Why must a "sectionalizer" be on the load side of an OCR?

A sectionalizer is a device used in electrical distribution systems to isolate faults and is designed specifically to work in conjunction with an Overcurrent Relay (OCR). When placed on the load side of an OCR, its primary function is to detect and interrupt fault conditions after the OCR has operated.

The correct answer emphasizes that the sectionalizer does not possess the necessary capacity to interrupt a faulted circuit by itself. This limitation is crucial because, in a fault situation, the OCR is responsible for detecting overcurrent conditions and tripping the circuit breaker to isolate the fault. The sectionalizer, therefore, operates on the premise that it will only deal with the residual current after the breaker has opened, ensuring it is only required to switch during normal load conditions and not under fault conditions.

This setup is strategic because if the sectionalizer were placed on the supply side, it would be exposed to full fault current, leading to its inability to operate effectively without possibly leading to catastrophic failure. Thus, positioning it on the load side allows the sectionalizer to function safely and effectively without the risk associated with fault conditions. In this configuration, the sectionalizer is used to re-energize the circuit after the fault has been cleared, ensuring system reliability and safety.