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The following systems which supply premises wiring shall be grounded:
1926.404(f)(1)(i) Three-Wire DC Systems
All 3-wire DC systems shall have their neutral conductor grounded.
1926.404(f)(1)(iii) AC Circuits, Less Than 50 Volts
AC circuits of less than 50 volts shall be grounded if they are installed as overhead conductors outside of buildings or if they are supplied by transformers and the transformer primary supply system is ungrounded or exceeds 150 volts to ground.
1926.404(f)(1)(iv) AC Systems, 50 Volts to 1000 Volts
AC systems of 50 volts to 1000 volts shall be grounded under any of the following conditions, unless exempted by paragraph (f)(1)(v) of this section:
1926.404(f)(1)(iv)(A)
If the system can be so grounded that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts;
1926.404(f)(1)(iv)(B)
If the system is nominally rated 480Y/277 volt, 3-phase, 4-wire in which the neutral is used as a circuit conductor;
1926.404(f)(1)(iv)(C)
If the system is nominally rated 240/120 volt, 3-phase, 4-wire in which the midpoint of one phase is used as a circuit conductor; or
1926.404(f)(1)(iv)(D)
If a service conductor is uninsulated.
1926.404(f)(1)(v) Exceptions
AC systems of 50 volts to 1000 volts are not required to be grounded if the system is separately derived and is supplied by a transformer that has a primary voltage rating less than 1000 volts, provided all of the following conditions are met:
1926.404(f)(1)(v)(A)
The system is used exclusively for control circuits,
1926.404(f)(1)(v)(B)
The conditions of maintenance and supervision assure that only qualified persons will service the installation,
1926.404(f)(1)(v)(C)
Continuity of control power is required, and
1926.404(f)(1)(v)(D)
Ground detectors are installed on the control system.
1926.404(f)(1)(ii) Two-Wire DC Systems
Two-wire DC systems operating at over 50 volts through 300 volts between conductors shall be grounded unless they are rectifier-derived from an AC system complying with paragraphs (f)(1)(iii), (f)(1)(iv), and (f)(1)(v) of this section.