A 240-volt, single phase fused disconnect that feeds a motor is being tested. Readings: Load side of Fuse A to ground - 0 volts; Load side of Fuse B to ground - 0 volts; Load side of Fuse A to load side of Fuse B - 0 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. Readings: Load side of Fuse A to ground - 0 volts; Load side of Fuse B to ground - 0 volts; Load side of Fuse A to load side of Fuse B - 0 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. Not enough information

Answer: (D)

The test shows that there is no voltage present on either load side to ground and no voltage between the two load sides. That alone does not prove whether either fuse is good or blown. A voltmeter reading zero in this setup can occur if the upstream supply isn’t connected, if both fuses are open, or if the circuit downstream is disconnected, so there’s no complete path for voltage to appear at the load sides. In other words, you don’t have enough information to determine the state of both fuses from these readings alone. To properly verify fuse condition, you’d need a continuity test across each fuse (with power removed) or measure line-to-line and line-to-ground voltages with the circuit energized, plus ensure the upstream supply and downstream connections are as expected.

The test shows that there is no voltage present on either load side to ground and no voltage between the two load sides. That alone does not prove whether either fuse is good or blown. A voltmeter reading zero in this setup can occur if the upstream supply isn’t connected, if both fuses are open, or if the circuit downstream is disconnected, so there’s no complete path for voltage to appear at the load sides.

In other words, you don’t have enough information to determine the state of both fuses from these readings alone. To properly verify fuse condition, you’d need a continuity test across each fuse (with power removed) or measure line-to-line and line-to-ground voltages with the circuit energized, plus ensure the upstream supply and downstream connections are as expected.