Which wire or wires in a four-conductor 240 VAC circuit should be attached to fuses or circuit breakers?

In standard American electrical wiring for a 240 VAC circuit, power is supplied by two 'hot' wires, while the neutral and ground wires serve as return and safety paths. Fuses or circuit breakers must be installed exclusively on these ungrounded, hot conductors. If a fuse or breaker were installed on the neutral or ground wire and it tripped, the appliance would stop functioning, but the internal circuitry would remain energized with dangerous voltage from the unbroken hot wires. By fusing only the hot wires, you ensure that all hazardous voltage is completely cut off from the device during an overload or short circuit.