Licq has nailed it.
It's exactly that, these are generating stations/sub-stations/transformer sites, where the on-site earth potential can become faulty and rise to (get this) 10,000V difference between that and our remotely transmitted Telephone Exchange earth !!
Have worked on lots of these sites and in a nutshell, there is a need to install LIU (Line Isolating Units) to protect the equipment being plugged into the broadband circuit. Now then, i have only ever worked on the copper side of things, so uncertain how this pertains to FTTP ??
As an aside, I remember one guy at our local generation station who took me for a walk around the place and it was a pretty damp day, you could 'feel' the static energy, quite scary if i'm being honest.