Many of our meters come with a "NCV" mode and sometimes a separate "Live" or "EMF" mode. This goal here is to present a description of what they are and how your meter does them.
DISCLAIMER: Working around higher voltage can result in injury or death. Please read your owner's manual or consult safety guidelines for working with or around mains or high voltage devices and cords.
NCV is an acronym for "Non Contact Voltage". It refers to a method to detect the strength of an "Electic Field" or "EF". Therefore, when in this mode, your meter looks for expanding and contracting magnetic waves, then reports the relative intensity with all sorts of excitement. It detects this using an antenna of some sorts inside the unit, almost always near the center of the head of the unit on a handheld multimeter. This means no leads need to be inserted for standard NCV operation.
For clamp meters, the NCV sensor is usually at the tip of the jaws, in the non-moving portion, often denoted with a visible raised bump or triangle. The wire being tested for NCV does not need to be inside the clamp jaws, in fact just leave the clamp closed for this.
Depending on your device, NCV may also be called Volt Alert, Hot Wire detection, or Fire Wire detection. The neutral or ground wire might be called the "zero" line. Unfortunately, there seems to be no standardization on this, so don't feel bad if you're confused!
Because the meter displays a result due to the presence of a changing magnetic field, which induces a voltage, it needs to be a constantly changing voltage source. Therefore, it only detects an AC voltage. The NCV feature on the multimeter is tuned to have the proper sensitivity near typical mains voltage around 110-220V AC, 50-60 Hz, and at a distance of only a few centimeters from a "Live" or "Hot" wire, even an insulated one.
In your supply wires, only the Hot/Live/Fire wire contains the necessary magnetic field to induce voltage in the antenna sensor. Knowing that the neutral and ground wires inside your device or cable are not supposed to radiate the magnetic field, you can use the device to differentiate between the hot versus the neutral & ground wires by the relative strength of the signal indicated on the meter. When near the hot wire, the bar display will be high, the beeps and flashing lights will be fast and loud. When near the neutral or ground, the bars should be low and the beeping and flashing will be slow. It won't likely be completely quiet, because the device is close enough to the live wire to register detect some EF.
Without ever touching the wires, you can determine if there is alternating voltage present where it should be, and predict whether other wires which could be unsafe. However, it is NOT a definitive determination. Verifying continuity and wiring standards is the best way to absolutely determine correct wiring.
It's been demonstrated that some devices with not-so-sensitive NCV detection might do nothing at all near a light switch or electrical socket. The problem is the sensor is not perpendicular to the magnetic field, not parallel to it. In this case the induction is poor. Sort of like trying to read the side of a pen when looking straight into the tip.
Furthermore, all the NCV function can do is positively announce that mains voltage is nearby, but it cannot for certain tell you that stray voltage of some sort is not present. There may still be DC voltage, lower AC voltage, or a intermittent connection. These can be dangerous as well, so continue practicing caution after no detection.
A few meters and devices have "EMF" feature which refers to "ElectroMagnetic Field". It works on similar induction principles to the "NCV" mode. However, EMF detection is tuned to more broadly detect the presence of magnetic field being radiated by a device while operating, such as motors, LED light bulbs, microwaves, and switch mode power supplies at a greater distance than NCV operates. This is less common in multimeters, and requires a meter which has special sensing and recognition programmed in.
EMF detection is important for several reasons. First, you want to avoid this type of electrical noise near your work area. This helps assure accurate and steady observances in your test results, receive radio signals, and so on. Second, it helps you determine the relative quality of equipment you are working around. Third, it helps alert you to potentially unhealthy environments that might slowly cook your brain.
The "Live" or "Livewire" mode is a little different than NCV. That is, different in procedure but similar in displayed results. Essentially, you do not hold your multimeter close to the insulated wire. Instead, the mode called "Live" typically requires direct contact with the hot wire. You connect the red lead to the required multimeter jack (usually the V jack), leaving the black input jack unoccupied. Select the "Live" mode. Then you touch the prong of the red lead to the metal conductor.
A few notes of safety here.
Perform with your full attention and with a steady probing hand, not sitting in a chair or leaning against anything.
Exclude disturbances, children, or pets anywhere close to your activity.
Ensure that only one bare wire is in proximity to your pronged lead to avoid a short.
Keep your other hand away while probing, to eliminate a direct current path through your heart in the event of a shock.
It's also best that the meter be set on a surface - not held in your other hand.
Wear shoes with rubber soles and if needed kneel, do not sit on the floor.
Consider thick rubber gloves and safety glasses as well.
Only use undamaged, appropriately CAT-rated leads of proper voltage rating
Keep your finger behind the guards on the probe pen.
This is not a complete list of safety, but some important ones to be highlighted.
If you don't see a "Live" mode on your multimeter, consult the owner's manual. In some devices, there isn't a separate position for "Live", it's typically active in the "NCV" mode setting. It may surprise you that a multimeter you bought that only showed "NCV" may also do direct connection "Live" measurements. What happens is the red lead simply becomes the antenna. I found just about all the meters I have access to with NCV mode also perform the "Live" type of measurement even if the manual doesn't say that it does. However, we cannot recommend the general public tries this because it may damage your meter -- or the inside of the meter was not designed well enough to protect the user from shock in the presence of high voltage. Therefore, if you decide to do so, it's at your own risk. If you need to use the "Live" feature, you really should use a meter that lists this mode in the specifications.
So "Live" a one-wire test. In the case of a pen multimeter, it's a no-wire test, the tip serves as the red wire and the black wire isn't connected. Some pen meters recommend to insert the black lead and cap the metal prong, but it's probably to prevent someone's hand from touching the recessed jack.
When using a pen multimeter in the "NCV" mode, the body might needs aligned to the wire, not the tip. Check out what the user manual says.
One final note, the NCV feature is so sensitive that mechanical bumps will temporarily set off the alarm. Knowing this, avoid physically contacting the meter against what you are testing so that false readings do not happen.
In summary, these features provide the user with safer ways to test and troubleshoot high voltage AC wiring, compared to the previously required 2-wire connection in the AC voltmeter mode.
There are new acronyms and terms to learn. The only way to really know how well these features work is to try it or find someone else who has. There aren't any sensitivity standards to be found, unlike voltage reading which need to be within a certain accuracy. This is why the reviews on the KeepOnTesting channel are so important!
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