Non-contact wire detection using a multimeter (121GW) - Page 1
Forum section: electronics / beginners

author: oliv3r
author_rank: Frequent Contributor
timestamp: August 26, 2023, 02:49:36 pm
content: Hey all,I am doing some home improvement and was looking at some wires in the walls, well not looking at them :pI know there's some multi-meters that actually support this, which i do not haveBut I do remember dave speaking of some multimeters being able to detect AC currents in live wires.So now I figure, is this something the 121GW (or fluke115) can do potentially? I figure i can hook up some (heavy) consumer (heater, vacuum) to the outlet and then see if the meter picks anything up? Is this even possible at all?I do also have a current-clamp extension from ebay, but obviously I wouldn't be able to run the wire through the clamp, as it is in the wall.Just looking for some thoughts on how to possibly measure this, if at all ... or is this something more for the RF nerds among us?

author: ejeffrey
author_rank: Super Contributor
timestamp: August 26, 2023, 03:01:40 pm
content: A NCV circuit isnt terribly complicated, you can find examples online. It's mostly a high impedance AC coupled amplifier with a lot of gain.However a stand alone NCV probe costs like $20 from a reputable manufacturer, no name ones are cheaper. It will also be more convenient to use and probably work better. If your goal is to complete a home improvement project i'd just buy one.

author: Veteran68
author_rank: Frequent Contributor
timestamp: August 26, 2023, 05:05:25 pm
content: First, some disclaimers to be aware of:1. Never trust any NCV probe with your life. Most any electrician can tell you a story of someone being deceived by a probe (they call them "idiot sticks" for a reason). They should be used only to verify that voltage IS present, not that it's NOT present. Always use a proper voltmeter to confirm there is NO voltage present.2. If these wires are in the wall and not easily accessible at a nearby wall plate, then there's even less confidence in a NCV probe to be able to reliably indicate the wire is live (or WHICH wire is live if there is more than one). Some won't have the range or sensitivity to detect through drywall or plaster especially if the wire is a few inches behind the drywall.That said, the probes do have their usefulness and I agree that you are probably better serviced by a dedicated NCV probe, in which case I recommend the Klein Tools NCVT-3P (I've owned a few and like this one best, and it was chosen over a Fluke in a review video I saw). I just tested the Klein on a wall around a light switch. On it's most sensitive (12v-1000v) range it detected voltage everywhere in the vicinity of the switch! However in it's least sensitive range (70v-1000v) it only detected voltage along where the romex ran down to the switch. Same with my Fluke 117 "Volt Alert" feature (on Hi it detected a wide useless area, on Lo it detected only around the switch and actual wiring). So marginally useful if you're trying to find a wire before cutting or nailing into drywall. Again, I wouldn't bet my life on it.As to multimeters with NCV, sensitivity and performance varies wildly across meters. Some work okay (ex. the Fluke 117) while others are abysmal and barely functional. The most sensitive I've seen is on a Kaiweets HT208D clamp meter, but it's so sensitive as to be practically useless. It will detect just being in the vicinity of voltage. I just tried it on the same wall/switch area -- it started detecting about a foot from the wall and indicated voltage in a 6 foot radius around the switch. No sensitivity setting that I can tell. So where you have multiple wires in one area you'll have no way of knowing which is live.The Fluke 115 and 116 do not have Volt Alert. You have to go to the 117 in that line.

author: EEVblog
author_rank: Unknown
timestamp: August 27, 2023, 12:03:46 am
content: The 121GW does not have NCV (Non-Contact Voltage) testing.The BM235 and BM786 do have it, and multiple levels.Some meters are a LOT better than others in this regard.Some also have a touch contact test feature with the negative probe.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 27, 2023, 01:02:10 am
content: Quote from: EEVblog on August 27, 2023, 12:03:46 amI have a Wavetek Meterman 35XP and it has NVC. I can use it as a "touch contact" by holding one hand over the NVC sensor at the top, and then poking one of the DMM probes into a wall socket. I sometimes use this to determine which of the holes in a mains socket is the live wire, and which is the GND return. Here in the EU wall sockets are unpolarized, and both wires are considered "live" for most practical wiring jobs.The way it works is that the insides of the meter will humm along with the 50Hz mains while your hand stays (more or less) at the same potential, so the NVC sensor sees a difference and it reacts to that.

author: oliv3r
author_rank: Frequent Contributor
timestamp: August 27, 2023, 02:14:12 pm
content: so no way to do it with the existing tools I have :pto be fair, the outlet is half way on the wall, and the PVC pipe (which olds the wires) are required by law to always go perfectly straight up. However, I just wanted to double check as I need to grind a second pathway about 30cm next to it, and didn't want to risk cutting the wire (not so much from a hazard pov, but more of an effort to having repair the damage).So I figured (and saw dave's video with de brymen one) about NVC, so hoped that I could just use the MM to follow the plastered pipe straight up, to confirm it truely runs there.> Some also have a touch contact test feature with the negative probe.Actually, I think I saw this being demonstrated... now I ponder, do I really need a nother MMDave, since you share little love for the 121GW nowadays, and always use the EEVBlog Brymen, please tell me I do not need more stuff

author: BeBuLamar
author_rank: Super Contributor
timestamp: August 27, 2023, 02:46:12 pm
content: I have a couple of Fluke non contact voltage testers but I found I can only trace the wires a short distance like a couple of feet. After that I don't know where the wires go but I have no signal.

author: Infraviolet
author_rank: Super Contributor
timestamp: August 27, 2023, 05:30:22 pm
content: Any hall effect based methods will only work if the live wire is delivering a large enough current to a load, in that case a hall effect sensor could feed a 50Hz (60 in some countries) tuned bandpass filter, and the filter's output could be envelope detected to give a DC level. But detecting a live wire which isn't delivering current would need one to measure the electric field from it rather than the magnetic.I have a cheap duratool meter of some model or another which lights up with an NCV detection when the top end of the meter is brought close to a live wire.But there are some mains wires, even slightly grounded shielded ones, it won't detect, and at other times it makes detections of low voltage AC or DC (maybe the ripple on it) voltages which have nothing to do with mains. Sometimes detects a "voltage" in a wire merely lying so that part of it is near a mains cable when the NCV meter is brought near a part of it a metre away.

author: Gyro
author_rank: Super Contributor
timestamp: August 27, 2023, 06:43:13 pm
content: There are a few products around that work by superimposing a higher frequency [Edit: actually short high current pulses] onto the mains wiring, such as the Martindale Fuse Finder. Sensitivity would depend on wall material, but would probably give you a better chance than 50/60Hz...https://martindale-electric.co.uk/product/fd650-digital-fuse-finder-kit/I don't know if anything similar, or cheaper, is available in the Netherlands.

author: oliv3r
author_rank: Frequent Contributor
timestamp: August 28, 2023, 08:37:33 am
content: well hooking up a load isn't an issue, if we are talking ~10 amps, via a heater for example.But was mostly just looking into using whatever I have at hand, which triggered my curiosity