Still, existence of external BFI signals may contribute to
spurious BF operations when testing the ZPR (a human error
of not isolating the BFI signals) or loss of BF protection
dependability (a human error of not restoring the BFI signals
after testing). These errors can be more or less likely,
depending on the location of the cut-out switches (outputs
from zone IEDs versus inputs to BF IED) and the applied test
procedure.