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

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Overcurrent protection is placed in the ungrounded conductors, so in a four-conductor 240 VAC circuit only the two hot wires are fused or breakered. Interrupting the neutral could leave equipment energised through the remaining hot conductor with no return, and interrupting the equipment grounding conductor would remove the fault path that makes the circuit safe.