Learning more about capacitive voltage sensors

Have you ever seen that tiny pen with a glowing tip? Do you know that it is called capacitive voltage sensors and it’s one of the technological breakthroughs in the metrology industry?

Wondering how it works? Here’s how:

AC voltage detectors work based on the principle of capacitive coupling. According to Fluke, to understand this, let’s return momentarily to electrical circuit theory and recall how a capacitor works. A capacitor has two conductors or “plates” that are separated by a non-conductor called a dielectric. If we connect an ac voltage across the two conductors, an ac current will flow as the electrons are alternately attracted or repelled by the voltage on the opposite plate. There’s a complete ac circuit even though there’s no “hard-wired” circuit connection. The electrical “field” inside the capacitor, between the two plates, is what completes the AC circuit.

We are actually missing the fact that our world is full of small “stray” capacitors. We always think that these are just individual circuit components like motor starting caps.

Here’s an example from the Fluke website:

Suppose you are standing on a carpeted concrete floor directly under a 120 V light fixture and the light is on. Your body is conducting a very small ac current because it is part of a circuit consisting of two capacitors in series. The two conductors or plates for the first capacitor are the live element in the light bulb and your body. The dielectric is the air (and maybe your hat) between them. The two conductors for the second capacitor are your body and the concrete floor (remember that concrete is a good conductor, as is shown by the use of concrete-encased electrodes as earth grounds).

The dielectric for the second capacitor is the carpet plus your shoes and socks. This second capacitor is much larger than the first. A very small ac current will flow because there is 120 V across the series combination. (As an aside, this current must be way below the shock threshold or we wouldn’t be living in a world of ac power-we definitely would not be turning on lights in the bathroom.)

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