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.
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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.