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Temporary Protective Grounds

Temporary Protective Grounds

Temporary Protective Grounds

(OP)
In sizing temporary protective grounds for distribution and sub-transmission voltages, I'm debating how to consider lightning. Would it be appropriate to estimate a current stroke and the associated voltage developed across the conductor/person? I'm leaning toward thinking that it is pointless to consider lightning however, as the Ldi/dt would be so high that no conductor could adequately reduce it.

If I'm mistaken though and you would consider voltage developed due to lightning, how do you determine inductance of the ground conductor? Inductance taken from data tables will always include self and mutual inductance, but as there is no second wire, there will be no mutual inductance. Is it appropriate to just consider the self inductance of the conductor?

RE: Temporary Protective Grounds



For purposes of understanding lightning behavior, it’s analysis and for specifying lightning protection devices, a common model of the lightning pulse has been specified by the Institute of Electrical and Electronics Engineers (IEEE). This model of the pulse is called the “The IEEE 8 / 20 Model” and is commonly used in specifying lightning protection devices. Figure 1 shows the current versus time graph of the pulse. It is generally thought
that this model is equal to or greater than 50% of lightning events. But, it should be remembered that the other half are more powerful than this
model predicts!

http://www.samlexamerica.com/support/documents/130...

In the class, I took at U of I, we used the 8/20 IEEE model. I don't know what the peak should be. The calcs are not too difficult and I posted examples below. In the class, we were modeling the lightning to determine what the grounding impedance would have to be so that you didn't have flashover when the lightning strike bounced off the ground and flashed over on the way back up, voltage doubling. You want your grounding wire to be as short as possible and for the connection to ground to be low impedance so that doesn't happen.


Example U of I lightning stuff

http://www.ece.uidaho.edu/ee/power/ECE524/Lectures...
http://www.ece.uidaho.edu/ee/power/ECE524/Lectures...
http://www.ece.uidaho.edu/ee/power/ECE524/

RE: Temporary Protective Grounds

(OP)
Thank you for the help, good references. In the example, the characteristic impedance is necessary. For that, is it more appropriate to use only the self inductance of a wire then for this case, as there is no other wire and so no mutual inductance between them?

RE: Temporary Protective Grounds

I don't think there would be much mutual inductance since you are doing straight runs with your grounding wires. You aren't going to have loops in them.
 

RE: Temporary Protective Grounds

I have not heard of temporary grounds being sized for lightning protection. The shock testing that formed the basis for IEEE 80 electrocution thresholds had a minimum duration of 0.03 seconds, so IEEE 80 is not applicable to the very short duration of lightning. As the duration of the electric shock shortens, the damage mechanisms change as well as the amount of acceptable current increases.

On a related note, workers performing live line maintenance on transmission lines sometimes use arc gaps to control the flashover location in the event of an overvoltage on the conductor.

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