Residual Current Monitoring (RCD / RCMU)
The detection of leakage current flowing to earth rather than returning through the intended conductor, which indicates an insulation fault and a possible shock hazard. Transformerless PV inverters include an internal residual current monitoring unit sensitive to both AC and DC leakage.
Because a transformerless inverter provides no galvanic separation, a fault on the DC side can produce a direct leakage current that an ordinary type AC residual current device will not see and that can even blind it. This is why PV installations specify type B or type A EV/PV-rated devices where an external RCD is required, and why the inverter's own monitoring is not optional. Persistent low-level leakage in damp weather usually points at water in a connector, a chafed cable at a roof penetration, or a module junction box, rather than at the inverter itself.