I also do the "Hot" test to verify the meter [tester] works, then open the circuit and re-test.
LOTO the heck out of any and all disconnecting means, even pull fuses out of switches if there's any possible jokers [Linked Image].

Got hit once from 277 VAC, and it hurt plus shook me up quite bad! Glad to say only hit once [also glad to be alive today to say I got hit once!]

Canadian Sparky,
As to your question about the 347Y/600 Wye system, you guys in the "Real Great White North" [the one that Bob and Doug are from, plus the one north of Washington, Idaho, North Dakota, Wisconsin, Etc.] are the only North American country with it.
Down here, we have a 3 phase 3 wire Delta, which is 600 VAC, but the only low voltage [under 1 KV] 4 wire Wye systems are 208Y/120 and 480Y/277.

Depending on the size - in square feet - of a commercial / industrial structure, the system voltage from the utility company could be 208Y/120, 480Y/277 and upto 4160Y/2400. All this to get the required KVA into the building.
Of course, normal office lighting would be dangerous at 2400 VAC - switches would be killing people left and right!, so the better lighting voltage for large structures is 277 VAC.

Scott SET

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!