A sectionalizer is a current monitoring device which isolates faulty sections of electrical systems. It is generally used alongside other protective devices such as breakers and reclosers. The sectionalizer contains a current monitoring device which senses the current.
The sectionalizer trips by both sensing the current and sensing the absence of voltage when the upstream breaker has tripped.
When the breaker closes and the supply resumes, the sectionalizer in the faulty line remains open. The supply is thus resumed to the other healthy feeders.
The sectionalizer is not a interrupting device. It cannot interupt a circuit. It can only isolate a de-energised circuit. Hence, it is is always used in conjunction with the immediate upstream breaker. Sectionalizers are relatively inexpensive equipment used to isolate and localise a fault.