Correct answer: B — Open conductor fault
Series fault and phase imbalance
A series fault changes or interrupts the normal path through which current flows along a conductor. When one phase conductor breaks, its series path becomes open. The other phase paths can remain connected, so the three phases no longer have identical circuit conditions. This makes an open-conductor fault an unsymmetrical fault.
For example, consider a three-phase feeder with one broken phase conductor. Current in the broken path is interrupted, while currents in the remaining phases depend on the load connection and available return paths. The voltage and current sets become unbalanced. An induction motor connected to such a supply may experience single-phasing, reduced torque and overheating.
This differs physically from a short circuit, which creates an unintended connection between phases or between a phase and earth. A series open circuit need not produce the very high current associated with a short circuit, so protection must also recognize conditions such as phase loss or negative-sequence current.
Opening one or two conductors produces unequal phase conditions. Simultaneously opening all three identical phase paths is a balanced interruption and is not the unsymmetrical case intended here.
