With power available at the fuses and the motor disconnected from line, the following voltage readings are taken: Top of "Fu 1" to bottom of "Fu 2" reads 450 VAC Top of "Fu 1" to bottom of "Fu 3" reads 450 VAC Top of "Fu 2" to bottom of "Fu 1" reads 0 VAC Top of "Fu 3" to bottom of "Fu 1" reads 0 VAC What condition is indicated? See illustration EL-0062.
• Three-phase 450 VAC systems: any two healthy phases will measure about 450 VAC between them. • What happens to the bottom terminal of a fuse when that fuse is blown versus when it is good (continuous). • How to interpret a meter reading of 0 VAC vs. full line voltage between two test points.
• If you read 450 VAC between the top of FU1 and the bottom of FU2, what does that say about whether FU2 is still connected to its phase or completely isolated? Apply the same thinking to the bottom of FU3. • If the top of FU2 is definitely hot, yet you read 0 VAC between it and the bottom of FU1, what must be true about the potential (phase) at the bottom of FU1? • Try each answer choice in turn: assume that specific fuse (or fuses) are blown, sketch which points are hot or dead, and ask yourself, "What voltage would I see between each pair of points?" Which assumption matches all four readings?.
• Make sure you decide which bottom terminals are energized (at line potential) and which are floating for each fuse condition you test. • Verify that your chosen fuse condition explains all four meter readings, not just one or two of them. • Double-check that a reading of 0 VAC means the two points are effectively at the same potential, while 450 VAC means they are on different healthy phases with a complete path to each point.
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