Which wire or wires in a four-conductor 240 VAC circuit should be attached to fuses or circuit breakers?
Why A four-conductor 240 VAC feed has two ungrounded "hot" conductors plus a neutral and a safety ground. Overcurrent protection goes only in the ungrounded conductors, because those are the ones that can deliver fault current from the source. The National Electrical Code prohibits putting a fuse or breaker in a grounded (neutral) or grounding conductor, since opening one would leave the equipment chassis or circuit energized with no return or no safety path.
Watch out The choice that fuses all wires sounds safer but is actually dangerous: if the neutral fuse blows first, the load stays connected to a hot line at full potential, and a broken ground connection removes the very protection that trips the breaker during a fault.
Fuse the hots, never the neutral or the green wire. Protection interrupts the source, not the return or safety path.