Aside from 120/240V, there is an onteresting North American variation similar to that used for British construction power. New in 1999, NEC Articles 640 and 530 ‘Technical Power’ for audio equipment uses an otherwise dedicated and isolated center-tap-grounded transformer secondary.
NEC ARTICLE 640 — Audio Signal Processing, Amplification, And Reproduction Equipment §640-7(b). “Technical Power” is a separately-derived system for reduction of 60Hz noise propagating through audio systems. [Center-tap grounded—termed “60/120V”. Center-tap conductor not permitted for serving loads—but locally, solidly-grounded only]
Referenced in 250-146(d) 640-22 640-7(c) 640-2 Technical Power System, and
530-73. Receptacles.
(2) All outlet strips, adapters, receptacle covers, and faceplates
shall be marked as follows:
WARNING — TECHNICAL POWER
Do not connect to lighting equipment
For electronic equipment use only
60/120 volt 1ø ac
GFCI protected
