When testing fuses in the power circuit supplying a three-phase motor, we use the crisscross voltmeter method.
The horsepower-rated motor contacts must be open and the three-phase disconnect must be closed in order to obtain proper readings.
There are three sets of measurements to perform on the line side of fuses to make sure that voltage is available. Measure across each pair of line terminals (L1–L2; L2–L3; L3–L1). In the diagram below, that would mean using a voltmeter to check between points 1-3; 3-5; 5-1. If any of these tests give a reading other than the phase-to-phase voltage, then check the incoming voltage upstream. If all three readings give a phase-to-phase voltage, then we know that voltage is present in the power circuit up to the fuses. The next test will confirm if the fuses are in good shape.
In the diagram above, all three readings give us a line-to-line voltage. To check the condition of the fuses, we measure from the line side of one fuse to the load side of another fuse.
Using the diagram above, that would mean using a voltmeter to check between points 1-4; 3-6; 5-2. The readings we get back are:
- 1-4 = line voltage, therefor Fuse B is good
- 3-6 = zero volts, therefor Fuse C is blown
- 5-2 = line voltage, therefor Fuse A is good
Because fuses A and B are in good condition, there is essentially no potential difference between points 1 and 2 and between points 3 and 4 respectively, and that is why the voltmeter reads line voltage on both sides of the fuse.
With the power contacts open and fuse C blown, the voltmeter lead connected to point 6 is completely isolated from any other part of the circuit and so experiences zero potential difference.
This method is called the crisscross voltmeter method because it is never necessary to check voltage through a fuse. If the fuse is in good condition as fuses A and B are, then we are measuring from points of equal potential, and if the fuse is blown, then our second voltmeter lead is isolated from the circuit, again giving us no potential difference. No useful information can be obtained from these measurements.