My employer: Hydro One.
Those are phase markers; important to ensure different phases are not inadvertently connected to one another should it be desired to transfer a load from one feeder to another; particularly important when two feeders are run on opposite sides of the same pole, especially when the phases on one feeder have been rolled while those on the other side of the same pole have not been.
See the pic @
where four 28 kV feeders leave their supply station heading load-ward; depending on how the transition is performed, any phase could end up in any position.
On a separate note: thank you, Bill, for your gentle reminder that terminology can vary from country to country, and not to ridicule the way things are done elsewhere.
As to Ontario Hydro, its official name at the time of inception was The Hydro-Electric Power Commission of Ontario [and many poles today still bear HEPC markers]; it underwent a demerger and ceased to exist in 1999, which, because I was working as a power system operator at the time, is how I ended up with H1. I was working in the Toronto area at the time, and was in regular contact at the time with the Toronto Hydro operators...who, incidentally, experienced the reverse fate when Etobicoke Hydro, York Hydro, North York Hydro, East York Hydro and the original Toronto Hydro et al were amalgamated...
The legacy of the generation mix supplying Ontario being almost 100% hydraulic generation was many years ago; that era ended post World War II.
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]