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Lightning Protection.

Former Community Member
Former Community Member
Good morning all.


Part of our electrical contracting services involve lightning protection testing and maintenance. On more than one occasion now we have come up against a situation where the resistance of 1 or more of a group of electrodes is greater than 10 Ohms, i.e  

R1 = 8.51

R2 = 22.4

R3 = 13.82

R4 = 71.3

R5 = 42.8

R6 = 38.4

R5 = 112.4


The IEE electrical maintenance publication calculates maximum resistance to earth should not exceed the number of electrodes X 10. On the example above 70 Ohms would be the maximum indicating R4 & R5 would not comply. When Lightning protection companies commission lightning protection systems their certificates use the resistances in parallel method: R TOTAL = 1 divided by (1/R1 + 1/R2 + 1/R3 + 1/R4 + 1/R5 + 1/R6 + 1/R7) In this case the total resistance of the system would be 3.28 Ohms.


Although the test complies with the overall resistance of the system the individual electrode resistance does not. The publication seems to contradict itself here. Please see attached highlighted sections. Improving the resistance of the earth is not practicable as the surrounding area in encased in concrete in most cases.


Should I be issuing a satisfactory certificate for the installation with comments on the individual electrode readings?


Kind Regards

Stephen Cherry

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  • 0
    Andy -  the "new" (2008 onwards) standard does retain the 10 ohm limit, with a note it may not be possible on a rocky base, and keeps the old way of one drop, one electrode (calling it 'type A' earthing), but  add an option to ring round linking all the electrodes near (above or below) ground level, to create a 'type B' earth, so in effect  the electrodes are all paralelled up at the bottom of the drop lines, to some extent equalising the worst effects of any resistance imbalance. In such a case, the fact that an odd electrode is a bit weak is then a lot less serious.

    Also the rolling sphere replaces cones, and a really complex risk assessment replaces a fairly complex one, but usually justifies retaining the existing system or only making small changes.

    Steve, is converting to type B easier than sinking new electrodes ?



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  • 0
    Andy -  the "new" (2008 onwards) standard does retain the 10 ohm limit, with a note it may not be possible on a rocky base, and keeps the old way of one drop, one electrode (calling it 'type A' earthing), but  add an option to ring round linking all the electrodes near (above or below) ground level, to create a 'type B' earth, so in effect  the electrodes are all paralelled up at the bottom of the drop lines, to some extent equalising the worst effects of any resistance imbalance. In such a case, the fact that an odd electrode is a bit weak is then a lot less serious.

    Also the rolling sphere replaces cones, and a really complex risk assessment replaces a fairly complex one, but usually justifies retaining the existing system or only making small changes.

    Steve, is converting to type B easier than sinking new electrodes ?



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