🔍 Key Concepts

• On-line fuse testing means the circuit is energized and you use a voltmeter, not an ohmmeter. • Across a good fuse there is almost no voltage drop; across a blown (open) fuse you will read full line voltage when referenced to the live side of another phase. • In a 3-phase fused disconnect, the top lugs (L1, L2, L3) are the line side and the bottom of FU1, FU2, FU3 are the load side going to the motor and control transformer.

💭 Think About

• Which test point on fuse 1 will definitely change its voltage condition (live vs dead) if that fuse opens, while the test point you compare it to stays live regardless of fuse 1’s condition? • For each choice, ask yourself: if fuse 1 is good, what voltage should I see between those two points? If fuse 1 is blown, what should I see? Do any choices give the same reading whether the fuse is good or bad? • Look carefully at which options compare the line side of fuse 1 to another phase, and which compare the load side of fuse 1 to another phase. Which of these actually depends on whether current can pass through fuse 1?

✅ Before You Answer

• For the pair of points you pick, one should always be on a known live phase that does NOT depend on fuse 1. • The other point must be on the side of fuse 1 that will go dead if fuse 1 opens, so the reading changes clearly between normal and blown conditions. • Eliminate any option where both points are on the line side of the disconnect, or both are guaranteed live even if fuse 1 blows, because those cannot tell you anything about fuse 1’s condition.