Earth Potential Rise risk with TT earth close to Pole Transformer.

Question

I'm looking at the risks associated with Earth Potential Rise Risk in TT an installation close to a Pole Transformer. I have read from the various DNO's they specify a distance between the HV earth and the LV earth, this I'm taking is to avoid the EPR from the HV side being transferred to the earth of the LV side. 
 This is based on sites being close to or within the distances mentioned above. 
 I have seen sites that have a DNO TN-C-S earth provided, but there seems a risk. In the event of the HV fault, even if the separation between the LV and the HV earths to reduce the EPR transfer to a safe potential. In the TN-C-S earth there will be a voltage on the earth from the HV fault, this voltage will be present at the DNO head cut out. Which will effectively cause a potential between the DNO earth and local earth. 
 I would consider a TT earth to mitigate the above. While the earth will bring in the Local earth voltage if there is one from the EPR from a HV Fault, it will effectively create a equipotential zone at what evert the EPR is at that point. 
 Any advise or references would be greatly appreciated.

mapj1 · Accepted Answer

Well there are a couole of points to note. First HV earthing is not simple - now in the UK, in a buried cable the HV earth usually comes with the supply and has a low impedance back to the transformer, where it is quite common to have a limiting impedance, say 1 ohm so that max fault current on a short run is about 6000A from an 11kV supply (each phase 6k5 to ground and 11k phase to phase) But overhead is normally 11kV phase wires only no earth or neutral, so the HV earth electrode then carries the full can to operate expulsion fuses or what is in effect the worlds largest RCD at the supply end. (though it is done with separate current transformers and special breakers) 
 At the end of a long run (upto perhaps 10km on an 11kV line) the potential fault current is a lot lower - think hundreds of amps. 
 During the time the HV ADS is operating the region around the overhead fed 11kV /400V transformer is distinctly hot - not the full 6000 volts of course but equally well over the few hundred the LV side would be able to shrug off. The solution to such a 'hot site' is to have separated HV and LV earth - the LV earth may be running down the next pole along to the one with the transformer on. Of course a town site with underground HV and a probbaly a pretty substantual earth mat is a 'cold site' where rise of earth is not expected to be much, Then HV and LV earth can share metalwork. 
 The field around any electrode of any shape dies off as the distance increases, and from far enough away all earth structures are point like - but this is like saying if they could grow long enough hair all animals would be the same size and spherical - great on the blackboard but not useful in theory, as you can almost never be 'far enough' away. 
 Equally most of the voltage drop is in the near field region and that is where we need to worry about step voltage etc. 
 (so put 230V into a typical 4ft rod, and (ignoring the electricity bill and the moral wisdom..) if you plot contours of declining voltage, once you get more than a rod length or so away, the less than 50V are left. (unless you have odd ground with buried pipes or something). 
 The earth mat under the substation may have dimensions of 10m or so, and the near field region is comparable, but very dependent on the detail of the shapes. 
 
 The trick is to keep these near field regions from overlapping if the electrodes are to be independant. 
 M.

Earth Potential Rise risk with TT earth close to Pole Transformer.

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