Earth Resistivity of 11 KV Transmission Lines

In each case, is the prospective fault current sufficient to operate the circuit protective device(s)? The settings for this/these device(s) cannot be seen from here.

How many ohms are the insulators from which the lines are hung? Why do the poles matter at all?

Good question. Perhaps the original poster will explain.

I presume you are referring to earth resistance of earth pit/s for each pole. If this is the case then the values are on the higher side. As per standards this value shall be within 10 ohm.

You can use 2/3/more number of earth pits for each pole and may use bentonite to improve the earth resistance.

Firstly , you details is very sketchy. No line configuration mentioned, nor if it is covered conductor or open wire. Span lengths? However, rock is not a great earth conductor as we all know. I have to assume that the holes were bored and the poles planted using the existing spoil. Was any mix added. i.e sand, cement, good stone free soil, conductive cement? If not, how was the pole earth installed ? Wrapped and coiled around the pole butt, crows foot or a simple 7m lead out? If none of the mentioned, then what is your primary reason for earthing each pole? Lighting protection in the area. Is it a high lighting area? Pole top fires will not occur on steel poles so that is ruled out. If it is for lightning, then run your earth above the line, connecting each pole to main earths each end of the line, or dropping it to a point where earth continuity is good in the ground, i.e. a field of at the bottom of the hill where the soil is offering good readings of 10 ohm and less. If it simply for earthing poles and offering some safety, simply string the earth below the OHL (Step and touch). Does the line need earthing? If it is for lightning, the earth will attract lighting. If you do not earth, the lighting will travel the line and be dissipated and controlled at the surge arrestors. Also if 85 ohms is the best you can get, then you have to accept that, if costs is a factor. So you need to know why you are earthing the OHL poles, the line configuration and insulator values and clearances. If you have lots of rain as you mention, then earthing should not be a big hassle for you. What soil resistivity do you get on the wet and dry readings of the pole back fill? Perhaps try putting in earth mats around the poles, especially where you have pole mounted switches, gang links, drop out fuses etc, for operator protection. And change the back fill to a less stone filled mix. Big costs will be involved, lots of digging up and importing of new back fill around poles, if you want it down to 10 ohms and less. Every steel pole has a earth tag, top and bottom, the jumper around the pole top should be connected to the tag, effectively earthing the earth conductor and pole as one single earth. And an earth wire to the pole base tag, the bare copper either buried directly in the ground or run out to a earth spike in good soil.

Acid rain may be creating conductive salts on the metal surface of the poles.

LOTS of WD40!

If they are new galvanised steel poles there will be acid washed off them as it rains and a white salt will appear when it dries. It will take about 3-5 years to weather the poles and all that acid will be in the surrounding ground. Possibly creating acid sulphate soil, and that will be fun. Good luck and it is an easy fix if costs is not a hassle. DO Let us know what you decide to do as a rectification . I always like to hear how it goes and what the solution is and why it was chosen. Looking forward to it. Cheers.

What standard are you working to as the information should be in there (and it varies from standard to standard, country to country)?

Sounds like you need to put more effort and expense in to creating better earth pits to lower the resistance (to whatever the applicable standard says), which seems too high.

Also see previous CR4 threads on the subject of earth pits for more information. There are techniques and earthing compounds you can use even in rocky terrain to get a good enough earth connection.