time. If the fault current has not been interrupted following a set time delay from circuit breaker trip
initiation, the CBF protection will operate, whereby the upstream circuit breakers are back-tripped to
ensure that the fault is isolated.
CBF operation can also reset all start output contacts, ensuring that any blocks asserted on upstream
protection are removed.
2.8.1 Circuit Breaker Fail Implementation
Circuit Breaker Failure Protection is implemented in the CB FAIL column of the relevant settings
group.
The circuit breaker failure protection incorporates CB Fail Timer. For any protection trip, the CB Fail
Timer is started, and normally reset when the circuit breaker opens to isolate the fault. If breaker
opening is not detected, the CB Fail Timer times out and close an output contact assigned to breaker
fail (using the I/O assignment feature). This contact is used to back-trip upstream switchgear,
generally tripping all infeeds connected to the same busbar section.
2.8.2 Circuit Breaker Fail Logic
Figure 6: CB Fail Logic
CBF elements CB Fail Timer can be configured to operate for trips triggered by protection elements
within the device or via an external protection trip. The latter is achieved by allocating one of the opto-
isolated inputs to CBF Init signal using the I/O Mask feature.
It is possible to reset the CBF from a breaker open indication from a protection reset. In these cases
resetting is only allowed provided the undercurrent elements have also been reset. The resetting
options are summarised in the following table:
CBF function Reset options Description
I<
CBF function will reset when the phase currents and earth current is less
set current
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