The IET is carrying out some important updates between 17-30 April and all of our websites will be view only. For more information, read this Announcement

Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 7 replies
  • Subscribers 31 subscribers
  • Views 3518 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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

Parents
  • 0
    I must admit I'd be wondering what was wrong with the second no 5.  and no 4 Is not that hot either.

    But there is no problem saying that it does not meet both the test conditions in the book.


    1) is the effective parallel resistance less than 10  ohms ?


    1/Rtotal  =   1/ 8.51   + 1/22.4  + 1/13.82 +  1/ 71.3 +  1/42.8   1/ 38.4 +1/112.4


    Rtotal = 3.8

    (note that if  all 7 electrodes had been 10 ohms each we would see 10/7 = 1.4 ohms)

    so YES

    and

    2) but resistance if any one electrode  < 7*10 = 70 ohms ?

    NOT TRUE


    What the rules are really saying is that Ideally the combination should be less than 10 ohms,  and also that the upper limit for planting a new one would be a reading over 70.

    you could, just, have 7 electrodes of 70 ohms each and pass both tests.

    Given the idea is that the electrodes share equally, and we do not get one side of the building at a much higher voltage than the other during the flash, I'm not that convinced that even if this one  only just met the requirement it would be that great. An additional/ replacement electrode is probably going to be  needed. How does this compare to historical readings - is it degrading or has it always been this poor ?

Reply
  • 0
    I must admit I'd be wondering what was wrong with the second no 5.  and no 4 Is not that hot either.

    But there is no problem saying that it does not meet both the test conditions in the book.


    1) is the effective parallel resistance less than 10  ohms ?


    1/Rtotal  =   1/ 8.51   + 1/22.4  + 1/13.82 +  1/ 71.3 +  1/42.8   1/ 38.4 +1/112.4


    Rtotal = 3.8

    (note that if  all 7 electrodes had been 10 ohms each we would see 10/7 = 1.4 ohms)

    so YES

    and

    2) but resistance if any one electrode  < 7*10 = 70 ohms ?

    NOT TRUE


    What the rules are really saying is that Ideally the combination should be less than 10 ohms,  and also that the upper limit for planting a new one would be a reading over 70.

    you could, just, have 7 electrodes of 70 ohms each and pass both tests.

    Given the idea is that the electrodes share equally, and we do not get one side of the building at a much higher voltage than the other during the flash, I'm not that convinced that even if this one  only just met the requirement it would be that great. An additional/ replacement electrode is probably going to be  needed. How does this compare to historical readings - is it degrading or has it always been this poor ?

Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines