Earth surface potential and earthing electrode shape 1

[hansforum]

Hi,

I am trying to build small earthing electrode in very limited space for a house that doesn't have it. It will be used with RCD (deltaI 300mA or 100mA) in system with 240V. I am choosing between straight horizontal stripe (around 7 m long) and two horizontal stripes (3.5 m long) crossed at the center of each other at 90 degrees. It would be at 0.8 m deep. Calculated with formulas form this link:

http://admin.copperalliance.eu/docs...-constructional-aspects.pdf?sfvrsn=6&sfvrsn=6

, formula number (7) it will be around 55 ohm with rho=300 ohmM. My question is how to minimize step potential voltage especially in longitudinal direction ? If I connect cable at the middle of the horizontal stripe would it be better to install vertical earth rods at the ends of the stripe 0.8 m deep to lower step potential in longitudinal direction or small V shape horizontal electrode. The same question is for two horizontal stripes crossed at the center of each other at 90 degrees.

 

[7anoter4]

If RCD will trip the supply within maximum 0.3 sec the Etouch50 –calculated according to IEEE 80/2000 ch. 8.3 -should be not more than 240 V-live to ground.
If two strips are employed- B as per Copper Table I for four-arm symmetrical-the electrode resistance will be only 34.8 ohm then the maximum trip time could be 0.5 sec.
However, in my opinion usually a RCD will trip in less than 0.1 sec.

 

[hansforum]

Does someone know formula for calculating earth surface potential for horizontal stripe earth electrode in longitudinal direction. I know how to calculate in transverse direction but I don't know how to calculate in longitudinal direction.

Is it safe to connect earthing electrode of the house with the earthing of the concrete utility pole that supports electrical wires (240V)?

 

[7anoter4]

EPRI presented a calculation way -explaining a software developed by EPRI in 1982.I have not this publication with me now. However, I tried to follow this way ten years ago. Attached here it is a copy. I hope this could help someway.

 

[7anoter4]

In my opinion, the equipment grounding conductor [the grounded conductor installed in the load part of the utility supply disconnected means] has to be connected to a grounding electrode existing in the supplied installation perimeter. So, the pole stands outside of this perimeter in the supply side of the disconnected means. However, a bonding jumper has to be provided between both grounding conductors.
If the grounding electrode it is a water pipe or the grounding resistance is more than 25 ohm- according to NEC- a supplementary electrode is required.

 

[Kiribanda]

hansforum,
If you are using 100mA RCCB for personal protection (normally should be 30mA), then why one should be worried about high step & touch potentials?
It looks like that you are following IEC60364 which allows a maximum touch voltage of 50V. That means the RCCB tripping current
multiplied by earth fault loop impedance = 50V. This earth fault loop impedance includes the earth (ground) electrode resistance
too. In other words, the usage of a RCCB allows you to have a fairly high electrode resistance, even you do not have to
measure it.