46 CFR §112.20-5

1. (a)If there is a reduction of potential of the normal source by 15 to 40 percent, the loads under § 112.15-1 must be automatically supplied from the temporary emergency power source.
2. (b)For systems in which a reduction of frequency of the normal source or final emergency power source adversely affects the emergency system and emergency loads, there must be means to transfer the loads under § 112.15-1 to the temporary emergency power source upon a reduction in the frequency of the normal source or final emergency power source.