Why Is Non-Contact Voltage Tester False Positive Results?

Non-contact voltage testers, or test pens, voltage sensors, and voltage tester pens as they’re also known, are a safe way to ensure that electrical conductors don’t have AC voltage present without the need to touch conductors and potentially getting an electrical shock.

Non-contact voltage testers work by detecting any changing electric fields that surround an object conducting an AC, or alternating, current.

The best part about these testers is they don’t need to make direct contact with the conductor to test the electric field.

Basically, a person using the tester only needs to touch the top of the handle to provide a ground reference.

If there is AC detected, the LED will light up.

The non-contact tester detects any voltage present through capacitive coupling. To understand this fully, it helps to understand how a capacitor works.

How does a capacitor work?

Every capacitor has two conductors that are separated by a dielectric, or non-conductor. If AC voltage is connected across the capacitor, the AC current will flow across the dielectric.

This system creates a complete AC circuit. Thus, there is no wire needed to complete the circuit.

How does a non-contact voltage tester work?

Now that you understand how a capacitor works, we can discuss how non-contact voltage testers work.

Let’s say there is a wire present that’s conducting an electric AC current. The wire will act as only one side of the capacitor.

The other side of the capacitor is the sensor tip of the test pen. The air between the wire and the tip acts as the dielectric. Thus, a small capacitor is formed between the wire and the tester pen.

The side of the tester pen that you are holding is also a capacitor.

When holding the non-contact voltage tester the person is considered the “conductive plate” of the large capacitor.

The second “conductive plate” of the large capacitor is the ground.

Carpet, insulation, the person’s shoe soles, etc act as the dielectric for the larger capacitor.

In essence, when holding a non-contact voltage tester around a live circuit, you are introducing the high impedance sensing element into a capacitively coupled series circuit.

If this is hard for you to visualize, there are lots of diagrams available to better help you picture it.

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So, a capacitive voltage divider is established. It consists of the parasitic capacitance between the sensor and the conductor, and then between the ground and the sensor.

The presence of AC voltage is detected when the tester finds a current flowing through this divider.

Smaller current flows through the pen tester and wallah. This makes the light sound and the buzzer buzz.

The tester is powered by a small internal battery.

The way the pen tester works to detect AC voltage is quite different from other mechanisms such as tone generators, probe kits, and electrical circuit tracers.

How to operate a non-contact voltage tester

Before trying to use any type of non-contact voltage tester, it’s recommended that you read, comprehend, and follow all warnings and instructions from the manual.

This will help you ensure you’re using the tester properly.

The best way to use a voltage tester generally requires these easy steps:

  1. Find a known system that’s live such as an outlet, port of a fixed lamp, or other electrical system. This will help verify your tester works correctly.
  2. Now, use it to test the unknown system by repeating the test you did on the known live system. A good non-contact voltage tester will use lights or sounds to confirm that voltage is present. Be sure to check every wire both before and after disconnecting it to make sure it is not energized.
  3. Remember when testing an outlet or electrical receptacle, the top of the tester pen needs to be positioned on the face plate of the smaller plug slot.
  4. It’s also a great idea to test all outlets just on the off-chance they were wired incorrectly, on different circuits, or simply differently.
  5. When the receptacle is attached to a wall switch, ensure that the light is on before you do any testing.
  6. If you are testing power switches, unscrew and completely remove the power plate. You’ll then place the nose of the tester on the side of the switch where the screws are.
  7. Testing a three-way-switch requires you to test all screw terminals on both switches before switching.
  8. If you want to test a light fixture, cut the circuit at the main panel, and make sure the light switch is in the ON position. You’ll also need to unscrew the light bulb before placing the nose of the tester pen into the center socket button.
  9. When working with a light fixture with two switches such as a three-way, test with one switch in the up and down position.
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Keeping a non-contact voltage tester in your toolbox can save a great deal of time when testing circuits. This will help ensure the outlets are working properly each time.

Also remember to use a quality insulation resistance tester if you need to measure the insulation that surrounds your electrical circuits.

Non-Contact Voltage Tester False Positive Results

Now that you have a better understanding of capacitors and how non-contact voltage testers work, it’s important to understand that they can provide false positive results sometimes.

Depending on which type of AC voltage tester pen you’re using, it may have different types of false-positive readings.

Every tester features different settings based on the manufacturer and the intended uses.

Here’s a look at the most common:

Static Voltage

Sometimes, the voltage tester will not show any voltage even when it’s held still. Though as soon as you move the tester around the surface of the component, it will provide a positive reading.

This may be a type of residual voltage present in your system. It’s most likely a sight that the bonding/grounding system is not functioning properly.

Stray Voltage

This is generally associated with dairies, but can also appear in some residential neighborhoods.

Stray voltage is associated with neutral connections that are present within the utility supply lines.

Ghost Voltages

These can be high enough to set off a tester device even though there is not current.

When this occurs you need to look for wires that are disconnected and run parallel to the energized wiring. Do you have experience with false positives from non-contact voltage testers? Comment now and let us know what you experienced, and why.

2 thoughts on “Why Is Non-Contact Voltage Tester False Positive Results?”

  1. While connecting three different circuits through a single tube of PVC conduit, two of which were light switch connections producing two hot lines each, I noticed a voltage signal on a totally open line using a Fluke VoltAlert 1AC-A2 sensor.
    After double checking my connections, I tested the open line with a volt meter, which read zero volts. It then dawned on me that the open line laying in close proximity with four hot lines produced an inductive “ghosting” onto the open wire.

    Reply
    • Yeah, don’t rely solely on Voltage tester, always double-check with multi-meter. The way how tester works and measure voltage presence, sometimes can give you false positive results. Thanks Michael

      Reply

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