Lightning Protection.

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