Hi can anyone help?

I've got a situation where we are installing a 2MVA transformer, and the Earth Nest array doesn't meet the 1 Ohm value set by BS7430 (as most earth nest can't), but we've been asked to confirm what the touch voltages / step voltages would be if we had a worst case value of 3.5 Ohm for the earth nest, and to check if these are within limits.

I've calculated the Fault current (40.917 KA or 29.41 MVA), and the relative impedance of the transformer (0.00587 Ohms) but any calculation I've found regarding the Touch Voltages seem to ignore the Earth Mat value (basically using the impedance of the circuit conductor Zp & the earth loop impedance Zel, which in essence is the Zs=ZE+(R1+R2) values).

If the Earth Matt impedance is considered, the figure is that much greater (i.e. in Ohms & not milliohms), that it throws the whole calculation out.

I assume that the calculations assumed that the Transformer is solid grounded, i.e. the impedance to earth is that low its can be ignored, but how does the earth nest value affect the Earth fault calculations or the touch voltage calculations?

Please help...

Hi there,
First calculate the rise of earth potential (RoEP) during the fault. This is the earth impedance of the earth 'nest' multiplied by the ground-return current. The latter is not the fault current figure that you have but the ground-return current, i.e. that portion that returns through the ground. The earth nest resistance should be taken into account when calculating this.

To calculate the touch voltage, you then need to know the potential on the soil, 1m away from the earthed plant of interest. This can be calculated using the formula in the standards. The touch voltage is then the RoEP - soil potential.

The permitted touch voltages are dependent upon your protection clearance times, e.g. for the 0.2second clearance time usually applied to 275kV and 400kV sites, the permitted levels are 1kV for soil ground cover and 1.4kV for chipping ground cover. For slower protection clearance times, the permitted levels are lower.

To calculate step voltages, you need to know the soil potentials at two points 1m apart.

Step and touch voltage calculations can also be carried out using earthing system simulation software in greater detail, which can take into account soil resistivity and a 3D model of the earthing system.

Hope that helps.

M. Taylor

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Matthew Taylor B.Eng C.Eng MIET PhD

Edited: 23 November 2006 at 11:12 AM by taylormj4

Matthew,

Thanks for your reply:

If its not too much trouble, do you have the formulas for calculating:

Ground return current - assume this is the earth fault current?
which standard, BS7430? and which formula for the potential on the soil?

cheers,

Rob

Hi Rob,
You have to be careful here. Earth fault current can mean the gross fault current feeding into an earth fault from the busbars. Some of this current can return to source via tower line earth wires, cable sheaths, healthy phases etc. As these portions of the earth fault current do not flow to earth through the earth grid, they do not cause a rise in earth potential. You need to calculate the earth return current, i.e. that part of the earth fault current that returns through the ground. If there are no parallel return paths, the earth fault current and the earth return currents will be the same.

Other standards that will help are ER S34, EA TS 41-24, IEEE 80, IEEE81 but the best way by far is to use computer simulation software as you can calculate touch and step voltages right across a site and then find the maximums of each, which are location specific with respect to the earthing system size and shape.

M. Taylor

-------------------------
Matthew Taylor B.Eng C.Eng MIET PhD

Robert,

M Taylor has effectively answered your question(s).
For further guidance you can also consult (incl. latest amendments):
EA Engineering Recommendation S34:1986 - A guide for assessing RoEP at SSs
BS7354:1990 - Code of Practice for design of high voltage open terminal stations
The Copper Development Association (www.cda.org)

Conwell N. Magoche