Effect Of Under Ground HV Transmission Cables On Earthing

Provided all cables are installed in accordance with the relevant national electrical code, there will be no effect.

If earth pit resistances are too high then among the solutions is tipping a conductive liquid, such as seawater, into the pit.

Thanks for the details

I am familiar on with US practices, and then only in the Southeast and Southwest areas. We were always careful to only ground the sheath at one point and monitor the current flow at that point. Many installation also monitored the voltage at the remote end of the cable also. If it is not done in this way, a circulating current can be produced that is basically a cable loss and can get high enough to trip relays due to leakage detection. The OP does not state what voltage he considers HV or HT levels. I think of 15000 volts as Medium Voltage (or MV) and when I think of HV I think of 69 KV and above, but usually above 138 KV. I would be concerned with connecting both ends of cable to ground, but I have no idea of the code or practice in Australia. I have thoughts that the HV or HT levels the OP is interested in are quite possible in the realm of low voltage (under 1100 volts) If that is the case, ignore my comments because I don't have much experience there, but I still think that grounding both ends of a sheath is a poor practice and can result in erroneous errors in relaying.

Thank you, well stated and accurate. Especially the last sentence.

I agree that grounding at only one end would be a better way to control ground loops, hence my comment re. the loops. That method is acceptable for short lengths of cable, but the practice allows quite high voltage differences to be present in the sheath on longer runs where the standing voltage between the sheath and earth at the unconnected end can be significant and will be a function of the length of the run, spacing of the cores and the current being carried. Ground fault conditions (even if only in the vicinity of the cable but not associated directly with it), and induction from trains, trams, lightning, etc can raise that figure significantly.

For this reason, single point bonded systems are generally limited to less than 500 metre length and employ an earth cable run parallel to and alongside the phase conductors to carry those fault currents and limit sheath voltage rise to acceptable limits. In these installations transposition of either the phase conductors or the parallel bond is employed to limit circulating currents, and a sheath voltage limiter must be fitted at the unbonded end.

Where circulating currents or cable heating are a concern then cross bonding techniques can be employed to reduce these.

Here is a link to one of our national authority discussions on the subject. Section 2 discusses screen earthing and 2.2.2 states their policy to bond both ends of cable sheaths. I would expect that they employ best practice which is likely the same in most developed countries.

You can decrease the Resistance value by adding some new earthing pits in parellel.

As we know that 1/R = 1/R1 + 1/R2 + 1/R3 + ....., where R= Total Resistance value, R1= Resistance value of Pit no 1, R2= Resistance value of pit no 2 and .....

And also you may decrease the Resistance value of Existing pits by pouring salty water in to the pits.

The different ground resistance measurements are nothing to do with HT cables proximity to your electrodes

The backfill around the cable is most likely sand and that's what your electrode is driven through. Try longer electrodes to get into the native soil below and you may start getting the same R readings as those in undisturbed soil situations where no cable is buried.

here soil resistance is very high, so to get the required grounding we auger the ground rod ("spike") holes minimum 100mm diameter and backfill with low ohm cement

If cables are directly underground laid the earth around the cable is drying due to the heat evacuated from.

This drying usually produces an earth thermal resistance rising but an electrical one could be expected also.

First link is not available

I see. However, if you will type the link in Google Chrome window it will open.