A non-contact voltage tester can easily give a false positive; if it does, the result can be misleading.
You often need to test the voltage in your home wiring, and a non-contact voltage tester seems to be a safe and convenient tool, but how do you know you’re not getting a false positive?
A false positive is usually the result of static voltage, induced voltage, stray voltage, or ghost voltage.
This article explains these reasons in more detail and gives tips on how to test safely without getting a non-contact voltage tester false positive.
Reasons for Getting a False Positive
When using a non-contact voltage tester, the reasons for getting a false positive are static, induced, stray, and ghost voltages.
I will explain these reasons and mention how you can avoid them.
Static voltage usually arises when there is no proper grounding/bonding system or impaired functionality.
While fixing a wire, if the voltage tester doesn’t show any current but does when you move it around a little, it might be due to some voltage remaining in the wiring.
This is an example of a positive reading due to static or residual voltage still present.
It is created when there’s an imbalance between objects’ negative and positive charges. Although there is no current flow, some static electricity is present at the site.
In short, if there’s a buildup of static electricity at the site where the non-contact voltage tester is being used, it could be one reason for getting a false positive.
The induced voltage is created when a long conductor, as in a long wire, forms an open-grounded loop that is parallel to a distribution line.
It might, for example, be under a distribution line above your house. This type of voltage can be dangerous because the voltage can be induced even if the wire is not directly connected to a power supply. Whether the voltage is induced completely or slightly, it can give you a false positive reading.
This shows that you cannot rely entirely on the result of a non-contact voltage tester.
Stray voltage occurs when two conductors have an electric field potential with no current difference.
Even if both conductors are grounded, current can flow due to this potential, i.e., when there is not supposed to be any voltage difference between them. The current can be quite high. Sometimes, this happens due to a fault, such as insulation failure or a weak connection, which creates the potential.
This type of incident, which creates an unintentional voltage to arise, is not uncommon, and when it happens, a non-contact voltage tester will detect a false positive.
Besides static, induced, and stray voltage, another kind can also cause a non-contact voltage tester to get a false positive.
This type of voltage is called a ghost voltage because the voltage is usually transient, the source is often mysterious, and it happens even if the circuit is open. It typically occurs under a high-load situation. There is always a cause, but it is not apparent. Technically, this happens due to “capacitive coupling,”; not the presence of a ghost. It happens when capacitance is generated that indicates a voltage even when there is no corresponding source. [Banerjee, 2012: 396]
Due to proximity, an attractive field is generated around the conducting wire. It remains in it, though usually only for a short period, as it finds a path to the ground or neutral. If the voltage is tested during this period, it will be detected as a false positive.
Tips for Testing Voltage Using a Non-Contact Voltage Tester
First and foremost, let us remind ourselves of important safety precautions:
Don’t touch bare hands – Never touch the wires or connectors with your bare hands to confirm that voltage is present when getting a positive reading that you suspect might be false. Wear protective gloves and shoes for extra safety, especially when working with high voltage.
Disconnect the supply – When testing for voltage to fix loose wiring or a weak connection, whether you get a positive or negative reading, it is always safer to disconnect the supply before working on it. An electric shock can be very dangerous or even fatal.
Now, I will give you more valuable tips on using a non-contact voltage tester to test voltage without getting a false positive reading or recognizing a false positive for what it is.
- Use a good quality non-contact voltage tester and avoid using a cheap one that will more likely give false positives.
- Also, ensure that the batteries have enough charge, i.e., are not weak.
- Check the non-contact voltage tester on a site where you know there is voltage. This is to make sure that it is working.
- Before testing the wire or contact, touch the outer cover, covering, or shielding with the tester. This is to make sure it is well insulated and not live. Only proceed with the testing if it is safe to do so.
- Having removed the cover, make sure the electrical system is properly grounded. The wiring in our homes is sometimes grounded and sometimes open.
- Look for any signs of damage (overheating, melting, or corrosion). If there are any, there is a problem with the wire, connector, or outlet, and it may need replacing.
- Check for any trace of moisture, as this can increase the risk of getting electrocuted. It is very risky working with electricity in a moist environment.
- When testing a particular part or component, clear all nearby parts that might get energized, i.e., conduct electricity. Don’t touch any of these parts while removing them.
To avoid the main causes of getting a false positive reading, you will have to take note of a few more things.
Preventing Ghost Voltage
To avoid the problem of ghost voltage, which may be caused by capacitance coupling, ensure that the leads are separate from one another as much as possible.
Using a Multimeter
Rember that you don’t have to rely entirely on the non-contact voltage tester. You can easily verify the result using a multimeter.
Also, a negative reading does not necessarily mean that there is no current or voltage, so the opposite of a false positive is possible. Again, you can easily verify that there is no current or voltage by using a multimeter.
Using a Load Meter
In addition to a multimeter, a load meter can be useful to avoid the possibility of ghost voltage.
A load meter has sufficiently low impedance to prevent ghost voltage and prevent you from getting a false positive. A load meter is generally useful when working with AC electricity or when you need precise voltage values.
A Contact Voltage Tester
If you need to ensure you’re not getting a false positive, why not use a normal contact current/voltage tester instead?
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