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Ghost voltage and triac leakage: why the meter shows voltage on a wire or board output that is really off

A high-impedance digital meter barely loads the circuit, so it can show voltage coupled capacitively from nearby live wires (ghost voltage) or the small leakage current through a solid-state switch such as a triac that is off. With the real load connected, or with a low-impedance (LoZ) meter function, those readings collapse toward zero; a real supply does not. Knowing this keeps you from condemning a good board or trusting a reading that is not there.

Cross-checked ~15 min moderateBrands: All

Before you start

  • A floating reading near line voltage can still shock you; treat the conductor as live until proven otherwise
  • LoZ functions load the circuit: do not use them on sensitive electronic signal circuits, and expect a LoZ test from hot to ground to trip a GFCI
  • Measure with power on only with insulated probes and one hand where possible

Diagnostic steps

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OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

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Ghost voltage and triac leakage: why the meter shows voltage on a wire or board output that is really off · OnCall Tech