General information
When a fault occurs on a branch (T-tap) of a 12 kV overhead distribution line or in a customer's network, the protection time delay of the incoming switching device cannot be properly coordinated with the protection delay of the substation's outgoing feeder circuit breaker — as a result, the outgoing feeder circuit breaker at the substation trips. If the fault is permanent, auto-reclosing at the substation is unsuccessful, and a local fault within a single medium-voltage customer de-energises the entire 12 kV line.
If an outdoor sectionalising circuit breaker is installed at the branch point or at the customer's service entrance (at the point of ownership demarcation), it detects that the fault (overcurrent, short circuit, single-phase earth fault, etc.) has occurred in the customer's network and isolates it automatically — customers on healthy sections continue to receive power.
