A major safety issue in grid-connected photovoltaics is to avoid nonintentional operation in islanding mode when the grid being tripped. Worst-case conditions under which islanding can occur are specified analytically. The circuit that is commonly used for testing is described. The issue of appropriate test conditions with regard to reactive power injection to the grid is discussed and the stabilizing impact of rotating machines and resonant circuits is evaluated in detail. Islanding test results for small inverters are presented. They confirm that very simple islanding protection methods that are commonly used, are likely to fail, if inverters are loaded with considerable capacitance. The obtained results allow the assessment of the islanding protection function. They emphasize important points when defining new certification procedures for upcoming guidelines and standards.