Ground Faults

A DC ground fault occurs when a live conductor — positive or negative — makes unintended electrical contact with the grounded system structure or earth. This creates a parallel current path that can bypass overcurrent protection entirely, since fault current flows through the earth rather than back through the fuse. In ungrounded or high-impedance grounded systems, a single ground fault may not trip any protection device, leaving it active and potentially sustaining arc conditions.

A developing ground fault almost always begins as an insulation degradation — a soft fault where resistance between conductor and earth is falling but has not yet reached zero. Riso testing at 250–1500V DC detects this early-stage degradation before it becomes a hard fault. Measuring RISO+ and RISO− separately identifies which pole of the array has the compromised insulation, significantly narrowing the search area for physical inspection.

Inverter ground fault detection circuitry is often more sensitive than a handheld Riso instrument, particularly for dynamic faults that appear only under operating voltage or temperature conditions. Check for intermittent faults by testing Riso at or near system operating voltage (1000–1500V DC rather than 500V), and test at different times of day when module temperature and therefore insulation behaviour differs. Also inspect connectors and cable entry points for physical damage.

Yes. In a shared negative topology (common in transformer-based inverters), a negative pole ground fault on one string creates a fault current path that involves multiple strings in parallel. The fault current magnitude depends on how many strings are paralleled and the fault impedance. In ungrounded floating systems, a single ground fault does not create a shock hazard or fault current, but a second ground fault on the opposite pole creates a bolted fault that can be very high current.