"But all
earthing electrodes are required to be bonded together into one system.
That includes any lightning rod system. "'

It may or may not have been in that case, I was never at the site. I did not wire the place. Thing is, with a large place there can be a ground gradient and even if bonded together like in the basement, there can still be a potential difference. I do not know the size of the place, the grounds may have been tied together, say with 12 guage wire, but it was so long it allowed a little bit of voltage drop. Video is generally 1 volt P-P. That means 100mV is 10 IRE, certainly noticable. Half that would be noticable. Double it and you are getting close to interfering with sync.

All I know is the isolator worked. I did not wiring on the place at all so it is not my problem. Whatever the situation, the guy is an electrician but that does not mean he wored the place. He could have bought it that way.