What Is A Non-contact Voltage Tester, And How Does It Work?


People often ask this question when they experience a non-contact voltage tester for the first time. If you are in the same shoes, you should check out this article as it clearly explains this tool’s structure, operating principle, usage, and weaknesses.

What Does Non-contact Voltage Tester Structure Include?

An AC voltage tester pen is usually shaped like a pen, thus, the name. It is quite small and convenient to bring anywhere you go. Typically, it must have the basic structure below:

  1. Battery cap: It is a battery compartment. When necessary, you need to renew battery life for the tester because a non-contact voltage tester is a kind of battery drain.
  2. Pocket clips: You can carry the tool with you by clipping it in your chest pocket.
  3. Voltage non-contact and illuminating tip/sensor: For checking voltage in lines.
  4. Light indicator: When detecting standard voltage, the handy flashlight is turned on for warning.
  5. Audible voltage indicators: For warning voltage in lines with beeping sounds.

You will find the structure of a non-contact voltage tester much more complicated than that of a direct-contact voltage tester (multimeter, clamp meter…). That’s because its design is to detect electrical charges from a distance.

The tester pen doesn’t need to touch electrical charges directly. As a result, this type of tester ensures higher safety than contact AC voltage, reducing the risk of damaging your actual health during the repair process.

How Does Non-contact Voltage Tester Work?

How Non-contact Voltage Pen Works

How Non-contact Voltage Pen Works

Voltage detector works based on detecting the change of electrical, magnetic field around conductive objects.

Voltage appears when there is an electromagnetic field, so the tester checks this field by capacitive couplings.

With this principle, it’s unnecessary for the tester to directly touch the voltage source to determine the electric charges.

Let’s assume that we have a wire with alternating current flowing through it. This wire is a part of the capacitor, and the other part is the sensor of the voltage tester.

These two parts do not need to touch each other, for there is already air acting as a dielectric in the middle. Thus, when we put the tester near the wire, we have a small capacitor.

Also, when you hold the tester, you accidentally create a second capacitor. You and the ground are the two main components of this large capacitor. Shoes or insulators under the feet act as dielectrics.

Accordingly, when you test with a voltage tester, you place a high impedance sensor in a special series circuit. This series circuit is coupled with the capacitance.

As a result, a small electrical current will flow through the voltage pen when you bring the tester close to a live wire. Thanks to that, a bright flashlight and sound will appear to warn you. That is the mechanism of a non-contact voltage tester, and perhaps it can answer the question, “What is a non-contact voltage tester, and how does it work?”

How To Use A Non-contact Voltage Tester?

How To Use Non-contact Voltage Pen

How To Use Non-contact Voltage Pen

There are three basic steps in the process of using a non-contact voltage tester.

Step 1: Checking the execution of the tester

It is necessary to test the voltage detector to make sure it works correctly. You should find an electrical system that you are sure is working. If the tester works fine, proceed to the next step.

Step 2: Determine electric system and check

#1 Check the power at the electrical panel

You should check the plug slot no matter how small or big it is because, in many cases, they are connected differently or wrongly wired. For the wall switch, make sure it is turned on during the test.

After removing the socket, you should check again to determine no voltage in lines. Check all the voltage in cables you see to make sure there’s nothing dangerous to touch.

#2 Check for the power switch

You should remove the cover plate, then put the voltage detector’s sensor tip on the internal screw connections. If there is no presence of voltage in the cables, you can safely remove the switch.

#3 Check for fixed electric devices

First, turn off the circuit, but make sure the light switch is on. You remove the light bulb and place the tester’s sensor tip into the center socket button.

If it has two switches, test the up and down positions to ensure no house voltage is present.

Once you have made sure that there is no circuit by the voltage detector, you can safely remove the devices.

What Are Non-contact Voltage Pen’s Weaknesses?

Non-contact Voltage Tester Weaknesses

Non-contact Voltage Tester Weaknesses

This voltage detector kind has two weaknesses.

First, you cannot use the tester for DC currents because its transformers and capacitors are not suitable for this current. Manufacturers make these testers for AC only. Thus, you should pay attention to the type of current you test to ensure the test is as safe as possible.

The second weakness of the AC voltage detector pen is the inability to measure the range voltage accurately. The fact remains that the bright LED flashlight or the beeping sound only indicates voltage exists.

However, today, an AC electrical voltage tester can accurately measure range voltage numbers, thanks to the sensor attached to the digital system. This type is uncommon and is mainly for scientific studies.


Perhaps you have found the answer to the question, “What Is A Non-contact Voltage Tester, And How Does It Work?”

With the above information, we hope that the usage of the AC voltage detector is no longer challenging, and you can ensure efficiency and safety while using it.

Further reading: 8 Types of Electrical Testers and Their Uses