Electrical Testing
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<p>Digital multimeter</p>

Tip of the Week: Using Your DMM to Safely Test for the Presence of Voltage

Follow these steps to avoid getting shocked.

Is it possible to use a DMM to verify that a circuit isn’t hot, and then get shocked anyhow? Yes, and you need to make that impossible. Here’s how you do it.

First, understand why the DMM might not read voltage that’s present. Several things can be wrong. For example:

• You have a broken test lead.
• The meter is selected for the wrong range.
• The meter is selected for the wrong function.
• The meter has been damaged and can’t read anything correctly.
• You think you’re measuring between hot and “ground,” but the point you’re touching with your black lead isn’t connected to anything (this sometimes happens with metallic raceway when the assumed “ground” isn’t there).

Except for that last problem, the following solution will work:

  1. Test the meter against a known live circuit. If the reading is close to the expected (nominal) voltage, proceed to the next step.
  2. Test the circuit for voltage. If you don’t read voltage, proceed to the next step.
  3. Test the meter against a known live circuit, again. If it reads voltage correctly, you can be certain it correctly read the lack of voltage on the circuit you just tested.
  4. Just to be double certain, repeat these steps again.

To avoid the “not connected” to ground problem, also measure between two phases or a phase and the neutral.

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