Voltage detector, how does it work?

Voltage detectors are essential equipment for electricians, but there is a great deal of discussion about the efficiency of voltage detectors, where some say that voltage detectors work, and others say that they do not. Due to the many doubts related to the effectiveness of voltage detectors, the World of Electrical explains how a voltage detector works, its respective characteristics, and whether or not we can trust voltage detectors.

In the image below, we can see that the multimeter indicates a voltage of approximately 113V because, in this test, we have a 127V network. To detect voltage with a pen-like this, you simply place the detector close to the cable. If it has energy, the detector will beep and turn on a led, right? However, when the voltage detector is released, it stops beeping and turns on the LED, even though it is leaning against the phase cable, and there is voltage, as we can see in the image below.

Voltage detector not working correctly?

After performing this test, can we trust the voltage detectors? It is important to note that this article is based on technical articles found on the manufacturers’ website and on the technical manuals for each product.

Capacitor – Operation and characteristics

Before we understand how voltage detectors work, we need to know how capacitors work, so let’s remember the theory of electrical circuits and 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 alternating voltage between the two conductors, an alternating current will flow as the electrons are alternately attracted or repelled by the voltage on the opposite plate.

The basic structure of a capacitor.

Thus, there is a complete alternating current circuit, even if there is no physical connection through electrical wires between the two plates. The electrical “field” inside the capacitor, between the two plates, is what completes the alternating current circuit.

In capacitors connected in series, the voltage is divided equally among them, according to the ohm law, that is, in series circuits, the highest voltage will develop through the highest impedance.

It is important to pay attention to some details, because with capacitors there is a difference, because of the smaller the capacitor (capacitance), the greater its impedance (also known as capacitive reactance).

Characteristics of the series capacitor circuit and series resistor circuit.

It seems a little complicated, but it is the opposite of what happens with resistors, that is, when two capacitors are in series, the highest voltage will be in the capacitor that has the lowest capacitance because this capacitor will have a higher impedance (capacitive reactance), as we can see from the following capacitive reactance formula.

The formula for calculating capacitive reactance.

What is capacitive coupling?

To understand how voltage detectors work, in addition to the need to know how the capacitors work, it is also necessary to understand what capacitive coupling is.

Capacitive coupling is basically the transfer of energy in a circuit through the current that travels between the circuit nodes, induced by the electric field.

Whenever we think of capacitors as electrical components, we already imagine those big ones used to start motors or those of electronic circuits. In reality, the world is full of small “natural” capacitors that we don’t normally notice, but which interfere with the functioning of the circuits.

Capacitive coupling example

Suppose a person is on a carpeted concrete floor directly under a 127V lamp, and the light is on. The body is conducting a very small alternating current because it is part of a circuit that consists of two capacitors in series.

It sounds strange, but it’s true. One plate of the capacitor is the lamp and the conductors next to it, while the other plate of the capacitor is its body. The dielectric is the air that separates it from the lamp, thus forming the first capacitor.

The first capacitor is smaller than the second capacitor.

There is also a second capacitor, where one of the plates in your body, and the other plate is the floor, the dielectric of this second capacitor being formed by the carpet, and the shoes that the person is using. It is important to note that this second capacitor is larger than the first.

The second capacitor is larger than the first capacitor.

With this configuration, a very small alternating current will flow because there is 127V in the combination of these two capacitors in series when you are in a location with alternating electrical power. An important observation is that this current is well below our threshold of perception, and therefore, we feel nothing.

In the example above, only a few volts develop between the person’s feet and the floor (largest capacitor), while the rest of the 127V will be between the lamp and the person’s head (smallest capacitor).

Voltage detectors – Operation

Voltage detectors work from the principle of capacitive coupling because when the person holds the detector and places the tip of the voltage detector close to a live conductor, the detector that is a high impedance electrical circuit is being inserted into a capacitively coupled series circuit.

As in the previous example, the hand and the body form a relatively large capacitor attached to the floor, and the tip of the sensor is a small capacitor attached to the active voltage.

The detection circuit is very sensitive and detects the voltage through this capacitive coupling, which does not need contact and indicates whether or not there is electrical energy.

The following image shows a capacitive coupling circuit.

Capacitive coupling with three capacitors.

Voltage detector – Tests

In the first test that was performed previously, where the voltage detector stopped detecting voltage in an energized cable when the detector was released.

This is because when the hand is removed from the voltage detector, the series capacitor circuit no longer exists, so if there is no closed circuit, there is no current to generate the voltage to be detected by the detector’s sensor.

For the use of the electrician, the detector is manufactured and calibrated to interact and capture specific voltages, for example, above 90VAC.

For this to be valid, we have to take into account the frequency and other characteristics of the electrical circuit, but this does not mean that other interactions with electrical fields cannot be detected, even if they are not alternating voltages in the range specified by the detector. For example, by rubbing the voltage detector against or hair, it will detect the electric field due to static electricity.

Voltage detectors – Observations

In order to use the voltage detector correctly, the electrician must read the manual and understand the correct way to use the tool, in addition, there are also other factors that are important to know before using the voltage detector.

The lack of voltage indication of the detector, even with voltage in the conductor, occurs if the test instrument cannot detect the presence of voltage, which can be influenced by several factors, such as:

  • Wiring Type / Shielded Cables
  • Insulation thickness and type
  • Voltage source distance
  • Fully insulated users that prevent effective grounding (and do not form the capacitive circuit)
  • Recessed socket receptacles/design differences between sockets
  • Condition of the test instrument and batteries.