AC voltage sensors

Knowing more about capacitive voltage sensors

Ever imagined how capacitive voltage sensors work?


AC voltage detectors’ functions are based on the principle of capacitive coupling. According to Fluke, to understand this, let’s return momentarily to the 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, AC current will flow through as the electrons are alternately attracted or repelled by the voltage on the opposite plate. The AC circuit can still be complete even without the occurrence of a hard-wired circuit connection. Given that the electrical field inside the capacitor, between the two plates, is what completes the AC circuit.


We are simply missing the fact that our world is full of small stray capacitors with the thinking that these are just individual circuit components functioning 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 small AC current as it becomes 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 using 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. AC current will manage to flow because of the 120 V across the series combination. (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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