Both the rms frequency and the rms deviation hide the low-frequency values. In this case study, the under-frequency values reached about 10 times the severity in amplitude of the rms frequency and lasted about twice the duration of the over frequency. The over frequency and under frequency show values which better reflect the severity. During the 10-s interval, part of the time was under frequency and part of the time was over frequency and this is well reflected in the over frequency and under frequency assessment method. The equation to use is function of the monitoring objectives.
Frequency is used to drive some motor driven clocks connected on the power system.
The precision of the frequency is not so important as long as the frequency is reduced or increased for adjusting the clock. In this case the average of value is the most appropriate factor to use. However, with the evolution of crystal clock, this factor is not so important in many countries. The ac power frequency is used today for synchronous motors in many industrial processes. In some cases, the over frequency could affect as much as the under frequency.