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How to Verify Electrical Test Tool Operation with a Proving Unit

A proving unit is a battery-powered portable device that serves as an electronic voltage source to safely verify the operation of an electrical test tools such as a digital multimeters (DMM), clamp meters or other electrical testers.

  Testing a digital multimeter with the Fluke PRV240 Proving Unit

Proving units can be used to satisfy elements of the 2018 edition of NFPA 70E 110.4E, which states, "When test instruments are used for testing the absence of voltage on conductors or circuit parts operating at voltages equal to or greater than 50 volts, the operation of the test instrument shall be verified on any known voltage source before and after an absence of voltage test is performed."

Typically, a "known voltage source" is a nearby power outlet or junction box or another panel. But in situations where no voltage is available, like up-tower in a wind generator or at a remote pumping station, a proving unit can serve as the voltage source. Also, once absence of voltage and lock out, tag out (LOTO) are complete there may be no available power for retesting the test instrument on a known voltage source.

Here is how to verify the operation of an electrical test tool at 240 VAC or 240 VDC with the Fluke PRV240:
  1. Select the ac or dc function with the selection switch on the side of the unit.
  2. Place the black probe of the unit under test into the (-) terminal and press down firmly.
  3. Place the red test probe of the unit under test in the (+) and press down firmly.
  4. Verify the meter reading of the tester is valid for the function tested.
  5. For low impedance testers the output voltage sourced should be >50 VAC or dc.
  6. It is recommended to test both ac and dc functions of your test tool.
If the power indicator led does not illuminate, check the condition of the battery and replace if required.

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