Earth resistence in MV/LV network

[bojleric]

Hello everyone... I would like to start a discussion about earth resistance in distribution networks. I work for a distribution company in Croatia. Due to our regulations we are obligated to perform measurement of the earth resistance for every single MV/LV distribution station every 4 years. By measuring the earth resistance we control whether the values of touch voltage are within the permissible range (according to the HD 637 S1:1999 Power installations exceeding 1 kV).Basically, all of our MV/LV stations can be divided into two groups:
1. the ones that are connected to the global earthing system
2. ones that are not connected to the global earthing system.
Group 2 are the stations that are powered with MV overhead lines, and whose LV network is also mostly overhead lines (rural areas). Measurement of earth resistance of such stations we perform using standard UI method.
Group 1 are the stations that are placed in the urban areas and are powered by MV underground cables. Such distribution stations are interconnected with PE conductors, cable sheets and other installations (gas, water etc). UI method for measuring earth resistance of such stations is particularly impossible due to the fact that you can't achieve „good“ neutral earth. In regulations it is stated that for such stations it is not necessary to perform measurement of earth resistance of the MV/LV stations but to effectively check it’s (MV/LV station) connection to the global earthing system. Each one of ours MV/LV stations that are powered with MV underground cables is connected with others MV/LV stations (and of course with HV/MV station) with PE conductor (copper cable 25 mm2) and cable sheath. How can, one easily check the connectivity of a MV/LV station to the global earthing system (beside visual inspection). What are regulations on this matter in other countries and how do you deal with this matter?

 

[Miguel Villarroel]

OK, Do you have one earthing systems for all, LV/MV and HV? Why every 4 years?

If there was no electricity there would be no internet. Good point, don´t you?

 

[bojleric]

As I said in my first post, the MV/LV stations in urban areas (cities), when speaking about earthing, are interconnected with PE conductors and cable sheets which means that in cities we have „one global earthing system“. Why 4 years… Well our new domestic regulation says that we are obligated to do inspections every 4 years.

 

[Miguel Villarroel]

What kind of connectors usually use?

If there was no electricity there would be no internet. Good point, don´t you?