The frequency indicates the health of the interconnection. In North America, 60 Hz represent the "healthy" value of the frequency.  Small variations on either the low or the high side are acceptable and are the result of the normal switching on or off of loads and the reaction of AGC control to increase and decrease the generation. However, following the occurrence of unexpected failure of equipment that results in large loss of generation, the frequency starts to decay quickly and, unless the system reacts promptley and effectively to arrest this decay, the disturbance can result in collapse of large portions of the interconnection.  The interconnection capability to succesfull recover from such disturbance depends fundamentally on the following three factors:

1. Interconnection  Self Regulation.

2. Interconnection Inertia

3. Settings on the Governors of the interconnection generators.