and misapply IEEE 80 as it is a dense guide that is not covered in typical university/college programs.

 

[waross]

In regards to industrial plants it has a lot to do with voltage levels.
Touch and step voltages are roughly a percentage of the faulted voltage.
The same percentage voltage developed by a 230,000 volt fault current will be almost 500 times less at 480 Volts.
EG: A 230 volt touch voltage from a 230,000 Volt fault will be 230 x 230.000/480 Volts at for a 480 Volt fault.
That is about a 1 Volt touch voltage for a 480 Volt fault. (If you want to use 277 Volts, then take the L-N voltage of 230,000V = 133,000V and the answer still comes out to about 1 Volt.)
Also, in a plant, the fault current often follows bonding conductors and structural members.
At plant levels, there is seldom much fault current conducted through the soil.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[7anoter4]

IEEE 665-1995 [ WITHDRAWN ]IEEE Guide for Generating Station Grounding
In preparation is the standard:
P665 Guide for Generating Station Grounding[PAR]

https://standards.ieee.org/faqs/pars/

 

[Kiribanda]

Most of the industrial installations, the distribution is by cables. Therefore, there is no way for the GF current to pass through soil.
As an example, if there is a ground fault inside a motor installed 100m away from MCC, the GF current is taken back to the MCC by the
bonding conductor (in IEC cables by the armour or PE, in tray cables by the grounding conductor laid with the cable). Therefore, there is no
STEP, TOUCH, GPR to be calculated using the soil resistivity.
But at the same industrial installation, it may be fed by a 25kV distribution line terminated at a small substation consisting bkr, disco switch
etc etc. At that setup if there is a GF at the sub, then 99% of that GF current is taken back to the Utility source through the soil. That means,
if there is person standing on the soil of that substation he is subjected to STEP, TOUCH & GPR due to soil resistance. Hence the modeling should be done
per IEEE 80 knowing the soil resistivity & the max. GF current.

 

[cuky2000]

It is common to find medium voltage an ocasional higher voltage in Power plants and industrial facility. The floor are build with concrete slabs with a lot of reinforce rebars and steel platform bonding columns, steel members and equipment with grounding conductors. The foundations also perform as a natural grounding electrodes.
From the practical perspective, the floor, foundations and steel floor behave as an equipotential mesh. Therefore, step and touch potential is a non issue for those applications since components above floor are virtually at the same potentials during a ground fault event.