Which device disconnects a generator from the system bus when its voltage is lower than the battery voltage?

The device that disconnects a generator from the system bus when its voltage is lower than the battery voltage is the reverse current relay. This relay monitors the voltage levels and ensures that when the generator's voltage drops below the battery's voltage, it prevents reverse current flow from the battery into the generator. This is critical as allowing reverse current flow can lead to potential damage to the generator and create unsafe operational conditions.

In this scenario, the reverse current relay effectively acts as a safeguard, maintaining system integrity by ensuring the battery doesn't attempt to "feed" the generator when it is not producing sufficient voltage. This operation is vital in maintaining proper voltage regulation and overall system stability.

The voltage relay, while related, typically serves to disconnect a circuit under specific voltage conditions but does not necessarily manage the interaction between the generator and battery in the same way as the reverse current relay. The current limiter is designed to restrict the amount of current flowing, and the load switch is used to control the distribution of power rather than managing voltage disparities between the battery and generator. Thus, the reverse current relay is the most fitting choice in this context as it directly addresses the concern of managing the interaction between a generator and the battery based on voltage levels.