Mike PleziaInside service technician, Clark Associates · Commercial Equipment, HVAC, and Refrigeration professional

Voltage Testing: Find Where the Circuit Opens

18 Oct 2026 · 1 min

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1:02

In this short instructional video, the presenter addresses newcomers to the electrical trades and explains a systematic method for diagnosing a non‑functioning load in a control circuit. The technique—voltage drop testing—starts at the load and works backward toward the power source, measuring voltage between the load terminal and common. If voltage is present, the load itself is faulty; if not, the tester moves the probe to the next upstream component (contactor coil, safety switch, thermostat output) and repeats the measurement. The point where voltage disappears marks the open circuit. The speaker emphasizes that this method is reliable, faster than testing each component individually, and useful for helping others troubleshoot control circuits.

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Hey, are you new to the trades? Here's something you may not know about using voltage testing to trace a fault in a control circuit. When a load isn't operating and you want to find out why, start at the load and work backward toward the power source. Put one meter lead on the load terminal and the other on common. If you read voltage, the load has power, the problem is in the load itself. If you don't read voltage, move one lead back to the next component in series— the contactor coil, the safety switch, the thermostat output. Keep moving back until you find voltage. The last place you find it is where the circuit is open. This is called voltage drop testing in a series control circuit. It's systematic, it works every time, and it's much faster than checking each component individually. Know where the voltage stops. That's where the fault is. Now that you know, you can help someone else know.