DC meter Plugged into AC outlet


  #1  
Old 04-28-21, 09:56 AM
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DC meter Plugged into AC outlet

Ever wonder what might happen if a VOM meter set to DC volts is inserted into an AC outlet?
Well I can tell you...BOOM!, FLASH, Lots of smoke and a stench that won't go away for hours. I had inadvertently set the meter to DC thinking it was AC. Go to test an outlet. It blow the red probe about 5 feet away and disconnected from the VOM. Surprisingly enough the outlet still works and the VOM meter seems to OK also. Just need to reconnect the red probe. Slight meting on the edge of the VOM case.
 
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Old 04-28-21, 02:09 PM
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I doubt that meter is certified for mains work. 🤯
 
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Old 04-28-21, 03:03 PM
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Well I certainly won't rely on it. But I'm going to put it through it's paces just for the heck of it. Then throw it out.
 
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Old 04-28-21, 06:14 PM
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Regardless of DC or AC, ideal voltmeter is infinite resistance. Therefore, all voltmeters will have very high resistance.
So, most likely nothing will happen. However, it is possible insulation on wire or circuitry might not be good enough for 120V and cause short circuit..

As for what happened with your VOM, are you sure you were in voltmeter mode? Not current measure mode? Or maybe you were in low voltage ranges?
All meters I have that can do both DC and AC never had such a issue. You just don't get any voltages in DC mode. Didn't try going down to low voltage ranges though.
 
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Old 04-28-21, 06:54 PM
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I've had my meter set in DC and accidentally measured AC mains. No problem other than an inaccurate measurement. It never let out the magic smoke.

Seems like your meter couldn't handle 120V (AC or DC) or you had the input jacks measuring current, which I've embarrassingly done too.
 
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Old 04-29-21, 04:06 AM
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This is what my setting were. Notice where the positive lead is plugged in?
 
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Old 04-29-21, 05:39 AM
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Yep. I've done that before. Just to note, it doesn't matter much where the dial was set, sending 20 or more amps thru a shunt rated for 10 amps is sure to burn out one or more parts.

Good thing is, these meters are as near to "one-time use disposable" as any you're going to find.

A few flames will remind you to never try this with an expensive meter!

 
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Old 04-29-21, 06:15 AM
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Thanks for sharing! Consider yourself very fortunate.

Cheap meters have very little, if any, input protection and your dmm is the cheapest of cheap. I'm shocked that it didn't blow up on you. I'm pretty sure that meter is toast whereas a good quality meter might just need a fuse or maybe a new PTC thermistor?

If you are now in the market for a new multimeter, check out the teardowns on youtube. The Flukes and Extechs are really good and safe but expensive.

I have a decent cheap-o multimeter, but I almost never measure current with it unless I'm dealing with voltages that I'd be willing to put across my tongue! Just the thought of accidentally leaving the leads in the current jacks and then going to check on a household receptacle makes me nervous, so I try to make it habit to remove them when I'm done measuring current. Even then, I've still forgotten and done what you did.
 
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Old 04-29-21, 08:54 AM
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I could have lost an eye once. Using a lowly neon test lamp on a 208v source. It quickly “dissassembled” itself, and shot the pointed top of the lamp across the room.
Use rated gear on high energy mains.
A half way decent DMM has high energy fuse protection on all ranges, even the DC shunts.
I have even non electronic Triplett VOM that has internal fuse protection.
 
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Old 04-29-21, 10:18 AM
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I wish those small test meters came without an amperage test section. Most users have no idea how to use it and don't need it. The dead short issue has been a problem for many users. You were lucky. Others have not been.
 
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Old 04-29-21, 11:01 AM
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Well at least I'm not the only one.

DINKO, I would consider a higher quality meter, but I don't really use it that much. This is the first time I ever did something like this.

Just shows that no matter how careful, the accident can happen.

And PJ, yes I realize it could been an injury type thing. I was lucky indeed.
 
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Old 04-29-21, 04:14 PM
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I doubt those cheap meters can even really handle rated 10A..
I am also scared to use that on high voltages. Insulation on the leads are very weak. Also, most of them lack required air gap between high voltage section and low voltage section.

Get $20~30 meters. It isn't foolproof, but safer to use.

High end meters are fused on both low current and high current part. Cheap ones are only fused for low current and even cheaper ones has no fuse at all.

My old Tektronix meter sounds alarm if I have leads plugged on current meter part and switch to voltmeter mode. This saved me few times.
 
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Old 05-06-21, 06:43 PM
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Meter in picture is good, reliable Gen-Tech Digital Multi Meter. Norm201 meter was damaged by miss use, not product defect.

Have used them for years without problem to measure resistance, AC and DC voltages and current.

To measure voltages one probe goes in bottom black socket. Other red probe goes in middle Volt, ohm, ma (milliamp) socket.

To measure DC current at 1 to 10 Amps use red probe in top socket labeled 10ADC . Do not use that socket to measure AC current. Amp/current meters are basically a short circuit when connected across a voltage source. They must be connected between voltage source and unit powered.

Another basic is do not check resistance with power on... it will damage meter. If in doubt check first for zero voltage.

Norm201 connected 10 amp probes to 120 Volt AC oulet. Meter was1200 watt load and went poof. Text and picture shows what happens when labels are not read and followed

Result is cheap $6.99 lesson in how not to do it: https://www.harborfreight.com/7-func...ter-63759.html
 

Last edited by doughess; 05-06-21 at 07:30 PM.
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Old 05-07-21, 05:10 AM
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Doug, you sound like Captain Obvious! LOL
Never said it was defective. I openly said I mis-used the unit. I've used that meter for many years and know all the basics. The pic obviously shows I plugged the positive in the wrong socket. I though I had it plugged in the right section. This was a pure accident, that could've turned deadly. What it proves is that my eye sight is not what it used to be.
In reference to the others who claim the unit is cheap and "waiting for an accident to happen", these unit are meant for the non-electrician and the occasional person who needs to just check voltage and ohms. It does not mean that you don't need proper electrical knowledge. And if you don't know or care about how electrical items work then you have no need for any VOM.
If you deal with electronics and electric devices on a daily basis, of course you do not use one of these units. But if you're like me who only needs the use of a meter perhaps 6 times a year or less, I'm not about to spend $$$ on a VOM. That's like telling the homemaker who needs to hang a picture on the wall to buy the most expensive hammer, laser level and a Mekita multifunction drill along with special screws and a book on how to hang pictures, when just a nail does the job.
 
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Old 05-07-21, 08:16 AM
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Amp meter is “short circuit” when connected directly to AC or DC power source, i.e. AC outlet or battery.

User does not even have to know difference between AC and DC or ohms and volts. Should think of it as putting finger in live bulb socket .. a NO NO

When Gen-Tech 10 ADC probe and ground probe is short connected to circuit. If circuit has power source and some circuit item will display current. If no item ...pow

Cost/price of meter is not issue, the user is.
 
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Old 05-08-21, 05:53 AM
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Once again, make sure the amp ranges have fuse protection. Yours eyes and face may thank you one day.
I spent some time watching arc flash hoods get tested. Molten metal and flying plastics will mess you up.
 
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Old 05-08-21, 07:09 AM
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Normally I agree that if you only use something a few times a year, there is no reason to "splurge" on quality equipment.

However, when it comes to safety, even just measuring household line voltage once a year is enough to kill you. Here's my analogy: If I only need to across country once a year, I don't take a motorcycle; they are just too dangerous to me (well, it's mostly the other cars that are dangerous, but I digress). Yes, I can get killed driving a "safe" car too, but a safe car allows me much more margin of error for my mistakes.

A multimeter without decent input protection just isn't worth even if I only measure line voltages once a year especially when you don't have to spend a lot to get adequate input protection. Some $20 meters are decent while some $50 meters are atrociously bad at protecting the user. If I'm just measuring low voltage hobby stuff, then any meter will suffice. I won't measure 240V with just any meter even if it were once a decade.

I don't own a Fluke but to me that would be more likely to save me even if I were careless. To use my driving analogy, it'd be like driving in a Volvo wearing a helmet (is Volvo a safe car or are the people who drive Volvos just safer drivers??). If I drove everyday for a living at 70mph (like being around 240V or higher), I'd want the safest car (or meter) I could buy.

I don't like to buy expensive tools either especially when I'm not making money from them. But when it comes to a tool that can kill me if not used right, I at least like to be aware of what I'm getting with my money and I don't always assume more expensive means safer.
 
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Old 05-08-21, 08:07 AM
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Look on the GenTech as a $6.99 fuse.

Key issue is not price of meter but user.

DH has many years electrical/electronic experience.

Cheap, light weight meters are expendible, accurate and disposible.

Haborfreight.com was giving Gen-Techs away. Have several.
 
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Old 05-09-21, 07:27 AM
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I like the idea of the alarm when leads are plugged in wrong for the setting. A neighbor's unit had a beep on the continuity setting so he didn't have to see the VOM in continuity mode.

Not to hijack the thread- can y'all suggest a good low to mid price point VOM for homeowner use?
 
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Old 05-10-21, 04:08 AM
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A clamp meter is probably more useful (and safer) for average home use.
You can go with a Fluke for $125, or a Klein for ~$50-75, or a $25 Chinese meter from Amazon.

If you only do low-voltage DC hobby work, most anything is OK. Just leave it in the drawer when you do AC work.

Sort of like owning a garden trowel and a shovel. They both dig in the dirt, but you use the appropriate one whenever possible.

BTW, if you plan to do solar, an "inrush" meter is a good investment, so that you can determine how much current your appliances like refrigerators and air conditioners require when starting up. This, in turn, determines how big an inverter you need to buy.

 
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Old 05-10-21, 02:11 PM
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I learned as a kid working on my train set that when you plug your volt meter into 120V AC on the DC setting that smoke comes out.
I keep wanting to get a nice fluke or something along those lines, but don't use a meter quite enough to justify it and those HF freebie coupons for the Gen Tech keep coming along lol.
 
 

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