Is it safe to stick my multimeter probes in the wall socket ? - Page 1
Forum section: electronics / beginners

author: openthomas
author_rank: Contributor
timestamp: October 22, 2012, 10:14:46 pm
content: If I set to measure AC voltage , can I probe ground to neutral safely from a wall socket ? Never worked with mains voltage unsure on this.

author: fozzyvis
author_rank: Regular Contributor
timestamp: October 22, 2012, 10:17:07 pm
content: Shouldn't be a problem.

author: c4757p
author_rank: Super Contributor
timestamp: October 22, 2012, 10:29:43 pm
content: Honestly, if you have a reasonably good meter and it's set to measure voltage, you can pretty much stick it anywhere. Most of them are OK up to 1000V - though I'd recommend for reasons besidesmultimetersafety that you avoid one kilovolt unless youreallyknow what you're doing.My best meter (Fluke 8050A) will take 1000V on the 200mV range (exceeding the range "maximum" by a factor of 5000) for up to ten seconds, and 500V on the resistance and conductance ranges. It's pretty hard to blow it up. (Stick it on the mains set to measure current and you'll be short a rather expensive fuse, though)

author: Unknown
author_rank: Unknown
timestamp: October 22, 2012, 11:08:50 pm
content: Make sure the meter is at least rated for CAT II 300 V (assuming residential installation not connected to overhead power cables) and that the manufacturer can be trusted to take safety seriously. VC99 would be out, for example, and I wouldn't trust any random Uni-T meter without checking either. Also check that the probes are at least CAT II 300 V and the insulation of the probes and leads is not damaged.

author: Psi
author_rank: Super Contributor
timestamp: October 23, 2012, 12:23:55 am
content: The main thing you need to check is make SURE the probes are in the voltage position.If you accidentally connect it to mains with the probes in the current (Amps) position it will...On a good DMM:Vaporize a chunk of metal off your probe tips and blow the HRC fuses. Maybe $80 to replace the fuse plus buy a new set of probesOn a bad DMM:Probe damage, plus the meter may melt, catch fire and perhaps explode (depending on household fuses/breakers). Maybe kill or burn yourself and completely destroy the meter.As long as the probes are connected correctly to the voltage terminals and the meter range switch is set to volts AC you shouldn't have any problems.Be very careful not to touch one probe end while the other is connected to mains.While it's highly unlikely to kill when configured for volts (as there is 1M or 10M in series) when the probes are set for current they're essentially shorted so it can kill instantly if you touch one probe while the other is live.Soalways treat both probes as live if one is live.

author: c4757p
author_rank: Super Contributor
timestamp: October 23, 2012, 02:10:39 am
content: QuoteThis. A thousand times this. Any time you are anywhere near lethal voltage, assume there's a voltage across your probes at all times. Never ever touch them.If you're working on an entire circuit with high voltages instead of just mains outlets, I'd recommend clamping one probe to a solid ground, then always probing with one hand off the desk (behind your back if possible). That way you avoid having a conductive path from one side of your torso to the other.

author: samgab
author_rank: Frequent Contributor
timestamp: October 23, 2012, 05:24:34 am
content: I'm of the opinion that if someone needs to ask this question, then the answer is most likely "no" as it reveals a lack of knowledge about electricity and electrical safety. But that's just my opinion. Certainly no electrician would be asking the question.

author: AndyC_772
author_rank: Super Contributor
timestamp: October 23, 2012, 06:52:05 am
content: One major problem with electrical safety - or safety in general, I guess - is that very few people are ever willing to take responsibility for saying what isisOK to do. I've lost count of the number of times I've read things which boil down to "if you need to ask, then the answer is no" - and whilst that no doubt keeps someone safe on that particular occasion, it doesn't advance anyone's knowledge.It's almost as though proper techniques are some kind of secret black magic that engineers aren't allowed to know until they've served their apprenticeship along side one of the privileged inner circle, and have finally proved themselves worthy of being entrusted with "the knowledge".There's a pretty comprehensive guide to working with mains voltages here:http://openbookproject.net/electricCircuits/DC/DC_3.htmlIf it were me, I'd pick one of my Fluke meters (rather than the crappy cheap one that lives in my toolbox), check the probes are plugged into the right sockets, then go right ahead and shove them in the socket. But then again, I'm not a member of the inner circle.

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 23, 2012, 09:13:24 am
content: I think it is more of an intuition thing, rather than a "secret black magic this", eg, if it feels RIGHT/OK to stick the probes in with the meter you have you probably have enough experience to judge if it is right or not

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 23, 2012, 10:03:06 am
content: I realise this doesn't apply to probing a wall socket, but the other thing to watch as well as the voltage, is the likely fault current of the equipment you are working on.It might only be a 230v circuit but if there are busbars involved (low source impedance) then any mistake on your part could ruin your day.So if you ever have to do any high power work, think first about what is going to happen if you do slip with your probes, and plan accordingly.

author: kg4arn
author_rank: Unknown
timestamp: October 23, 2012, 11:09:51 am
content: Here is an interesting publication, from the CDC in Atlanta, on Electrical Safety.There is a table on page 7 showing current vs. injury level.http://www.cdc.gov/niosh/docs/2009-113/pdfs/2009-113.pdfA sobering quote from the document (emphasis is mine):According to the Bureau of Labor Statistics Census of Fatal Occupational Injuries Research File for 1992–2005, electrocution is the fifth leading cause of work-related deathsfor 16- to 19-year-olds, after motor vehicle deaths, contact with objects and equipment, workplace homicide, and falls. Electrocution is the cause of 7% of all workplace deaths among young workers aged 16–19, causing an average of 10 deaths per year.

author: Psi
author_rank: Super Contributor
timestamp: October 23, 2012, 11:16:54 am
content: Some pretty gruesome burn pictures in that pdf.

author: Lightages
author_rank: Unknown
timestamp: October 23, 2012, 04:19:00 pm
content: One helpful tip for measuring higher voltages. Do not use both hands. If at all possible clip one lead to one side of the circuit and use one hand to probe with the other. If probing a wall socket, hold both probes in one hand and insert them that way. This way, if there is a fault the current only flows through your hand and not across your chest.A good general rule for working on high voltages/unknown voltages, keep one hand behind your back.....

author: G7PSK
author_rank: Super Contributor
timestamp: October 23, 2012, 05:30:41 pm
content: Ascertain the neutral first if at all possible and connect to this first then put the other probe to the live with one hand and if at all possible withe meter resting on something and not in your other hand.

author: Neilm
author_rank: Super Contributor
timestamp: October 23, 2012, 07:11:34 pm
content: Another tip is to remove any rings from your fingers or wear gloves if you have to put your hand in a restricted space. One of the worst injuries I've ever seen (thankfully in a picture not real life) was the remains of a guys finger who had a ring on when it shorted out a bank of batteries.Neil

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 24, 2012, 07:03:33 am
content: If you're working on mains, never be alone, safety first, never rush, ignore the deadline, it doesn't matter if you're dead.

author: AndyC_772
author_rank: Super Contributor
timestamp: October 24, 2012, 07:32:13 am
content: Quote from: kg4arn on October 23, 2012, 11:09:51 amHang on a minute... not only am I much more likely to be killed in a car crash going to or from work than they are to be electrocuted while working - I'm actually more likely to bemurdered by a co-worker...?!OK, I'm not in that age bracket any more, and don't live in a country where ordinary citizens carry guns, but even so it seems like a good reason to have a lot of confidence in electrical safety procedures. Maybe I should overcome my general fear and apprehension when it comes to working with the mains after all?

author: ciccio
author_rank: Frequent Contributor
timestamp: October 24, 2012, 07:39:21 pm
content: Quote from: Neilm on October 23, 2012, 07:11:34 pmAgree.And another one is to never wear metal bracelets or all metal wrist-watches...You can lose an entire hand if the metal touches a live part.I always wear an cheap all plastic wrist-watch when working on mains circuits.Best regards

author: Unknown
author_rank: Unknown
timestamp: October 24, 2012, 07:51:22 pm
content: Do you really take all those safety precautions just to measure the voltage of a wall outlet? It's important to be aware of the dangers, but decent probes should be fine as long as you keep your fingers behind the finger guard.

author: c4757p
author_rank: Super Contributor
timestamp: October 24, 2012, 09:03:19 pm
content: alm, I work with high-ish voltages a lot (vacuum tube audio equipment), and when you're used to doing that, yeah, those safety precautions become second nature. I would never wear metal jewelry or a metal watch around anything high-energy(so car batteries are right out, too - I've heard horror stories from my nurse mother about metal jewelry completing a circuit on one of those). I avoid putting both hands at once near the device, I use a clamp probe to attach to ground if possible to free a hand, and so on. I've been shocked many times at voltages much higher than 120/240 and I'm all right, but I'll be damned if I put unnecessary electrodes on my skin and touch unnecessary voltages because I can't be bothered to move a hand away and take my jewelry off.

author: ivan747
author_rank: Super Contributor
timestamp: October 24, 2012, 11:28:12 pm
content: I heard from one electronics teacher that one of his friends was tightening a nut on a lead-acid battery with a wrench when he touched both contacts. The wrench welded into the contacts and they had to hit it with a hammer to separate it. Lesson: be careful with high energy circuits not just high voltage ones. 12V at hundreds of amps can produce nasty stuff like this.

author: Psi
author_rank: Super Contributor
timestamp: October 25, 2012, 12:03:39 am
content: When dad worked at the airport someone dropped a small wrench in the 120V ILS backup battery bank (entire room of batteries.)(We assume it landed across the full 120v bus bars.)After the flash no amount of looking could find the wrench and everything around the area was plated with metal.

author: SeanB
author_rank: Super Contributor
timestamp: October 25, 2012, 04:54:09 am
content: Wrong spanner for the application. There are regulations for battery room tools, which include full insulation of the tool with only one end of the metal at the working face exposed. I wear a metal banded watch, and always take it off when working with the car, not only for the battery, but to keep it clean and not to have anything that can catch. Rings have the same warning as well. I have met mechanics with piercings and studs, but none of them work with them in.

author: amyk
author_rank: Super Contributor
timestamp: October 25, 2012, 05:58:15 am
content: Quote from: ivan747 on October 24, 2012, 11:28:12 pmThis is also why the negative lead should be the first to come off and last to go on when working with a car battery, to reduce the chances of shorts to the grounded chassis. (Positive-ground vehicles should have the order reversed).

author: Unknown
author_rank: Unknown
timestamp: October 25, 2012, 09:21:17 pm
content: No argument that these precautions are necessary when working on high-energy circuits. I'm aware that vaporization is a perfectly normal failure mode for metal tools when working on large battery banks. But assume you use suitable probes and DMM and plug the probes into a wall socket. What would it involve to get these kinds of failures? Depending on the plug your country uses, it's barely more dangerous than plugging in a real plug: make sure you keep your fingers away from the metal parts.

author: ptricks
author_rank: Frequent Contributor
timestamp: October 25, 2012, 09:27:01 pm
content: The deaths associated with electric shock are mainly not from the electricity , it is from the reaction to the electricity. What is the first thing you do when you get shocked ? You jerk back your hand or whatever is contacting the power. If you are on a ladder changing a light bulb and get shocked, you jerk back, you fall off the ladder. Or you are working on something small, not really dangerous, you get shocked jerk back and hit your elbow on something hard, doesn't kill you but injures you more than the shock alone did.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 26, 2012, 12:10:02 am
content: Certainly things like falling off a ladder can kill you even if the shock doesn't, but 115 or 230 VAC can certainly kill you by electrocution, and saying anything that implies otherwise is just reckless, stupid, and wrong. There are many factors that affect electrocution danger like the condition of your skin, your heart health, and what path the current has to take to ground, but never just dismiss the risk.

author: Psi
author_rank: Super Contributor
timestamp: October 26, 2012, 12:15:39 am
content: He never implied 115 or 230 VAC cannot kill.All he implied was that deaths/injuries from incidents involving an electrical shock are more often from other factors than the shock itself.Which could be true since not all shocks approach the current needed to kill but can still give you a big fright and result in a fall or injury.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 26, 2012, 12:20:54 am
content: Quote from: alm on October 25, 2012, 09:21:17 pmThe great thing about Australian standard sockets (GPOs),apart from the fact that they are all switched,is that standard DMM probes can be plugged into them,& will be retained in the socket.(Tough titty to all you non-Oz people!)It then becomes easy:Make sure you are connected to the volts inputs on your DMM,switch the thing to AC Volts,place the meter on a convenient surface,if you are really paranoid,then switch on the GPO.Now read the voltage---Easy! caveman can do!!

author: psycho0815
author_rank: Regular Contributor
timestamp: October 26, 2012, 01:22:45 pm
content: Quote from: vk6zgo on October 26, 2012, 12:20:54 amI'm pretty sure you could fit standard 4mm bananaplugs into an german schuko-plug, that would give you a solid hands-free connection. Most of them aren't switched though, so you win.

author: G7PSK
author_rank: Super Contributor
timestamp: October 26, 2012, 05:40:53 pm
content: The UK also has shuttered and switched power socket that you can put your probes into and they stay there.

author: T4P
author_rank: Super Contributor
timestamp: October 28, 2012, 05:59:32 pm
content: Sockets in UK have a fantastic invention mostly no other sockets have : A Switch.That said, you can prepare yourself if something big is coming but the BS1363 sockets themselves have really huge conductors and great plug retention system, it takes effort to yank a plug out by accident and if it does, a weak wire will snap instantlyWell it's also quite hard to accidentally poke something into the live and neutral terminals ... you can't, at least without pushing a chisel screwdriver into the earth terminal

author: dr_p
author_rank: Frequent Contributor
timestamp: November 16, 2012, 11:49:06 am
content: I remember when I probed 220V mains for the first time: thorough test cables inspection, triple checked the 500V AC range, two sets of gloves %-B, rubber sole shoes. Nearly wet my pants when I saw the reading on the LCD.

author: jeroen74
author_rank: Frequent Contributor
timestamp: November 16, 2012, 02:18:49 pm
content: A long time ago, somewhere in the eighties, when I was still a kid and quite ignorant, I was in the workshop of my father's computer shop, and I decided to measure how many amps the mains could deliver.So I stuck the probes of the IIRC, Fluke 77, into the nearest socket I could find, a bang... and everything went dark, the whole shop, the whole street. Somehow a fuse in a distribution cabinet in the street failed, not just a local breaker. My father did not appreciate my little experiment

author: LAZLOT
author_rank: Contributor
timestamp: November 16, 2012, 04:47:33 pm
content: Check Dave's Blogs on cheap multimeter's... He has a lot to say re: safety of some of the cheap Chinese units- food for thought. I have five m-m's, only one of which I trust with high power. I have previously checked line-voltage (in Aus. it's 240V, with less current potential, but same power overall) with cheap m-m's with no ill effects. BUT be ABSOLUTELY CERTAIN you have the m-m set correctly- i.e AC Volts in the right range setting, depending on the m-m. Different m-m's have different range maximums. The safest way is to start at the highest setting (750 volt?)- or whatever your m-m has, then work down to increase resolution, stopping before you go too far! Also, you should ensure the red lead goes to the active terminal for maximum safety. BOTTOM LINE... DON'T do it until an electrician has shown you how to do it and keep safe, AND checked your m-m! Frayed lead's, poor quality leads (my oldest m-m leads have smooth handles with no way to stop your fingers sliding down onto the probe tips), or a dodgy m-m are just a few of the potential hazards. View ep's. 6.1&2, 10.1&2, 66, 91 and esp. 94-and some tutorials, not mentioned here, but, again GET PROFESSIONAL ADVICE FIRST!

author: Monkeh
author_rank: Super Contributor
timestamp: November 16, 2012, 05:30:30 pm
content: Quote from: LAZLOT on November 16, 2012, 04:47:33 pmLess current potential than what?

author: LAZLOT
author_rank: Contributor
timestamp: November 16, 2012, 06:11:40 pm
content: Quote from: LAZLOT on Today at 04:47:33 PMin Aus. it's 240V, with less current potential, but same power overallLess current potential than what? - From MonkehGetting a bit late here, and I'm noodling up my terms... What I should have said was -we have higher wattage items, but same power overall; i.e. same current (compared to England's 120 Volts). Eg; Aus 240V @ 2400W=10A, GB 120V @ 1200W=10A. Pls excuse my dylsexia!

author: Monkeh
author_rank: Super Contributor
timestamp: November 16, 2012, 06:18:26 pm
content: Quote from: LAZLOT on November 16, 2012, 06:11:40 pmUhm.. The UK operates on 240VAC 50Hz. The reason you operate on 240VAC 50Hz is.. well, because we do. That's actually why most of the world uses 240VAC 50Hz or near enough..And power is measured in watts, not amps.

author: SeanB
author_rank: Super Contributor
timestamp: November 16, 2012, 06:42:23 pm
content: I think you mean USA, not GB.The problem with mains is that the potential current is incredibly high. A 16A socket outlet ( or 10A, 15A or whatever is the local socket) generally is fed by a cable with a resistance in the order of milliohms. Typically 0.2 to 0.3 ohms resistance for a typical socket outlet. The supply point to the house can have a cable resistance of under 100 milliohms, and the supply network typically can be considered to be zero or pretty close to it.Thus you have the potential to have short circuit currents in the order of 1000A for a typical socket for the time it takes the protective device to react and open the circuit. Thus the need for 1kA rated fuses in power distribution in the home, and for a class 3 meter to have 1ka rated fuses.If you are measuring at the point of supply you have the potential short circuit current being higher, 10kA or more until the protection opens. Thus the class 4 fuses being rated for 10kA. Your supply fuses are rated for 20kA breaking typically, and are large and expensive ceramic blocks. They have to be big to withstand the pressure from the failing element inside. The linesmen typically wear blast shields and blast aprons when putting these into the panel when replacing them, as they can let go explosively.If you are measuring at the upstream side you need training, and a meter certified to be used on these lines. These typically have fused leads as well, as the internal fuses may be inadequate.

author: Monkeh
author_rank: Super Contributor
timestamp: November 16, 2012, 06:51:43 pm
content: 1kA and 20kA? What is this, the 50s?Fault currents are often much higher than 1kA. Where they'relowyou need supplementary protection to prevent fire before the normal protective devices can operate.

author: T4P
author_rank: Super Contributor
timestamp: November 16, 2012, 07:06:01 pm
content: 1kA only? It would be fine if your substation transformer is roughly about 240kW but it's not in my case ... it's roughly about 2.4MWLinesman wear Arc Flash PPEs for a reason when going anywhere near fuseboards or switchboards ... if anything happens it's not them getting blown into pieces/burnt into crisps

author: SeanB
author_rank: Super Contributor
timestamp: November 16, 2012, 08:48:37 pm
content: 1kA at the plug outlet. All bets are off at the breaker box, the meter point or the substation. An assumption is that there will be no faults within 3m of the distribution panel, which is used to calculate the breaking capacity of breakers. Here you get a residential breaker that is 2.5kA rated, but newer ones are 3kA. Light commercial come in 5 and 10kA ratings, with a common breaker range from 5A to 80A for non specialised loads. If it is hard wired then you can go up to a 100A breaker with 15ka rating. Transformers still have fuses, as they are more reliable, and always fail open circuit. At work I have a little 250kVA substation that provides power, and an unused backfeed to the 2MVA transformer 50m away. Connected but no fuses, the Metro would have to come and place the 3 800A fuses to use it, though both poles of the fuse holder are live to show cable damage, using 200A fuses at the other end. The 11kV cable has had a few incidents involving pickaxes, but the protection relays did work to cut the power, or the 11kV line fuses did the same on the ring.

author: Monkeh
author_rank: Super Contributor
timestamp: November 16, 2012, 08:52:19 pm
content: Quote from: SeanB on November 16, 2012, 08:48:37 pmSorry, that won't cut it.QuoteWhat's this betting about, then? It's easily calculated and tested.QuoteUuuhhm, that's a ridiculous assumption.QuoteSo very 70s.

author: SeanB
author_rank: Super Contributor
timestamp: November 16, 2012, 09:08:20 pm
content: Not in the UK............Most fires are caused by exploding paraffin stoves and illegal wiring, not from home wiring.

author: Monkeh
author_rank: Super Contributor
timestamp: November 16, 2012, 09:13:39 pm
content: Quote from: SeanB on November 16, 2012, 09:08:20 pmLaws of physics are the same everywhere, and no matter where you are, it's generally considered a good idea to get with the times.

author: G7PSK
author_rank: Super Contributor
timestamp: November 16, 2012, 11:06:45 pm
content: I get the impression that some people on here are scared shitless by the mains. As long as reasonable precautions are taken there is no reason to be scared of a few hundred volts in a system nor of high amps for that matter. The highest amperage that I have worked with is ten thousand, quite interesting as when switched the field in the conductor would make 200 KG of steel jump.

author: Psi
author_rank: Super Contributor
timestamp: November 16, 2012, 11:32:16 pm
content: Quote from: G7PSK on November 16, 2012, 11:06:45 pmEven with all the precautions taken, people should still be a little scared of it and should definitely respect it.The moment ya start to be causal about it is the moment you make a mistake.It's pretty much the same as handling a loaded gun. It has the capacity to injure or kill you.Except that the trigger is potentially any metal object.

author: nixxon
author_rank: Frequent Contributor
timestamp: November 17, 2012, 12:36:00 am
content: If this compares to aviation pilots, the safest people are either the newly graduated or the senior workers. Worst off are the ones that are neither rookies, nor seniors. These are the ones that believe they are the best of the untouchables...

author: G7PSK
author_rank: Super Contributor
timestamp: November 17, 2012, 10:13:35 am
content: I meant to put caution to be used, but it is caution not fear, if you are afraid that is when it is most likely you will make a mistake.

author: jeroen74
author_rank: Frequent Contributor
timestamp: November 17, 2012, 10:35:59 am
content: I once accidentally touched a fully charged photoflash capacitor (330uF@380V). That will teach you some respect. My whole arm hurt for two days.

author: madires
author_rank: Super Contributor
timestamp: November 17, 2012, 12:12:24 pm
content: Quote from: jeroen74 on November 17, 2012, 10:35:59 amOuch!

author: ptricks
author_rank: Frequent Contributor
timestamp: November 18, 2012, 12:45:21 pm
content: Quote from: jeroen74 on November 17, 2012, 10:35:59 amI have done that too. Was taking apart a digital camera and the cap voltage ran through a circuit board far away from the cap, touched it and damn that hurt

author: G7PSK
author_rank: Super Contributor
timestamp: November 18, 2012, 09:13:31 pm
content: I once got a 10K jolt from an electric fence that did no hurt as much as the landing on top of the metal gate. I was opening a metal gate while sitting on a horse and baked the hose into the electric cattle fence, I got zapped the horse got zapped and jumped forward about ten feet and upwards to boot so I got launched and landed on the gate one leg each side.

author: M. András
author_rank: Super Contributor
timestamp: November 19, 2012, 07:19:57 pm
content: Quote from: G7PSK on November 16, 2012, 11:06:45 pmi can imagine the fences and blast proof walls around each socket in their house and the long ceramic rod which they use to plug in the plugs