The timing chart in Figure below reflects the timing sequence of 138 kV breakers in some substations. 
At Power Plants, the required critical clearing time was calcultated to be 10 cycles. The recommended total clearing time was 9 cycles as shown in Figure below. In order to achieve this total clearing time, it was necessary to purchase a breaker failure relay that had a short fault detector dropout time. The special BF relay is selected because its instruction manual reflected a dropout time of less than 1 ms. 