Electric plug sockets being replaced by USBs?

Factual Questions
21

Well, the UL approved outlet is certainly going to be less likely to fail, but my point is that if it fails you can yank the $25 eBay device out of the wall and immediately stop the smoke. The outlet with the built-in electronics may be less likely to fail, but it’s an electronic device with numerous components, and isn’t anywhere near as rugged and simple as a regular 120 volt outlet, which is just a few big pieces of metal (and yet can still fail). The key here is that if it does fault, there’s nothing you can do at the outlet to stop the smoke.

Outlets in the UK have an off switch right at the outlet. We don’t do that in the U.S. If the USB outlet has an off switch built into the outlet, then I wouldn’t have so much concern over it, but that’s not the way that they are typically built.

Nope. That’s not what breakers do. Well, it is sorta what they do, but the details are very important in this case. A breaker typically won’t protect you from this type of fault.

There are three types of breakers.

The first is a regular breaker. This is designed to prevent your house from catching on fire if something tries to draw too much current. If a regular outlet shorts out, that generally will trip a breaker. However, since breakers usually trip somewhere around 15 amps, it’s entirely possible that that a small electronics short won’t draw enough fault current to trip a breaker, but will draw more than enough current to start a fire. Similarly, frayed extension cords can easily start a fire, as you have more than enough current going through a small fault to generate heat but not enough current to trip the breaker. A breaker is not designed to protect you from shorts in small current devices, and it is not designed to protect a human being in the event of a short. Humans can easily be killed by the fault current available on a typical breaker-protected circuit. 15 amps is a lot of current.

The second is a Ground Fault Circuit Interrupter. These have been required since the 1970s or so, but are only required in bathrooms and other areas where there is typically water around. A GFCI works by measuring the current in the hot wire and the neutral wire, and trips if they aren’t equal, the assumption being that if the currents are mismatched, then some current has managed to find an alternate path to ground. A GFCI’s main purpose in life is to protect human beings from ground faults. As long as the currents are equal, a GFCI won’t trip, so a GFCI isn’t designed to protect you from a short or something causing a fire.

The third is an Arc Fault Circuit Interrupter, or AFCI. As I said above, a frayed extension cord can easily start a fire without tripping a breaker. Since this is a very common type of electrical fault, they created the AFCI, which will detect the varying current from something that is arcing, like through a frayed cord. This is designed to stop fires. For a USB charger, it might trip the AFCI, or it might not. It depends on exactly what the fault is. If the transformer in the charger shorts out and starts arcing, there’s a good chance it will trip the AFCI. If one of the semiconductors in the charger faults and starts burning, that probably won’t trip the AFCI because the current going through the transformer will still be relatively constant and the AFCI won’t see any arcing. If the charger shorts out internally anywhere on the low voltage side of the transformer, there’s a good chance that the most it will do is draw more current from its transformer and won’t cause enough of an arcing pattern on the main voltage side of the transformer to trip the AFCI.

AFCIs have only been required in a the past couple of decades, so many older homes don’t have them.

GFCIs are often installed in outlets, but may also be combined with a regular breaker at the breaker box. AFCIs can be separate devices, but are most typically combined with breakers at the breaker box.

If your house is old enough to have fuses, they function like a regular breaker. It’s just that you hae to replace them if there’s a fault instead of just turning them back on. Protection-wise, fuses just break the circuit if the current gets too high, again, typically above 15 amps, though some circuits that old might be lower (12 amps is typical).

22

I’d be interested to know if there’s any non-anecdotal data about USB-equipped wall receptacles actually succumbing to any of these hypothetical dangerous failure modes. I assume (perhaps naively) that a UL-listed device would have built-in protection circuitry that’s a little more robust than a counterfeit charger from eBay.

23

I just bought a power strip that has six standard three-prong outlets, four 1.0 Amp USB jacks, and two 2.1 Amp USB jacks. Works very nicely.

24

There are receptacles with a USB recharge port, such as here

25

Unless there has been a quantum leap in power supply technology for this type of receptacle, the hazard they present is not hypothetical, and the assumption that (if) it possesses a UL listing makes it safe, is wishful thinking. Fires are frequently caused by the wall-wart type of power supply that have UL listings. Stuffing one inside the wall of a structure seems inherently dangerous, IMHO, as previously pointed out by engineer_comp_geek.

I have yet to read up on changes in the 2017 NEC, but if this situation isn’t addressed there, it would not surprise me if before the 2020 NEC is issued, these little firebombs have been banned.

26

How frequently? I’ve never heard of this, and in my brief 35 years on the planet I’ve never seen a wall-wart cause any dangerous problems. (I have seen old and worn out receptacles, switches, and light fixtures create dangerous conditions, though.)

According to the National Fire Protection Association (who maintain the NEC) the large majority of electrical fires are caused by

Granted, these USB receptacles have only been around for a few years, so we don’t know how they will perform over the long term. But I am pretty confident that such devices from reputable manufacturers are reasonably safe to stick in your wall.

27

Given the potential for harm, I’m not sure if “reasonably safe” is the proper standard.

From a purely statistical perspective, Russian Roulette could be considered reasonably safe. And if the potential consequences were minor, instead of major, it would be.

28

Learned a new word–wall wart.

I guess my main question now after reading all the above (unusually interesting) is whether a thermostat mightn’t offer the best protection against this kind of failure.

One more comment: while power cubes (what I have called wall warts till now) can be pulled out of the socket if they catch fire, I don’t sit around watching them. Typically, I will plug my phone or my toothbrush or whatever in and then go on doing what I was doing, including sleeping. I leave my laptop plugged in all the time (save when I go on vacation). So the protection offerred by the possibility of pulling it out of the socket is essentially delusional.

29

Friedo: Sorry, the stats I’ll leave up to you, if you’re interested in a dispute. However, I would surmise that, if you have never (in your 35 years existence on this planet) been aware of a power supply related fire, you are, um, unaware .

Your cite: “Roughly half (48%) of home electrical failure fires involved electrical distribution (emphasis mine) or lighting equipment in 2007-2011.” would include power supply failures. Gross statistics can be tricky.

The fact that you have not personally experienced, or are unaware of, power supply failures does not mean they do not exist.

30

Wouldn’t happen since USB is an evolving standard. If you outfitted your house with USB a few years ago, some items would charge very slowly, if at all.

31

Anything that uses USB. Iphones and iPads mostly.

32

Good point. While we all may leave an external power supply unattended, the chance of a devastating fire resulting from it’s failure is much less when mounted externally upon the fireproof face of a receptacle and the relative safety of a sheetrock backplane. Stick it inside the wall, and you have the additional hazard of contact with more flammable (pine studs) material, and the heat retentive aspect (of a hole in the wall) surrounded by various insulation factors causing a heat build-up, that is not experienced in an externally mounted fixture.

33

FYI - Be aware there are counterfeit electrical products being manufactured by China and these have counterfeit UL labels. That includes counterfeit circuit breakers and extension cords which have caused fires.

For information on this, google; counterfeit electrical

34

You can all base your reasoning and/or your faith concerning electrical safety, upon the regulations imposed by the NEC OR or your “personal expertise”…

The fact is: Sometimes, the NEC, due to the bureaucratic nature of the organization, is many years behind the times… Otherwise, one’s “personal expertise” is a highly subjective matter.

My personal experience on this subject, for what it is worth, cause me to err upon the side of caution, and distrust in the internal wall mounted power supplys.

35

I’ve heard of people wiring homes for DC. I take it there’s no standard for this?

IIRC it’s typically off-grid situations, e.g. solar + batteries or an RV. But I suppose you could just rectify once and run AC and DC separately. I’d like to read more if someone can point me in the right direction

36

Something of interest…

The NEC (National Electrical Code) comes from NFPA or…

The National Fire Protection Association!

And home’s power distribution, was in the early 1900’s, DC and AC! They had not yet decided what was the best. So you can find some old books on google.com for wiring a house DC.

I have an old set of books called the “International Library of Technology” 1926. A book from this series; “Distribution of Electrical Energy” on section 48 page 3 [Interior Wiring] discusses “Choice between direct and alternating current”.

This is on google books, but I can’t get it to show the whole book? Here is the beginning of that…

37

What does that mean? Surely there is no way to answer this question, except from statistics. Do we just choose the most convincing anecdote from everyone posting here?

Established safety regulators (like UL) are willing to bet, with their reputation and at the risk of litigation if they’re wrong, that these power supplies are safe. You are claiming that the entire class of products–not just certain bad designs that slipped through the cracks, or imported knockoffs with fraudulent safety marks, but any conceivable product of this form–is dangerous.

The regulators have made mistakes before, like with aluminum wire, but you are making an extraordinary claim. These power supplies, in-wall or otherwise, are pretty safe to begin with. If one’s going to catch fire, then it’s not obvious to me that I’d rather have the fire start outside the wall, hanging over my carpet, wooden floor, etc., than in a metal box that holds sparks in and limits the oxygen supply. Where’s the evidence?

For those who prefer statistics to anecdote, here’s a review of US home fires. Annual average from 2007-2011, there were 240 fires attributed to a “Battery charger or rectifier”, and 120 attributed to a “Low voltage transformer” (Table 7.A). Average fatalities were zero. So, the odds that your power supply will cause a fire big enough to get reported to the fire department seem like they’re roughly on the same order as the odds that you’ll get struck by lightning.

I don’t see an obvious reason why hard-wiring the supply in a metal box makes it more dangerous, since as Hari Seldon notes, most wall warts are left unattended, with no chance to unplug them early if they start to fry. There’s less cooling by natural convection, but the thermal designer knows that, and will size heat sinks etc. accordingly. As to the question about thermostats, power supplies and other power electronics routinely include overtemperature protection, sometimes with semiconductor temperature sensors on the same die as the power transistors, sometimes with a discrete temperature sensor of some kind.

And the above statistics make no attempt to exclude old, incompetently repaired, noncompliant, etc. power supplies. So, the risk if you buy only power supplies with reputable safety marks from reputable vendors is probably lower. In the scheme of risks, this one just doesn’t seem too important.

Finally, here’s some testing from a fire and arson investigation magazine. The authors find that line power supplies that comply with safety standards are safe, and that power supplies that don’t comply sometimes aren’t safe, not too surprising.

38

Any conversion from DC to AC (or vice versa) loses some efficiency. If you’re starting with DC, as you will be if you’re running from batteries, then DC wiring saves efficiency, and saves the cost of the inverter. If you’re starting with AC, then that advantage isn’t present.

A surprising range of products designed to operate from 100-240 VAC will run from DC. Incandescent lights, heaters, and other resistive loads obviously work fine. Universal motors work from DC, so a lot of power tools work. Many switching power supplies (like in electronic equipment) still work; the bridge rectifier just always conducts in the same direction. Others will blow up. Induction motors, and anything with a 60 Hz transformer, will pretty much always fry.

39

In addition to Tommy’s post, I’ll point out that while USB receptacles are somewhat new, putting DC electronics in electrical boxes isn’t. Fancy programmable dimmers, timers, and home-automation nodes have been going in switch and outlet boxes for decades, often with substantially more complicated circuitry than just s simple rectifier for USB power.

There’s nothing wrong with erring on the side of caution, of course. And nobody is going to make you install the things. But I prefer to investigate what the actual risk factors are - if known - before deciding whether a given level of caution is reasonable when weighed against other factors, like the convenience of having a USB plug at my kitchen counter. (FWIW, I don’t have one. But I might get one.)

40

Replacing existing outlets with integral USB outlets is a dumb idea. Wiring new houses with USB outlets in addition to AC outlets, including data lines would be a good idea except USB may not last that much longer anyway. Something will evolve over time, AC won’t change, but DC power and data wiring in some form will become more common.