As best I can tell, this spec does not address the issue of grounding, specifically the relationship of the 30vdc to earth/ground/EGC potential (of the building AC lines powering the driver which generates the DC voltage). In other words, does this refer simply to the potential between the two conductors (in a two-wire cable), or is some reference to ground (i.e. the EGC in the building supplying the DC voltage) implied. Presumably the latter, since otherwise the 30vdc could have an arbitrary offset from ground. Given that, must the '-' conductor (or either conductor) be tied to EGC potential, or is it simply required that neither conductor be more than 30v different from ground or the other conductor ? (For example, in an adjustable system, the '+'conductor 30vdc above ground, and the '-' conductor variable between ground and 30vdc).
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.