low-voltage wet locations and EGC
Article 100 of the 2017 NEC has the following definition of voltage:

Voltage (of a circuit). The greatest root-mean-square (rms) (effective) difference of potential between any two conductors of the circuit concerned.

Article 411 covers installation guidelines of equipment falling under UL 1838. It states the following:

Where wet contact is likely to occur, the limits are
15 volts ac or 30 volts dc.

Therefore, the voltage between any of the conductors (including ground) must not exceed the maximum of 30 V.
Article 411 also addresses grounding and isolation requirements:

(A) Grounding. Secondary circuits shall not be grounded.
(B) Isolation. The secondary circuit shall be insulated from the branch
circuit by an isolating transformer.