Due to deregulation and the use of renewable power, especially solar power, the proportion of PV system is rapidly increasing and most of them are connected to distribution networks to supply power to the grid as well as local loads. There are many power quality issues to be considered with PV systems and one of the main issues is islanding. It is important to estimate and detect an islanding situation quickly and accurately as it can lead to serious plant damage if the grid is suddenly reconnected. This paper investigates the characteristics of ROCOF relays which affect the operation responses and analyze the ROCOF performance for PV systems islanding detection. The equivalent impedance value V2/P is introduced as an interlock function to cooperate with ROCOF relays to avoid false operation during the non-islanding situation. It is shown that it can block nuisance tripping signals quickly and accurately. The range of effectiveness is established for the small power imbalances where there is a small potentially non-detection zone due to the overlap range of the impedance value.