Methods for measuring insulation resistance in a photovoltaic (PV) array may include partitioning the PV array into groups of PV panels, isolating a group of PV panels selected for an insulation resistance measurement from other groups of panels by setting bypass selectors on each PV panel in the PV array, and making an insulation resistance measurement for the selected group. If a measured value of insulation resistance for a selected group corresponds to an insulation problem in a PV array component, a separate measurement of insulation resistance may be made for each PV panel in the selected group. Insulation resistance measurements may be made accurately and rapidly for large PV arrays without disconnecting and reconnecting cables between panels. Measurements may be made at frequent, regular intervals to permit changes in insulation resistance to be detected before damage from dielectric breakdown occurs.