Probes catch on fire when measuring 120vac  - Page 1
Forum section: electronics / beginners

author: stallvalds
author_rank: Newbie
timestamp: June 19, 2023, 12:08:23 pm
content: Hello!So i'm trying to measure 120vac with a manual multimeter. i set it to 200vac and used the 10a unfused terminal. but when i plug it in to a surge protector the probes started melting right away until the breaker tripped :'( Can it be the surge protector?Thanks so much in advance!

author: dmills
author_rank: Super Contributor
timestamp: June 19, 2023, 12:24:40 pm
content: STOP, and learn some more before you have a serious accident.You should be learning on things powered by zinc carbon batteries, not the frigging mains (Which can and will kill you if get unlucky).The unfused 10A hole on the meter is for measuring CURRENT, never VOLTAGE, and is as it says UNFUSED, so there is little to limit the current flowing.

author: janoc
author_rank: Super Contributor
timestamp: June 19, 2023, 12:26:30 pm
content: Quote from: stallvalds on June 19, 2023, 12:08:23 pmDude, be glad you didn't kill yourself or set something on fire. I suggest you learn how to use a multimeter and what is actually the difference between current, voltage and what each of the ranges of your meter does - esp. before you come even close to any mains voltages with it.I do wonder what were you trying to measure there.If you stuck the probes into the surge protector (I presume one to live and the other one to neutral) and have used the 10Acurrentterminals, you have literally made a short circuit. On mains.No wonder the probes and wires were melting.There is nothing wrong with your surge protector - or rather - there wasn't before you attempted this. No idea whether it has survived your probing unscathed.

author: Zero999
author_rank: Super Contributor
timestamp: June 19, 2023, 12:28:47 pm
content: 'A' stands for amps i.e. for measuring current and any terminal marked with that letter i.e. 200mA, 10A, should never be connected to a voltage source, such as the mains. It's highly likely you've damaged the meter and possibly the surge protector.

author: Ian.M
author_rank: Super Contributor
timestamp: June 19, 2023, 12:28:54 pm
content: My first reaction was you are trolling us, but then I checked and manual ranging DMMs with an unfused 10A range are still available in 2023so its possible you did what you claimed.STOP any further experiments with mains electricity until you understand *that*, *how*, and *why* you nearly killed or seriously injured yourself!The 10A terminal is for measuring current, and only current, and does not go through the range switch (though the correct range must be selected to display the current). By connecting the leads to it and Com/-, then probing the mains you short-circuited the mains supply, drawing a grossly excessive current, far above the rating of the test lead wire.Something in the circuit had enough resistance to prevent the breaker tripping quickly - probably the cheap and nasty test leads (as no modern DMM with an unfused 10A range is high quality, and the leads are usually nastier than the meter).You are lucky not to have started a fire in the wall, and that the melting leads didn't compromise the probe insulation badly enough to electrocute you, and that the multimeter didn't explode in your hand.At this point, the leads are trash, the multimeter may be as well, and I wouldn't trust the surge protector till its been opened up and checked for any signs of overheating or serious arcing. The wall socket, wiring in the wall back to the breaker panel and circuit breaker may also be compromised - if your home has passed a fairly recent electrical inspection, its probably 'man enough for the job' and if after resetting the breaker, the socket appears to be working normally with no signs of any burning or overheating, its probably OK - if however it hasn't been inspected in living memory and your home wiring is a hodgepodge of DIY repairs and modifications, that overload could easily have been the last straw that overheated a loose wire-nut or terminal, or burnt the breaker, and the circuit should be thoroughly checked for peace of mind!Why are you trying to measure your mains supply, and did you read the instruction leaflet that came with the multimeter?

author: rsjsouza
author_rank: Super Contributor
timestamp: June 19, 2023, 12:29:43 pm
content: Before anything: this is 100% expected as per your description.What are you trying to measure? If you want to measure the voltage, be sure to plug the leads to the correct input jack. The 10A input jack is only suitable for current measurements, which is not what you want to do. You should have plugged your leads to the COM and the V/Ohm jack.I recommend you to carefully read your multimeter's user's manual before attempting to measure anything - this will help keep your equipment intact and potentially saving you from any harm.Quote from: stallvalds on June 19, 2023, 12:08:23 pm

author: Zero999
author_rank: Super Contributor
timestamp: June 19, 2023, 12:34:42 pm
content: Quote from: Ian.M on June 19, 2023, 12:28:54 pmThe thought didn't cross my mind. I do wish they would ban unfused meters, but even then, there will still be dodgy imports and beginners using old meters. Putting a meter set to amps, across a voltage source appears to be a very common beginner mistake.

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 19, 2023, 01:26:24 pm
content: Protection aside I sometimes wonder why such large current range even still exist. It causes more headaches than it might worth IMO. Large current measurement is rarely needed, and when you need one you rarely use the built-in shunt. None of my meter even have current range any larger than 440mA.Heck, making such large current shunt an "external add-on" just like a thermocouple "plug" makes more sense if you really need to put the feature.

author: themadhippy
author_rank: Super Contributor
timestamp: June 19, 2023, 02:10:36 pm
content: Quotebecause some of us want to measure a bit more than 440ma,making it an external box is just something else to loose or set up wrong

author: TimFox
author_rank: Super Contributor
timestamp: June 19, 2023, 02:23:01 pm
content: or mis-spell...

author: golden_labels
author_rank: Super Contributor
timestamp: June 19, 2023, 03:48:57 pm
content: After telling OP, how bad choice they made, let’s become constructive and make this lesson count.In the past devices had schematics attached and measurement equipment happened to even have simplified block diagram drawn on the panel. These times are now gone, but it is still worth seeing your meter as something more than just a black box. I donotencourage you to disassemble a modern DMM. Old analog meters had a simple and obvious construction, but the current tools are too complex to deliver a clear image. But you may still have the right mental model.Voltage measurement port — the COM and V jacks — may be seen as a resistor in megaohms range. You connect it in parallel to the measured points:Code:,---------.| DMM || ,-- +--------------------.| ,-. | || | | MΩ | || | | | || `-' | || `-- +---. |`---------' | || ,---------. |- - ------V--| element |---V----- - -`---------'That means, this MΩ “resistor” becomes a part of the circuit. Most elements will exhibit V/I much lower than MΩ, making this “resistor” negligible. But not always. If you put in parallel with equally big resistor, or measure across very tiny capacitor, or in a crystal resonator circuit, that is enough to both affect the reading and upset your circuit. So: be aware you are not only reading voltage, but also putting a big resistor in the circuit.Current measurement ports — the COM and mA/A jacks — is usually(1)a very low value resistor. The DMM then measures voltage across it and — knowing resistor’s value — can calculate the current. The resistor is in ohms to tenths of ohm range. You connect it in series, in the path of the measured current:Code:,------------------.| DMM || ,----->,-----------------.| cal- ,-. | || cula- | | Ω…mΩ | || tions `-' | || `----->`---------. |`------------------' | || | ,---------.- - ----------V V--| element |-------- - -`---------'This means, that you basically short two ends in the circuit. And all the current goes through that tiny resistor. That is often not something to be ignored. Aside from mistakes, like the one you just made (connecting a tiny resistor across 110 V), there is some voltage drop on this resistor. This voltage is called “burden voltage” and is unavoidable. As long as impedance in the measured circuit is considerably higher, the effect is small. But otherwise it makes voltages and currents change in the circuit.(1)Let’s ignore clamp meters.

author: inse
author_rank: Super Contributor
timestamp: June 19, 2023, 08:22:45 pm
content: I am tending to rate this as a hoax post.When you connect an unfused ampmeter to the outlet, things usually happen quickly, in a puff it would have ended.No time for wires to start melting until the breaker trips…

author: Ian.M
author_rank: Super Contributor
timestamp: June 19, 2023, 08:59:49 pm
content: Quote from: inse on June 19, 2023, 08:22:45 pm"puff" is too gentle a description. *FLASH* *BANG* would usually be more realistic, but as the O.P. has the US flag selected, we can't rule out sh!tty DIY wiring that would shame a 3rd world country, that resulted in a short circuit current barely enough to trip the breaker, hence a slow trip, and even if the O.P wasn't in North America, we couldn't rule out sh!tty CCS wire in the leads, under-sized enough that its resistance limited the current!

author: Benta
author_rank: Super Contributor
timestamp: June 19, 2023, 09:29:02 pm
content: Let's wait for the attempt to measure the internal resistance of the power station...

author: Ian.M
author_rank: Super Contributor
timestamp: June 19, 2023, 09:47:55 pm
content: We wouldn't hear about that, except possibly on the local news, as the O.P. would be unlikely to survive the resulting arc flash & blast.@O.P. Mains electricity is *DANGEROUS*, the closer to the source of supply, the more dangerous. If you are (electrically) close to a large substation transformer, a short circuit fault can release enough energy to turn you into a crispy critter in milliseconds. A closed casket funeral is the *good* outcome - if you survive, you'll wish you hadn't!

author: bdunham7
author_rank: Super Contributor
timestamp: June 19, 2023, 11:04:00 pm
content: Quote from: inse on June 19, 2023, 08:22:45 pmI've actually managed to do this with an unfused Harbor Freight meter (not the free one) and perhaps the OP has the same model. In my case is it was carelessness and being tired, not a lack of understanding--but the sparks were the same. The meter got the traces blown off the board, the probe tip was burned off and the breaker tripped. There's nothing crappy about my house wiring. I fixed the meter and ground the probe tip back to a point, but I had to replace the wall socket because the internal metal contact was a round blob of brass. The breaker will trip off in <50ms, but available fault currents can be hundreds of amperes in a situation like this, more if you are close to the service panel.

author: bdunham7
author_rank: Super Contributor
timestamp: June 19, 2023, 11:08:31 pm
content: Quote from: Ian.M on June 19, 2023, 09:47:55 pmIf by 'electrically close' you mean sitting on top of, then perhaps. Arc flash isn't really a thing on a household single-phase installation.

author: MMMarco
author_rank: Regular Contributor
timestamp: June 19, 2023, 11:15:12 pm
content: Quote from: stallvalds on June 19, 2023, 12:08:23 pmThe fact that you're asking such a question here is proof that you shouldn't even come close to anything high-voltage, let alonemains voltage.Get an Arduino and measure there, you won't be able to kill yourself accidentally.

author: CaptDon
author_rank: Super Contributor
timestamp: June 19, 2023, 11:29:54 pm
content: Benta, you crack me up as I was thinking the very same thing!! Years ago I had a super nice Philmore analog meter that had colored scales and was so nice and handy. I lent it to a friend who was making resistance measurements and forgot to switch scales and dropped it across 300vdc. The inside face of the meter had those black with blue streaks across it where the meter coil blew apart. I nearly cried when he gave the meter back to me, it was a discontinued model and I bought the last one from a local distributor. Glad the O.P. wasn't close to 480 3ph hehehe. You should see the stupid conclusions I have seen 'experienced' electricians get boondogled over with 480 3ph ungrounded open deltas!!! Don't know why but the rooftop 5hp ventilators where I used to work were open delta fed. I suppose if the circuit were perfect you could touch ground and any 'one' point of the system and not get killed? Cheers!!

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: June 20, 2023, 12:49:18 am
content: First post and hasn’t been back. Trolled!

author: MMMarco
author_rank: Regular Contributor
timestamp: June 20, 2023, 01:28:48 am
content: Quote from: Ground_Loop on June 20, 2023, 12:49:18 amGlad you're admitting to the trollingI'm just kidding

author: golden_labels
author_rank: Super Contributor
timestamp: June 20, 2023, 03:35:54 pm
content: With a faulty joint in cheap probes or an already damaged electrical installation? I say it’s believable the current was not high enough to trip breakers, but enough to overheat and cause cheap plastic to bubble or deform. May be rare, but the other outcomes are mostly invisible to us.In a topic of this kind I find it less important if OP is a troll or not. Sure, they may be in need of psychiatric help or some corrective actions. But in here it’s weighted against a chance of somebody’s safety being at risk. Not only OP’s, but orders of magnitude more people reading this topic later. So if anybody suspects it is a troll, my take on it is: do not respond to them directly, but to a potential future reader.

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 20, 2023, 04:35:56 pm
content: You must use AWG 5 wires to keep them from burning when testing the pole transformer current

author: Terry Bites
author_rank: Super Contributor
timestamp: June 20, 2023, 05:11:25 pm
content: Yes. A lot of us did that as kids and never did it again. DMM + DUMM= Boom.That'll learn ya. Hopefully, now you know volts from amps. Dont stick meter probes in the mains outlets unless you know what's at stake.Dont fix this nasty. Spash out on a decent CATIII one with HRC fuses and one that has some kind of real aproval mark on it.Dont buy off brand crap.A dodgy DMM could cost you an eye or worse. Dont risk it.

author: TimFox
author_rank: Super Contributor
timestamp: June 20, 2023, 05:24:50 pm
content: And when one purchases a good DMM with a proper CAT rating and internal HRC fuse, one should not complain about the high price of replacing that fuse after probing a stiff voltage source with the 10 A ammeter input.

author: RJSV
author_rank: Super Contributor
timestamp: June 20, 2023, 08:41:23 pm
content: If that'd be TROLL, would be believable; stepping on a subject that's a notorious pet-peeve...that being the mis-connection of a voltage, into a current or into a resistance measuring channel.Then, just watch the little Engineers scurry, in indignity.

author: Zero999
author_rank: Super Contributor
timestamp: June 20, 2023, 10:40:44 pm
content: Quote from: Terry Bites on June 20, 2023, 05:11:25 pmI've never done that intentionally. The thought of connecting a meter set to current across a voltage source, be it the mains, or a battery never occurred to me. I always have to restrain myself from calling anyone who said they've done so a retard. On the other hand, I do admit, I've accidentally left my meter set to current and connected it to a battery or bench supply and blown the fuse, which I consider to be careless, rather than stupid.

author: MMMarco
author_rank: Regular Contributor
timestamp: June 20, 2023, 11:17:18 pm
content: Quote from: Zero999 on June 20, 2023, 10:40:44 pmMaybe it's too long ago for you to remember. I certainly did it when I first learned about electronics - so did almost everybody else. 😂

author: BeBuLamar
author_rank: Super Contributor
timestamp: June 21, 2023, 12:15:21 am
content: I did that often enough but it always blew the fuse. I never use an unfuse meter.

author: Ian.M
author_rank: Super Contributor
timestamp: June 21, 2023, 07:35:35 am
content: Heh. Last week, I dug out and repaired my old Altai M102BZ analog multimeter specifically so I could use its unfused 10A range, as I have been working on a LiPO powered device that cold resistance checks lead me to suspect could draw an initial surge current of over 10A, and I'm too cheap to risk a Fluke 11A 1KV HRC DMM fuse unnecessarily. As it turned out, it peaked at around 6A (I suspect a PWMed soft start) which gave me the confidence to use the Fluke to get the actual peak current figure I needed.Like most of us, I've blown a few 300mA range fuses over the years, mostly when I've done something dumb downstream of a meter hooked up to monitor supply current.I've even deliberately briefly shorted a few batteries with a meter on a current range, but only when I was certain the short circuit current would be lower than FSD of the highest current range available, and never for more than a couple of seconds so have never melted test leads doing so, nor blown any meter fusesHowever I find that working on higher energy and higher voltage circuits concentrates the mind - when one is aware of the dangers, and know that if you drop a tool that lands unluckily at best you are likely to get a big and expensive bang, and if you are not so lucky will be far too close to an exploding fireball of burning steel vapour, or worse - you tend to develop the habit of checking meter connections twice before powering the circuit. Others who have a less cautious disposition should probably stay away from high voltage or high energy circuits, and over the years I've encountered a few individuals*you couldn't trust with anything higher energy than tenZinc CarbonAA cells in series!* e.g. One burnt his leg badly enough to need a trip to A&E by putting some Alkaline AA cells in the same pocket as his keys!

author: Zero999
author_rank: Super Contributor
timestamp: June 21, 2023, 08:09:09 am
content: Quote from: MMMarco on June 20, 2023, 11:17:18 pmNo because I knew about short circuits and Ohm's law, so intuitively knew connecting an ammeter across a voltage source would be a stupid thing to do.

author: MMMarco
author_rank: Regular Contributor
timestamp: June 21, 2023, 08:28:56 am
content: Quote from: Zero999 on June 21, 2023, 08:09:09 amGood for you then.