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Earth rod resistance

AlunW
Hi

Hoping someone can point me in the right direction!

On site we are trying to review our existing distribution earthing arrangements.  We have 11kV or 6.6kV distributed to local substations where we drop to 415v locally.  We generally have PNB systems with common HV equipment and LV neutral earth arrangements.  Typical one shown below
ebe497219bf639b0954936dc05b950cf-huge-substation-earthing-arrangements.png

I have reviewed several 'local authority' documents which suggest a maximum of 20ohms for earth rod resistance, but looking through BS EN 7430 it suggests on a similar arrangement in A.2 a maximum of 1 ohms?  Could anyone kindly advise what it should be? or point me in the right direction?

Many thanks

Al
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  • Former Community Member
    0 Former Community Member
    The earth resistance mentioned in the standard is for combined electrode resistance in that substation say for example if that substation consists of only four earth rods then each rod should not be greater than 4 ohms i.e R1= 4,R2= 4,R3= 4,R4= 4,  then the equivalent resistance is              1/Req = (1/R1+1/R2+1/R3+1/R4) = (1/4+1/4+1/4+1/4) = 1, therefore  Req= 1 ohm, but still this 1 ohm value should not be taken into consideration for all cases because the earth resistance totally depends on ground potential rise, step and touch potential which has to be calculated, you can refer IEEE-80 for calculation, but it is always advisable to maintain earth resistance to a very less value because in the distribution substation there will be frequent earth fault and due to this  high current will flow through ground which increases the ground potential rise and due to this step and touch potential also gets developed,if the earth resistance is very less then the ground potential rise also will be less and so the step and touch voltage,hence it is always good to maintain very less earth resistance.
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  • Former Community Member
    0 Former Community Member
    The earth resistance mentioned in the standard is for combined electrode resistance in that substation say for example if that substation consists of only four earth rods then each rod should not be greater than 4 ohms i.e R1= 4,R2= 4,R3= 4,R4= 4,  then the equivalent resistance is              1/Req = (1/R1+1/R2+1/R3+1/R4) = (1/4+1/4+1/4+1/4) = 1, therefore  Req= 1 ohm, but still this 1 ohm value should not be taken into consideration for all cases because the earth resistance totally depends on ground potential rise, step and touch potential which has to be calculated, you can refer IEEE-80 for calculation, but it is always advisable to maintain earth resistance to a very less value because in the distribution substation there will be frequent earth fault and due to this  high current will flow through ground which increases the ground potential rise and due to this step and touch potential also gets developed,if the earth resistance is very less then the ground potential rise also will be less and so the step and touch voltage,hence it is always good to maintain very less earth resistance.
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