A 240-volt, single phase fused disconnect that feeds a motor is being tested. Meter readings indicate: Line side A to line side B - 240 volts; Line side A to load side B - 240 volts; Line side B to load side A - 0 volts; Load side A to ground - 120 volts; Load side B to ground - 120 volts. With 100% certainty, what is the state of both fuses?

A 240-volt, single phase fused disconnect that feeds a motor is being tested. Meter readings indicate: Line side A to line side B - 240 volts; Line side A to load side B - 240 volts; Line side B to load side A - 0 volts; Load side A to ground - 120 volts; Load side B to ground - 120 volts. With 100% certainty, what is the state of both fuses?

• A. Fuse A and Fuse B are good
• B. Fuse A is good, Fuse B is bad
• C. Fuse A is bad, Fuse B is good
• D. Fuse A and Fuse B are bad

Answer: (C)

The readings point to one fuse being open while the other remains intact. Line A and Line B show the expected 240 V between them, and Line A to Load B also reads 240 V, which means the fuse on the B side is passing power to Load B. The key clue is Line B to Load A reading being 0 V. With Load A isolated by a blown fuse on that side, Load A is no longer tied to Line A; the motor windings connect Load A to Load B, so Load A is effectively pulled to the same potential as Load B, making Line B to Load A read 0 V. The measurements from Load side to ground—120 V on both Load A and Load B—fit a split-phase supply where each leg is 120 V to neutral. This confirms that Load B is still connected to the live Line B while Load A is floating due to the open fuse. Putting it together, Fuse A is open (bad) and Fuse B is closed (good).

The readings point to one fuse being open while the other remains intact. Line A and Line B show the expected 240 V between them, and Line A to Load B also reads 240 V, which means the fuse on the B side is passing power to Load B. The key clue is Line B to Load A reading being 0 V. With Load A isolated by a blown fuse on that side, Load A is no longer tied to Line A; the motor windings connect Load A to Load B, so Load A is effectively pulled to the same potential as Load B, making Line B to Load A read 0 V.

The measurements from Load side to ground—120 V on both Load A and Load B—fit a split-phase supply where each leg is 120 V to neutral. This confirms that Load B is still connected to the live Line B while Load A is floating due to the open fuse.

Putting it together, Fuse A is open (bad) and Fuse B is closed (good).