What is Ghost or Phantom voltage and low-Z measurements?
What are Ghost or Phantom Voltages?
When a 0–10v circuit or other low voltage control is run in the same conduit as a line-voltage circuit (especially over a long distance), this causes capacitive coupling of the control circuit and line-voltage conductors. In this case inaccurate abnormally high voltage can be read with a typical digital meter with a high-impedance input, which is typically 1 MΩ (megaohm) or greater. This is known as “Ghost Voltage," which has little to no current and generally disappears when a low-Z meter is used to take the measurement.
Making Accurate Voltage Measurements on 0–10 Volt or other Low Voltage Control Wires
Making accurate measurements of voltage created by capacitive coupling on control wires requires use of a meter with a low-impedance (“low-Z”) input, typically 3 kΩ (kiloohm). Many modern digital multimeters provide a selectable low-Z setting. Examples of such meters are the Fluke 114, 116, and 117.
- If voltages on the control circuit are greater than 20 volts when measured with a low-Z meter, there may be a poorly isolated or non-isolated driver connected to the control circuit, or there may be an excessive number of drivers connected to the control circuit. Treat the control wiring as potentially live.
- If voltages on the control circuit are less than 20 volts when measured with a low-Z meter, the voltage is a "Ghost Voltage" and is not a concern because there is little to no current.
NOTE: Why not measure with a low-Z meter all the time? A low-Z meter is less accurate because it loads the circuit with 3 kΩ added in parallel to the circuit reading, which can create a significant voltage drop across the circuit. For certain types of measurements, high-impedance meters provide greater accuracy because the meter itself does not load the circuit with an impedance lower than 1 MΩ.
For general measurements use a high-impedance meter. If thing seem a little strange, switch to a low-Z measurements to see if you are getting a ghost / phantom voltage.