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.