5.1 loss of utility voltage
To prevent islanding, a utility connected PV system shall cease to energize the utility system from a de-energized distribution line irrespective of connected loads or other generators within specified time limits.
A utility distribution line can become de-energized for several reasons. For example, a substation breaker opening due to fault condition or the distribution line switched out during maintenance.
Of inverse (single or multiple) have DC SELV input and have accumulated power below 1 kw then no mechanical disconnect(relay) is required.