13

I was recently told that the common 120V electric receptacles (non-GFCI) found in North American homes can be extremely dangerous to measure using a multimeter. FYI, this came up while I was checking proper connectivity of the wiring directly using the three-prong outlet slots.

The specific concern is not that the operator will mistake hot, neutral or ground slots and stick the multimeter leads into the wrong slot(s). Rather, the stated concern was that the internals of an electric receptacle are not completely isolated from each other and by sticking a single lead into one of the slots, the operator may actually short two or more of these (e.g. Sticking a prong into the ground slot may result in the lead touching both ground and also either hot or neutral).

I am a DIY hobbiest that enjoys upgrading my home for smart components. This warning about the internal structure of a basic electric receptacle set off many warning flags. It's hard for me to imagine that this would be true due to the potential danger but I do not understand how these common electric receptacles have evolved over the last 60 years. Maybe way back in the day there were outlets where this actually was a concern. I hope that this is not possible in modern day receptacles but this is where I'd love to lean on this forum to help set me straight!

JACK
  • 72,186
  • 15
  • 68
  • 170
Travis
  • 131
  • 1
  • 4
  • 3
    Even if true, how is this more dangerous than using a multimeter misconfigured for amps instead of volts? I've got one that says it can measure 100 amps. (It actually doesn't because the fuse is blown.) – Joshua Feb 14 '22 at 15:56
  • Yeah, I once blew out a meter by having it set to ohms while probing a 120v power outlet. But just about anyone can make such a mistake -- I'm sure that electricians and appliance service guys do this fairly often. – Hot Licks Mar 24 '22 at 21:19

9 Answers9

29

Unless you shattered the insides of a normal outlet, you wouldn't be able to short out the outlet using a multi-meter. Looking at the picture below,

outlet with cover removed

each female part of the socket is in its own chamber that will be fully enclosed when the top part of the outlet is in place. The ground connection is also in its own enclosed chamber with the tops pressed into the top cover. You would literally have to punch through the chamber walls or pry through the top of the outlet to get through to other connection points.

JDługosz
  • 1,101
  • 5
  • 12
JACK
  • 72,186
  • 15
  • 68
  • 170
  • 12
    It might be worth adding a caveat that this is true of modern (as in, last 20, maybe 30 years) receptacles, but I wouldn't confidently say that every receptacle in use is like this. I've definitely retired devices from my old house that were much less well designed. – nexus_2006 Feb 13 '22 at 19:34
19

Don't handle the probes carelessly. Given that the probe metal contact has some length, it's possible to bridge them across two contacts or cross them (although this would be fantastically difficult while probing a NEMA socket).

Meters don't short things.

Meters are designed to be very high resistance. They are designed to minimize "observer effect", i.e. change the thing they're measuring as little as possible.

Except in ammeter mode, when they definitely short things!

However most meters have an "ammeter mode" in which they are intentionally a dead short through the circuit. Generally on most meters, you have to move one of the probe(s) to a special port used only for ammeter measurements.

If your probes are in the "special ammeter port", then yes, it will create a dead short across whatever you are measuring, even if you are set on volts or amps! Do you have any idea how expensive the fuses in a Fluke meter are? I know because I've blown a few from forgetting the probes were in the amps port.

Cover up the amps port. You'll never use it.

The novice is best putting a piece of clear tape across that port, as it should never be used (for AC power). To measure amps in AC power, we have a much better way to do that called a "clamp meter". This is non-contact. It takes advantage of AC's ever-changing electro-magnetic fields, to sense current inductively (the way a transformer works, in fact, those are called "current transformers"). Doesn't work on DC.

Now if you're thinking "OMG all my electric wires throw these big magnetic fields", relax. In properly done wiring, the current in any given cable is equal and opposite - current going out one wire comes back on another wire in the same cable. As such, the magnetic fields cancel each other out and are nullified. This removes the magnetic fields. It also means a clamp meter won't work on a whole cable. (that's properly wired).

Harper - Reinstate Monica
  • 276,940
  • 24
  • 257
  • 671
  • 6
    A multimeter probe can definitely short things, if you touch two different parts of the circuit with the *same probe tip* at the same time. The way I read the OP's question, that's what they're worried about. Your answer seems to be about creating a short circuit *through the meter*, which is not what the question seems to be asking about. – Ilmari Karonen Feb 13 '22 at 02:48
  • 1
    @Ilmari fair enough, I added a bit at the beginning. – Harper - Reinstate Monica Feb 13 '22 at 03:34
  • 2
    I saw a friend connect an analog multimeter on an amps range to a charged 12v car battery. There was quite a lot of very smelly smoke and the meter was dead after that. – Michael Harvey Feb 13 '22 at 11:53
  • Not all meters have detachable leads. For those that don't, setting the dial to A instead of V is a realistic hazard. I'm not just referring to cheap no-name junk, because my one like this is a Wavetek – Chris H Feb 14 '22 at 12:55
  • @Harper Why yes, I do know how expensive those fuses are, and why: it's because the good folks at Fluke specified a fuse with a ridiculously high "interrupting rating". And who ever even heard of "[interrupting rating](https://en.wikipedia.org/wiki/Breaking_capacity)" as a specification for a fuse? (I'm sure you have, but I hadn't, until I opened my meter up and found that fuse. First time I ever saw the unit "kiloamp" used in practice, too. ☺ ) – Steve Summit Feb 15 '22 at 17:28
  • @Harper, No, I don't know how expensive fuses are, because my Harbor Freight meter didn't use them for the higher amp range. Instead, I got to find out how much an entire new meter costs, since they didn't have replacement leads, and the internals of the old ones had decided to migrate to the outside of the insulation. – James Feb 16 '22 at 00:38
12

It is possible in any given outlet that behind the contact for one prong there is an uninsulated metal component leading to a different prong. But to short circuit the two together with a probe would require a bizarrely designed probe combined with recklessly aggressive, almost purposeful, insertion of it. If you use an ordinary probe and insert it with even the minimum amount of focus and caution that would be due when working with an energized outlet, I would say this is not one of the dangers you need to be worrying about. You hold the plastic part of the probe, and insert it carefully with the intent of just making contact and of not breaking the outlet or ramming the probe through it and into the next room.

jay613
  • 28,850
  • 2
  • 36
  • 112
6

Use a multimeter designed for what you're using it for!

Your multimeter has a category rating, from I to IV, and a voltage area. This tells you something about what kind of systems it's intended to be used on.

A Cat I multimeter is intended to be used on systems with very low energy, such as a low voltage apparatus powered by a wall wart. Even if something goes horribly wrong, the short circuit levels and voltages are low, so the total amount of energy is low.

Cat IV is when working with high energy systems, such as distributions busbars with a large feed capacity.

In addition to the category there's a voltage marking, from 150V to 1000V. This indicates the applicable voltage at which you can use the device. Commonly a device can have multiple voltage and category ratings; it may be CAT IV - 600V and CAT III - 1000V, meaning that it's category IV up to 600V, and III up to 1000V.

The rating is applicable to leads as well, and indicate how well they will handle a fault situation. In your case, measuring at the wall socket, I'd go for Cat III. Cat II may be applicable, and Cat IV is probably overkill. If used properly (e.g. fingers behind finger guards on probes), even a short circuit at the tip of the probe should not place the operator in danger.

Using a multimeter with a higher than necessary category rating is not dangerous or detrimental in any way.

Another common misunderstanding is that a Cat III 1000V is better than Cat IV 600V. This is not so. They are intended for different use areas.

vidarlo
  • 1,505
  • 9
  • 16
  • It seems that there is no commercially adapter to allow convenient connection of multimeter probes to a standard 120 V receptacle. This would be useful to me. – Jim Stewart Feb 12 '22 at 18:15
  • 3
    The problem with the high category probes is they tend to have very short and fat tips. Fine if you have large industrial terminals to probe, not so useful if you are trying to probe the contacts of a socket. You could take the socket of and probe the terminals instead but i'm not at all convinced that would be safer. – Peter Green Feb 12 '22 at 18:29
  • 1
    @PeterGreen [Fluke twistguard](https://www.fluke.com/en/product/accessories/test-leads/fluke-tl175) is a good solution to that problem, and allows you to vary the probe between a Cat IV 600V and Cat II 1000V... :) – vidarlo Feb 12 '22 at 18:31
  • My issue with the basic 2-mm dia sharp pointed probes is that I have to force them into the receptacle and the grooves catch on i section and removal. Why are there no adaptors with blades to go into the receptacle and holes sized to receive the probes? – Jim Stewart Feb 12 '22 at 23:54
  • It might be useful to say more about what the Cat ratings mean. My understanding is that, for the same nominal voltage rating, higher category meters are designed to withstand much larger transient voltages, in case of a fault. A Cat II or a Cat III meter rated for 600V should keep you equally safe from a normally-functioning 120V outlet, but the Cat III meter is expected to protect against high-voltage faults that could reach the outlet, whereas the Cat II meter is not. See e.g. https://www.digikey.com/en/blog/what-are-multimeter-cat-safety-ratings . – Glenn Willen Feb 13 '22 at 06:07
  • 1
    @GlennWillen That link is included in the answer. The answer also gives enough context for reading up on category ratings using external literature. I don't see a big need to duplicate that detail here. – vidarlo Feb 13 '22 at 07:27
  • @JimStewart You could use a kill a watt-style device, or you can get a plugin voltmeter. Or did you want to measure something other than voltage? – Andrew Morton Feb 13 '22 at 12:31
  • 3
    It's strange, over here in the UK every major vendor of electricians test equipment vendor sells test adapters that plug into a regular UK domestic socket and give you 4mm shrouded sockets for your test leads., but I can't find any for american sockets. – Peter Green Feb 13 '22 at 14:42
  • Residential outlets are CAT II, so almost any cheapie meter is fine. You don't need to worry about CAT III or higher until you're in a commercial/industrial facility OR if you're working on lines outdoors. – J... Feb 14 '22 at 14:01
  • @J... Your outled is probably Cat II, but your incomer may very well move into CatIII territory. – vidarlo Feb 15 '22 at 09:57
  • @vidarlo OP wasn't asking about testing the neighborhood transformer... – J... Feb 15 '22 at 10:11
4

If you are a European reading answers to this US-oriented question, the following applies. If you are one of the other 90% of people who live elsewhere, it may or may not.

These are made for the purpose and are cheap:

Mains socket tester

Mains Socket Tester £3.59

You plug it into the wall outlet and then plug your appropriately rated (usually 1000 V) 4mm banana plug leads between this and your appropriately rated (Cat II or better) voltmeter or approved test device.

Most multimeters use sockets for 4mm banana-plugs and many accessory lead sets have what appear to be the appropriate cables:

test lead set

As a tangent to this tangent. If all you want to do is check that the outlet is wired correctly, there are a variety of specialised plug-in "outlet tester" tools intended for that purpose. They have limitations and some of the indications are misleading, but they don't have some of the issues issues with poking multimeter probes into outlets.

Outlet tester

These are available in the US, Europe and probably most places. This one does more than the basic ones but there others that will also test your GFCI/RCD protection at the outlet.

Obviously if you want to check the exact voltage, this won't do that job.

RedGrittyBrick
  • 24,033
  • 11
  • 46
  • 88
  • 2
    That doesn't apply to a receptacle in North America, about which the question was asked. – isherwood Feb 14 '22 at 14:47
  • 3
    I could have sworn I'd seen the equivalent for USA - but maybe I imagined it. Anyway the answer may be applicable for much of the other 95% of the world. – RedGrittyBrick Feb 14 '22 at 14:51
  • 2
    This is exactly what I would like to have for US 120 V receptacles. – Jim Stewart Feb 14 '22 at 17:18
  • For european market there are simpler, and better designed products, that you just plug in the socket. – JB. Feb 15 '22 at 13:29
  • @JB Those do a different job, but its a good point and I'll add something to the answer. – RedGrittyBrick Feb 15 '22 at 16:28
  • I could combine the UT-E01 interface with an adaptor to US blade plug. Are the UK and European plugs the same? – Jim Stewart Feb 15 '22 at 18:59
  • @Jim no. See https://en.wikipedia.org/wiki/AC_power_plugs_and_sockets#BS_1363_(Type_G) and the following section. UK and IE use same outlets/plugs. The rest of Europe have some interoperability with each other but not with UK/IE – RedGrittyBrick Feb 15 '22 at 22:30
  • A device like the top one could be useful in the US so you can solidly plug in, and leave plugged in, your meter leads and go around plugging the adapter into various sockets to test them. If that's what you need to do. It would be especially useful in the UK where the socket holes are blocked by little plastic doors, and opening those with the meter leads would be awkward and dangerous. – jay613 Mar 24 '22 at 21:05
2

I'd question your question: why are you using a multimeter to probe receptacles? This is prone to bogus readings as the tiny amount of capacitance between wires, combined with the meter's high internal resistance, will often give bogus voltage readings that make you think the outlet is energized when it is not. Unless you get the electrician's multimeter with the LowZ mode, of course.

In addition the probes don't stay stuck properly inside the holes, and you need to hold the multimeter and perhaps flip a circuit breaker, so you'll need four hands, with one of them reaching to the electrical panel.

For that I usually use either a simple lamp or something that makes noise, like a radio. The latter is useful when checking if a particular circuit breaker corresponds to a particular wall receptacle which is in another room.

If you want to know which side is live, a tester screwdriver is a better tool than a multimeter. It's faster to use than a multimeter. It's also a screwdriver. Iif you use this screwdriver to work on electrical installations it only takes a second to test if it's live before messing with it, just in case. Sometimes you get surprises, especially on old installations, where the wires don't go where they should, and even if the breaker that should correspond to this circuit is off, the wire at the socket side is still live...

bobflux
  • 8,484
  • 22
  • 28
  • Would someone who has a Fluke with LowZ mode ask the question here?... And I don't see how you'd short a socket with a tester screwdriver. Well you can always get the electronic one which has an insulated tip, but then it isn't a screwdriver. – bobflux Feb 14 '22 at 16:17
  • Ah OK lol. I've got a set of old Stanleys that never want to break, but yeah, the cheap no-name new ones I bought are good for about one half of a screw tightening and then the blade breaks. – bobflux Feb 14 '22 at 20:27
  • 2
    Multimeters are sold and advertised for exactly such functions (probing receptacles for whether it's hot or dead). It should be expected that homeowners/DIY users will use it for that function. – TylerH Feb 14 '22 at 21:17
  • A properly functioning lamp or radio can't measure the hot-to-ground or neutral-to-ground voltages. – Mark Feb 15 '22 at 00:17
  • @Mark sure, but quite often it's not needed to actually measure voltage – bobflux Feb 15 '22 at 07:41
  • @bobflux Lots of multimeters have a LowZ mode, e.g., my $40 Kaiweets clamp meter. – Andrew Morton Feb 15 '22 at 17:37
  • 2
    Meters used for LOTO must be tested both before and after the zero energy verification of the item to be locked out. One easy source of mains AC is to use a nearby still-powered outlet (remember, you must test functionality on the same range as you do zero energy verification). So, yes, lots of people stick multimeter probes in outlets all the time. – Jon Custer Feb 15 '22 at 19:22
1

When measuring voltage in 120VAC receptacles with a multi-meter in good working condition, your biggest shock hazard is inserting one probe into the "hot" side while you are making physical contact with the tip of the other probe. If you are grounded (or not insulated from ground) that will result in a circuit being completed through your body (you'll get shocked.)

Steve White
  • 11
  • 1
0

It's worth mentioning that in the US at least there are many grades of outlet receptacles ranging from 2 for a dollar to tens of dollars in price. This YouTube video discusses the two lowest grades, "residential" and "spec"/"commercial" grade and shows examples of their internal structure. As shown in the video, the cheapest "residential" (or "contractor") grade has design shortcuts that make it more susceptible to having shorts.

In particular, the residential grade outlet shown in the video has one-piece "V" contacts for a plug's blades (rather than multi-piece wiping contacts), which can more easily separate with use, leading to arcing and damage. It also carries ground on a metal strip running on the face of the receptacle between the hot and neutral blades, with only thin flexible insulation between the hot, ground and neutral metal. The commercial grade outlet has molded hard plastic cavities that more securely isolate the hot, ground and neutral, and it carries the ground on a metal strap on the back of the outlet, not the front.

Armand
  • 3,834
  • 5
  • 22
  • 3
    In accordance with SE rules, please include some details from the video so that this is useful info if, err.. when the video disappears. – FreeMan Feb 13 '22 at 20:42
  • 1
    [Wayback Machine archive of the video](https://web.archive.org/web/20220215094836/https://www.youtube.com/watch?v=kX6xnOksQTc), which should become viewable within a few days. – Vikki Feb 15 '22 at 09:54
0

@Peter Green and others asked about a US tool that plugs into a standard outlet and provides safe access for multimeter probes etc. An "AC Line Splitter" would seem to do that job and others.

Essentially, it runs the hot wire along one side and the neutral along the other so that a clamp can inductively measure current flow on just one wire when a device is plugged into the end. It also provides multimeter probe access holes for voltage measurement.

enter image description here

Armand
  • 3,834
  • 5
  • 22