Log in to the online community

Want to post a reply? You'll need to log in  Having trouble logging in? Read our help guide
Lightning electrodes
lyledunn
641 Posts
Question

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. 

32 Replies
broadgage
977 Posts

I would be inclined to try a new earth electrode in place of the worst one, a reading that high might suggest that most of the electrode has rotted away.

Alternatively, could all the down leads  be connected together, in a ring. If buried bare copper tape is used for these connections then that will provide an extra connection to earth, in addition to linking together all the existing down leads.

lyledunn
641 Posts

I am seeking the possible consequences rather than a fix. So what if one electrode is well outside code? 

broadgage
977 Posts

If lightning strikes the rod with a high resistance to earth, then the down lead and anything attached thereto will briefly be at a very high voltage to true earth.

Anyone touching the down lead would probably be killed.

The high voltage will force some of the current through the walls and via the foundations to earth. This can dissipate several megawatts as heat in the walls, albeit for a tiny fraction of a second. Any moisture in brick or stone will flash into steam with explosive force, perhaps bringing down the wall.

Wooden parts such as roof timbers may be ignited. Metal rainwater goods and metallic roofing materials will conduct dangerous voltages to other parts of the building, destroying electrical equipment and perhaps starting fires.

 

mapj1
4321 Posts

Are the collection electrodes connected at the top, for example by a ridge wire ? - if so, the effect is nothing like as dramatic - just the current does not divide evenly and the ground bounce is highly asymmetric. There  is greater chance of damage to things like telephones and so on due to induced voltages. The maximum strike that can be tolerated before damage occurs is reduced.

In practice a side strike to a well connected electrode is likely.

Mike.

 

 

broadgage
977 Posts

Agree that if the different electrodes are interconnected at high level, then a high resistance connection on one is unlikely to have dramatic consequences.

My reply was presuming that each lighting rod, down lead, and earth rod was “stand alone” and not interconnected.

 

Zoomup
4098 Posts

His wonders to behold…..

The biggest implication is probably the insurance cover. Assuming that, as it is a parish church, it is Ecclesiastical Insurance, I believe that they require the lightning conductors to be tested every three years and a report issued to show it is being maintained in a satisfactory condition. If this is not the case (i.e. no report or not maintained satisfactorily) then the insurance may not cover damage caused by a strike. As the code (I assume you mean the published BS EN) is used to define ‘satisfactory’ then if it doesn't meet the code the insurance is questionable.

I am guessing that the ‘small parish church’ refers not only to the size of the building but the size of their finances also and that you are trying to keep the cost to a minimum for them. While this is commendable, if there were to be a lightning strike and then problems with the insurance you wouldn't have done them any favours so replacing the dubious electrodes is probably the best option. Also remember that they only get worse, and so proving that they will probably still protect the church today doesn't mean that there will be the same level of protection one year down the line.

The idea of lightning conductors is being misunderstood. Whilst a direct strike is possible, it is very unusual. The purpose is to reduce the electric field above the building, which is why the electrodes on high points have sharp “points”, and therefore reduce to chance of a nearby discharge. Direct hits on most systems will produce damage to some level. Many kA will pass along the conductors and the potential will be dangerous whatever one does. Ohms law of several kA and 10 Ohms is? It is unlikely that the electrode resistance will make all that much difference, at these voltage and current levels the electrode resistance is highly non-linear anyway. The COP is as usual a guide to new systems, as most of you realise, getting a 10 Ohm Earth is difficult at the best of times, and the electrodes may well need to be multiple and long! Modern systems tend to use the foundation concrete, but this will not be present in an old building. 

For some reason too the lightning in the USA tends to be more violent, probably due to temperature and humidity differences. The voltage required to discharge through a km of the atmosphere is enormous, although the capacitance to a cloud is small. 1 million volts and dryish conditions needs a gap of about 1 metre on a bad day, the grid at 400kV has spaces of about 3 metres worst case, so is resistant but the arc horns will flash over even with a nearby strike. The insulators provide considerably more distance than this although crackle a fair bit when it rains.

lyledunn
641 Posts
5291b486ded760689cdadfb4c37ec751-origina
lyledunn
641 Posts

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?

lyledunn
641 Posts

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! 

 

lyledunn
641 Posts

 

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. 

lyledunn
641 Posts

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

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

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

Zoomup
4098 Posts

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

Denso tape perhaps.

Z.

mapj1
4321 Posts

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.

 

broadgage
977 Posts

Historically there was a reluctance to fit lightning conductors to Churches, on the grounds that the lightning was sent by God, for His reasons, and we mere mortals should not attempt to defy the will of God.

A proponent of the then new lightning conductors stated that cold winter weather was also sent by God, but that the Church accepted the use of overcoats in winter !

ISTR reading of a huge disaster in Italy. A vast store of gunpowder was being kept ready for a war, and was stored in the church, this being considered a suitable place to store explosives.

During a thunderstorm the local populace sheltered in the church, ringing the bells to ward of the lightning. The church was struck by lightning, the gunpowder exploded, and most of those sheltering therein killed.

I read somewhere that Benjamin Franklin ,who studied lightning, had opposition to fitting lightning conductors to churches for this very reason.

lyledunn
641 Posts

Thanks Mike. Yes it is a complex subject. I rather think that is the attraction for me but unfortunately reports, plans, invoices, quotations and general day to day business prevent me from any deeper diving. I am glad that this forum has people like you who are infinitely more clued-in than me!

I have test instruments that allow the FOP and stakeless methods, as well, of course, as the standard earth loop. Just out of interest, we used all three with results being fairly similar. Comparing the FOP result with the loop results we figured that the DNO connection to earth was around two ohms or below. 
So our results show an electrode well outside acceptable as far as code recommendations are concerned but the overall resistance below that required 10 ohm value. As David Z indicated, the resistance may only be a lesser component in what is a non-linear arrangement. 
With the greatest respect to the LPS contractors, I doubt whether they could offer an answer as to the seriousness of the situation other than to say it’s outside code and here’s a quote for fixing it. 
 

 

 

lyledunn
641 Posts

I should also say that Alasdair‘s advice is likely what will be followed. I was just interested in the actual physical risk rather than codes and insurance requirements. 

gkenyon
1853 Posts

lyledunn: 
 

 I suspect that previous tests were done without removing the test link at each down conductor! 

The test link would not be removed if the 2-clamp (or double-clamp) method is used? But I agree it's far from good practice not to inspect the connection of the tape to the electrode.

Share:

Log in

Want to post a reply? You'll need to log in  Having trouble logging in? Read our help guide