Lightning electrodes

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.

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

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.

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.

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.

Frightening……

Grand Isle St Gerard church hit by lightning - Bing video

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.