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 3517 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

    We have had instances where large industrial buildings are built on ground made up of crushed hardcore and concrete which we have excavated and introduded additional electrodes or plates encased in Marconite with little or no improvement in soil resistivity. 



    Sounds like you might have to go (a lot) deeper then to get a decent resistance. Making better contact with upper layers is likely to be defeated if a layer below is dry/airy enough to pose a high resistance to Earth proper.

      - Andy.
Reply
  • 0

    We have had instances where large industrial buildings are built on ground made up of crushed hardcore and concrete which we have excavated and introduded additional electrodes or plates encased in Marconite with little or no improvement in soil resistivity. 



    Sounds like you might have to go (a lot) deeper then to get a decent resistance. Making better contact with upper layers is likely to be defeated if a layer below is dry/airy enough to pose a high resistance to Earth proper.

      - Andy.
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