Depending on the product, it is possible to measure the difference in voltage in the touch area. What's more, you don’t need to be over-concerned about safety issues.
Let's take the NPort 5610 HW Rev.3, as an example, to shed more light on why it is possible to measure the difference between the signal ground and the earth ground as well as the theory behind it.
Before measuring the voltage, the circuit diagram of the device is illustrated in figure 1. Zeq is equivalent to the impedance of the NPort 5610; Zin is the input impedance. Neutral (N) is grounded together with the shield ground and the earth ground (PE). The NPort 5610 is powered by an AC power source.
When we measure the signal ground and the shield ground, the impedance of the voltmeter (Z) is serial connected, as shown in figure 2. Usually, the impedance of the voltmeter (Z) is extremely large, such as 5 or 10 mega ohms. The large impedance contributes to a voltage drop based on the voltage divider rule, even if only a small current leakage occurs between the two nodes.
For example, it’s possible to measure a difference of 90 V between signal ground and the shield ground. This value might seem alarmingly high, but, in fact, it is derived mostly from the impedance of the voltmeter. In other words, if the impedance between the nodes becomes smaller, the divided voltage becomes smaller as well. This 90 V cannot power up another device, and the small current leakage is not fatal to humans, either.
If you have any further concerns regarding this issue, please don’t hesitate to consult MOXA technical support for more information.