When Must Systems Be Grounded? T&B.

Abstract: This article explains the difference between grounded and ungrounded electrical systems. In a grounded system, a ground fault will cause the circuits to open. In an ungrounded system, a single ground fault will not interrupt the circuit. Ungrounded systems pose a hazard though, because another ground fault in a different phase of the system occurring at the same time could cause a phase- to-phase short. The National Electric Code suggests using ground fault detectors with ungrounded systems.

This article also lists the National Electric Code requirements for grounding wires. Every interior wiring system with alternating current rated 50 volts to 1,000 volts should be grounded. The article contains the individual specifications for different volt levels.