When does a sectionalizer operate to interrupt fault current?

A sectionalizer operates to interrupt fault current when the backup device is open. This mechanism is designed to work in conjunction with protective devices like circuit breakers. When a fault occurs in a system, the sectionalizer detects this fault through current flow. However, it will only trip and interrupt the current if it recognizes that the backup device, which is typically a circuit breaker, is open.

If the backup device is closed (which would mean it is still in the process of trying to clear the fault), the sectionalizer will not trip. Its primary function is to ensure reliability and minimize unnecessary disruptions in the system, particularly when there is already a protective device engaged. By waiting until the backup device is open, the sectionalizer helps in isolating the fault and clearing the line without causing further complications or damage to the network.

The other options suggest scenarios where the sectionalizer would operate differently. For instance, if the backup device is closed, the sectionalizer would not activate; it does not operate at any random time and should not activate while the circuit is under load since this could lead to unwanted system interruptions.