The scheme shown in Figure 18.17 is unusual since it uses the breaker auxiliary "a"
contacts in the trip circuit to control the backup timer in case the fault current level is below
the pickup of the fault detector relays (50BF/2 and 508F/3), which might be the case for a
low level transformer fault or low level unbalanced currents. This is a bit unusual since the breaker "a" contacts may have been partly the cause of the breaker failure. In this case they
are required for low-level faults.