Is it possible to dangerously short a U.S. power outlet with multimeter probe?

Question

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!

Answer 1

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,

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.

Answer 2

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).

Answer 3

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.

Answer 4

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.

Answer 5

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 £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:

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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.

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.

Answer 6

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...

Answer 7

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.)

Answer 8

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.

Answer 9

@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.