A lightning conductor is only as good as its earth, so the resistance between the tower's tape and the ground is measured every year with a tester and three spikes, and it is written on a certificate. A good earth is under ten ohms. This tower reads lower every year, and the trend goes through zero in about three years, and zero ohms into clay is not a thing.
I test lightning protection, and churches are most of my round because they are tall and old and insured.
How an earth is tested.
The conductor runs from the spire down the tower to an earth electrode, a rod or a plate in the ground. You disconnect it at the test clamp, put a tester across it, and drive two spikes into the ground at set distances, and the tester passes a current and reads the resistance of the earth. It should be under ten ohms and it should be stable, because ground is ground: the number moves with the seasons, a little, and that is all.
The reading goes on the certificate with the date, and the certificates are kept, so the history of an earth is a column of numbers.
One tower's column reads: eight point one, seven point four, six point six, five point nine, five point one, four point three, three point six, two point eight, two point one, one point three, and this year, nought point four.
Ten years. A straight line, near enough, downwards.
An earth does not improve. It gets worse, as the rod corrodes and the soil dries, and a falling resistance usually means water, a broken drain, a rising table. A falling resistance that falls in a straight line for a decade means something else, and I do not know what.
Nought point four ohms is a superb earth. It is better than the earth at a substation. The next reading, on the line, is about zero, and after that the line goes negative, and a negative resistance is not a thing the ground can be.
What has been checked.
The tester. Two of them, calibrated, and both read the tower the same and read every other church on my round the way they always have.
The electrode. Dug out in 2020 for inspection. A copper rod in clay, corroded a little, connected to the tape, with nothing else in the hole. Reinstated. The reading that year was five point one, on the line.
The clay. Sampled at the electrode and at a control point across the yard. The two samples have the same resistivity in the laboratory, which means the ground at the electrode is ordinary clay and should read what ordinary clay reads, which is what the control reads: eleven ohms, every year, flat.
So an ordinary rod in ordinary clay reads a number that is walking down to zero at a rate you could set a calendar by, and eight feet away the same clay reads eleven.
What a good earth means for the tower.
A strike goes to earth cleanly. That is the point, and the tower is, on paper, better protected every year. The insurer is pleased. The architect is pleased. I have the only face in the room that is not.
What is done about it.
The certificate is issued, nought point four, pass, and the trend is in my file and nobody else's, because a certificate has a box for the reading and not for the ten before it.
I have shown the column to the diocesan engineer. He said it was a very good earth.
What I do.
I test it.
The one thing I have changed is that I do not hold the tester's leads with bare hands at that tower.
The tester passes a small current and you feel nothing, ever, at any earth. Gloves are not in the procedure.
At the last test, at nought point four, with the leads in my hands and the spikes in the yard, I felt it. Not a shock. A pull, in the wire, towards the ground, the way a fishing line pulls when something has taken the bait a long way down, steady, and the tester read nought point four, and I let go of the lead and it lay on the grass and did not move, and I put my gloves on and finished the test and I will do it in gloves next year, and I have decided that I will not do it the year after.