46 CFR §111.05-19

1. (a)If the voltage of a distribution system is less than 1,000 volts, line to line, a tank vessel must not have a grounded distribution system.
2. (b)If the voltage of a distribution system on a tank vessel is 1,000 volts or greater, line to line, and the distribution system is grounded (including high-impedance grounding), any resulting current must not flow through a hazardous (classified) location.