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Lightning electrodes

A relatively small parish church has 8 down conductors connected to rod electrodes. Each electrode measured separately ranged from 215 ohms to 12 ohms with the overall value being around 8 ohms. Now I am aware that the overall value should be less than 10 and that each individual electrode should be no more than 8x10. We have one at 215 and one at 135, the others meeting that requirement. It is easy for me to advise that the system does not meet code but I do not have the expertise to assess the implications of the situation. I would appreciate your opinion. 

  • 5291b486ded760689cdadfb4c37ec751-original-194504c8-b288-4f87-ab2b-772e95db72a1.jpg
  • Been in 21 year, tested by a contractor every 3 years each electrode tested separately at around 4 ohms! 

  • Can you explain what that shows Lyle, I assume that the resistance is zero?

  • The invitation to tender required the periodic inspection of the fixed wiring and the testing of the lightning earth electrodes. It was not within our remit to deliberate on the effectiveness of the LPS. However, I must say that my interest in the subject was very much kindled and it is that which needs scratched.

    The lids of the inspection pits were extremely difficult to lift. Even the church warden said that he didn’t think they were ever lifted before. I suspect that previous tests were done without removing the test link at each down conductor! 
    As for experts who ply their trade in this field and having glanced through the DEHN technical document on LPS, I have no hesitation in elevating them in regards to esteem.

    However, on another church, the expert company are looking £2500 to reduce the electrode resistance from 10.8 ohms to what they say is an absolute minimum of 10 ohms! 

     

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    Sorry David, indeed it doesn’t show much. I was attempting to provide a picture of the arrangement at each pit. As you can see, the top of the electrode isn’t much bigger than what would be used in a wee domestic TT job. The down conductors are flat approx 38x3mm covered  in a plastic material. This goes below ground and is attached to the top of the rod some how. There appears to be a hessian type material wrapped around the connection which prevents inspection of the final connection. Rod resistance at this one measured around 215 ohms using the electrode resistance test on my megger 1741. Much  the same result was achieved using the loop method and the DNO earth. 

  • I should also say that we filed clean the top of the rods and also tested from here. Made no difference. 

  • Former Community Member
    0 Former Community Member

    Electrodes for for lightning protection is outside the scope of BS 7671

  • The LPS is more EICR type money for old rope, valueless!

  • There appears to be a hessian type material wrapped around the connection which prevents inspection of the final connection.

    Denso tape perhaps.

    Z.

  • the diameter is not unusual - what we do not know is how deep it goes - it may well have failed at a threaded joint, as such things are often driven in stages, and deeper then a house electrode, or there is a buried mesh and again it has come detached. 

    If the small parish church is supplied by a small parish transformer on a pole in the small parish churchyard be aware that a test using a loop tester includes the DNO electrodes, which on a single pole pig may be comparable resistance  or even higher than those of the LPS. 

    Foundation electrodes are very good for TT and small surges. They can have the weakness of the concrete cracking if the worst happens in terms of LPS.

    In this case, 200 odd ohms has ‘broken’ as the unwritten label as i is a figure suggest something has come off under ground leaving just a short rod.

    Lightning voltages and the megavolt/metre rule do not scale for lightning - rather the discharge has more in common with a chain of unevenly charged capacitors breaking down, where only one in the chain has to be stressed enough to fail, and then overstressing the next weakest.

    Clouds are 5-10km up and at a megavolt per metre, you may think gigavolts are involved, but not really, more like about 20- 50MV and a lot of jumps of a few tens of metres at a time, as short regions of space are stressed and then break down, until a continuous discharge is formed.  This more modest voltage is part of the physics justification of the rolling balls model where the decision of what is within side-strike distance is modelled as being within a sphere of radius proportional to strike voltage.

    It is complex.

    Mike.