Which device is used to disconnect a circuit during fault conditions?

The device used to disconnect a circuit during fault conditions is the RCD, or Residual Current Device. RCDs are designed to protect against electric shock and electrical fires caused by earth faults. They monitor the electrical current flowing through a circuit and can detect imbalances in the current that may indicate a fault, such as a short circuit or ground fault. When an imbalance is detected, the RCD quickly disconnects the supply, typically within milliseconds, thereby reducing the risk of serious injury or damage.

In contrast, while a fuse serves a similar protective function by melting and breaking the circuit when there's excessive current, it does not specifically detect earth faults or differentiate between a fault condition and normal operation. A switch is used for manual disconnection of the circuit but does not respond automatically to fault conditions. Lastly, a transformer is used to change the voltage of electrical power but does not serve any protective function during fault conditions. Thus, the RCD is the most appropriate device for automatically disconnecting a circuit when a fault occurs.