Sub Feed Panel

[don_resqcapt19]

Re: Sub Feed Panel

crossman,
I think that you are thinking of the earth as a single path of resistance, but it reality it is multiple paths and each of these paths are in parallel with each other and that is way the human in really in parallel with the other earth paths.

Question: The current that flows through the person's body... where is it going?

The same place that all current goes...back to its source.
Don

[ January 04, 2005, 12:37 PM: Message edited by: don_resqcapt19 ]

 

[crossman]

Re: Sub Feed Panel

WOW! My mind is changing and my eyes are opening! I made another drawing and am beginning to see what is going on thanks to the help of you guys. Very cool!

What I see here is that person just MAY be in parallel with a large percentage of the electrodes resistance to ground. Before, I was assuming that the resistance of the earth was spread out evenly all along the path to the xfmr. If most of that resistance is indeed around the 30" of ground rod, then I see what yall are saying.

In this instance, he is definitely in parallel with 30 ohms of ground resistance and in for a possible nasty shock.

Obviously the possible voltage depends on the nature of the earth and the electrodes. Down here in our wet black clay soil, resistances of or 2 or 3 ohms on a ground rod are typical.

Seems that if the guy is standing right on the ground right on top of the electrode, he will be safer than if he is further removed from the electrode.

Also, the more electrodes you have and the better their continuity to earth is, the less the shock hazard. Right?

 

[crossman]

Re: Sub Feed Panel

Calculation of above with a 4 ohm connected load on one phase of the panel gives a shock of 67 volts.

Now that is with the high impedance to ground.

With a low impedance of say, 2 ohms in the 30" sphere, then the shock voltage is reduced to about 9 volts.

So I am seeing that a grounding electrode has the potential to reduce the "touch voltage" if the soil and electrode characteristics are good, but if they aren't so good, then ZAP! WATCH OUT!

I really appreciate you guys bearing with me on this and I definitely have a better understanding of the whole thing now. Awesome.

 

[iwire]

Re: Sub Feed Panel

Crossarm I really do like the drawings they add some fun along with some clarity to the subject.

These forums have taught me more than all the classes I have attended. It is a great place where we all learn.

I would like to add something about your latest drawing you posted at posted 01-04-2005 12:44.

It is true the power is heading back to the center tap of the source transformer however due to the utilitys use of a single MGN for everything and the bonding at each house the current path through the earth is likely to be in all directions.

Bob

 

[crossman]

Re: Sub Feed Panel

iwire, thank you for the additional info.

This thread ended up being entertaining and educational to boot. I very much enjoyed it. Sometimes it pays to leave the ego at home and listen to and think about what others are saying.

The key to that paragraph^ is "and think about". I am not one to rotely accept what someone else says. I want to know the reasons WHY it is the way it is. You guys certainly helped get my brain to churning.

Now, for the next round of theoretical discussion, I'm gonna talk about the parallel paths around the neutral. Should be fun! But not tonight... maybe tomorrow.

Thanks again to everyone for all the comments.

 

[iwire]

Re: Sub Feed Panel

Originally posted by crossman:
Now, for the next round of theoretical discussion, I'm gonna talk about the parallel paths around the neutral. Should be fun! But not tonight... maybe tomorrow.

Crossarm feel free to call me Bob and I am glad you found your way here.

Now you got me on the edge of my seat as I always get involved with 'parallel paths around the neutral' type discussions.

You want to talk about 250.32(B)(2)?

How about metal raceways between meter sockets and service panels?

Or how about the insistence of some people to run a EGC between a meter socket and service panel when PVC is used as a raceway?

Bob