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Why has grounding influence on the result for non-contact voltage tests or single-pole voltage tests?

There are many testers on the market that can indicate voltage by connecting only one probe or even completely without having contact to the voltage carrying part. Also HDT has such products like the 312x non-contact voltage testers and the 303x 2-pole voltage testers which have single pole measurement implemented. But for this kind of tester, it is often noticed that the behavior on different locations and/or on different voltage sources is different. It seems like the sensitivity is not stable. But this is not a sign that the tester is not working properly. It is indicating that the grounding of the different voltage sources or of the user himself is different.

This happens because on such “single-pole measurements” the user is “part of the measurement circuit” and provides a reference potential for the equipment. A voltage is always the difference between two points with different potentials. Because of that it is not possible to detect a voltage only with one pole (even this impression is generated by the name of the testers). Because of that the second potential for such testers is coming from the user himself.

This results for a non-contact voltage tester in a similar schematic as shown above:

  • In this circuit the green capacity and resistor are the schematic of the tester (CT and RT) while CM and RM are the capacity from tester to the user (tester is held in the hand of the user) and the body resistance of the user.
  • The test device needs to measure a current to “know” that there is a voltage (difference in potential). Because without this difference in potential no current flows at all. The picture above shows very well also that the current depends on the resistors and capacities in the circuit. This means if user wears good isolated shoes, the resistance to ground will increase. The same happens when user doesn’t hold the tester firmly. This would decrease the capacity CM and would decrease the current as well.
  • The schematic also shows very well that there can not flow any current at all if the voltage source has no connection to ground. In this case the non-contact voltage tester would not indicate voltage.

Because this type of measurement is so sensitive to environmental conditions, it is much more reliable to measure voltage with a 2-pole voltage test. With this kind of test there is always a defined impedance between the 2 measured points and the tester can show the voltage without dependence on external factors like shoes of the user, grounding of the source, specific body resistance of the user or how tight the user holds the test equipment.