Question

The company Fluke makes a DMM that they claim can measure AC voltage by simply clamping the connector around a live wire, and using the E-field to induce a voltage/current into the capacitor.

I can understand the concept if we assume the person holding the DMM is grounded and the measuring loop is sufficiently sensitive. But what if the person holding the DMM is not grounded, or is in a vacuum, etc. Some people on the internet claim it works even if the DMM is hanging from the wire in open air. I have my doubts... So, I am wondering if someone can explain to me if current can flow in a capacitor that has no path to ground. It seems like an obvious no.

Answer 1

A DMM can measure AC voltage using non-contact voltage detection through capacitive coupling, which does not require the user to be grounded. The electric field from AC voltage induces a detectable current in the DMM, and grounding is not essential for this process but is important for safety.

Step-by-step explanation:

The concept in question is related to a digital multimeter (DMM) using capacitive coupling to measure AC voltage without requiring a physical connection to the ground. This method, known as non-contact voltage detection, uses the principle of electromagnetic induction. In this method, the electric field from the AC voltage source induces a current in the measuring device, which is proportional to the voltage of the source.

The ability of a DMM to measure voltage through capacitive coupling does not require the user or the device to be grounded. This is because the voltage measured is a result of the changing electric field causing charges to move back and forth (but not to flow in a continuous circuit) within the capacitor created by the DMM's internal sensor and the live wire. This alternating movement of charges is what allows the DMM to infer the voltage present on the line.

Grounding typically provides a reference point and helps stabilize measurements, but it is not essential for the operation of non-contact voltage detection. Even in a vacuum, the lack of grounding does not prevent capacitive coupling from occurring—although the efficiency of induction may vary with the medium between the wire and the DMM.

It is important to note that while grounding is not necessary for a DMM to make these types of measurements, safety recommendations generally advise grounding electrical cases to prevent hazards due to leakage current or induction on the case.