How Does a Non Contact Voltage Tester Work?

Non-contact voltage testers (also known as voltage tester pens, test pens, or voltage sensors) are the safest way to detect the presence of AC voltage. Non contact voltage testers work by detecting the changing electric field around objects conducting an AC current (AC stands for “alternating current”). They do not actually need to make direct contact with the conductor to achieve this. So long as the person using the voltage tester is touching the top of the handle to provide a ground reference (through a stray capacitance to ground), the indicator LED will light up if a live circuit is detected.

The voltage tester pen detects this voltage through capacitive coupling. To understand this phenomenon, we first need to understand how a capacitor works.

A capacitor has two conductors that are separated by a non-conductor (known as a dielectric). If an AC Voltage is connected across the capacitor, an AC current will flow across the dielectric. This makes a complete AC circuit – even though technically there is not an actual wire completing the circuit. You can see a diagram of a capacitor below:

Capacitor Circuit Diagram

Now that we understand the fundamentals of a capacitor, we can understand how a non-contact voltage tester works.

Let’s say we have a wire conducting an electric AC current. This wire acts as one side of a capacitor (a “conductive plate” in the diagram above). The the other side of the capacitor is the sensor tip of the voltage tester. The air between the tip and the wire acts as the dielectric. Hence a small capacitor is formed between the voltage sensor and the wire.

Now on the other side of the voltage tester (the side you’re holding) – we also have a capacitor. The person holding the voltage tester is the first “conductive plate” of the large capacitor. The ground forms the second “conductive plate” of the large capacitor. The person’s shoe soles (or carpet/other insulation) acts as the dielectric for this large capacitor.

So when you hold a non-contact voltage tester near a live circuit, you are inserting the high impedance sensing element into a capacitively coupled series circuit. This can be hard to visualize – so have a look at the diagram of a voltage tester pen below:

How a voltage tester pen works

A capacitive voltage divider is established, made up of the parasitic capacitance between the conductor and the sensor, and between the sensor to ground.

When the voltage tester detects a current flowing through this divider, it indicates the presence of an AC voltage. A small current flows through the voltage pen and wallah – the light sounds, and the buzzer buzzes (powered by a small internal battery). This is how a voltage tester pen detects the presence of an AC voltage. The mechanism is quite different to electrical circuit tracers or tone generators and probe kits.

How to Use a Non Contact Voltage Tester

Before using any non contact voltage tester, make sure to read, understand, and follow all instructions and warnings in the manual. This will help you more familiar with properly using your tester.

The best practice of using a voltage tester requires three steps. First, find a known live system such as a port of a fixed lamp or an electrical system or outlet. This helps you verify that the tester is working correctly.

Then, use it to test the unknown system and then repeat the test on a known live system. The best non contact voltage testers will light up and/or make a sound to confirm that it has detected the presence of a voltage.

It’s important to check each wire before and after disconnecting it to ensure that it’s not energized. When testing an electrical receptacle or outlet, the tip of the non contact voltage tester needs to be placed on faceplate of a smaller plug slot.

But it’s a good practice to test all outlets just in case they were wired differently, incorrectly, or on different circuits.

If the receptacle is connected to a wall switch, make sure that the light is on before you start testing. When testing power switches, unscrew and remove the power plate. Then place the nose of the tester on the screw sides of the switch.

When testing a three way switch, test all screw terminals on both switches before switching.

When testing a light fixture, turn the circuit off at the main panel, and ensure that the light switch is turned on. Unscrew the light bulb, and place the nose of the non voltage tester in the center socket button.

If the light fixture has two switches as with a three way switch, test with one switch in both the up and down position. Having a non-contact voltage tester in your toolbox can save a lot of time when testing to ensure that your electrical outlets work every time. Make sure to use the best insulation resistance testers if you need to measure the insulation surrounding your electrical circuits.

Non Contact Voltage Tester False Positive

Non contact voltage testers have different types of false positive readings. Each tester has different settings based on the intended use and manufacturer

  • Stray Voltage is usually associated with dairies but it might show up in some residential neighborhoods. It’s associated with neutral connections within the utility supply lines.
  • Static Voltage. On some occasions the voltge tester won’t show any voltage even if its held still. But as soon as you move the tester through the surface of the component, it will give you a positive reading. This might be some kind of residual voltage in your system and is a sign that the grounding/bonding system isn’t working properly.
  • Ghost Voltages can be high enough to set off the device’s tester but have no current. Look for disconnected wiring that’s running parallel to the energized wiring.

Voltage Tester vs Multimeter

As its name suggests, voltage testers are used to test for the presence of voltage. Volt testing devices can detect AC, DC, or RF voltage.

Non-contact voltage testers are used for electrical engineering and household appliances, but have fewer applications than multimeters. Voltage testers are typically used to test for the presence of voltage, while multimeters can not only detect the presence of voltage – but also measure and display the exact amount of voltage present.

The best digital multimeters can measure a variety of variables including the resistance, current, and voltage to humidity, inductance, and frequency.

Multimeters can be used in multiple scenarios, from industrial work, laboratory testing, power domestic appliances, electrical equipment, motor controls, domestic wiring systems and power supplies.

How to Test Christmas Lights with Voltage Tester

Start by plugging in the lights into an electrical outlet. Sit near the outlet, so you can access the lights from the outlet to the other end of the string.

Next, press the power button to operate the non-contact voltage detector. Place it near the lightbulb to gauge its voltage quality. You don’t have to touch the lightbulb for the device to work.

You will know if there is electrical current if the voltage detector lights up. If it lights up, that means electrical current is present and the bulb works properly. If no lights are present, then the light bulb needs to be replaced.

Replace the faulty light bulb and try the lights again. If the lights are still off, test the bulbs and replace the bad ones. If this doesn’t solve the problem, then it might be a problem in the wiring of the string.

Remove the string of lights if they are not working due to faulty wiring. Once you’re done testing the Christmas lights, plug them in back in the sockets and they should work properly! If you’re looking at doing other electrical work, make sure you have the proper electrical tools for the job.

About Blake Sutton

Blake has worked as an electrician for over 10 years, receiving his Journeyman Electrician license in 1998. Looking to take his professional electrical career further, in 2008 he received his Bachelor of Science in Electrical Engineering (BSEE) from the University of Texas in Austin. Blake now works full time as an electrical engineer, specializing in power systems.