Electricity, how does it work?
September 26, 2010 12:17 PM   Subscribe

What are the potential risks of leaving things plugged in on one end (to an outlet, to a computer) and free on the other end?

There are three times I run into this regularly, and I imagine the risks are: next to none, don't do this, and none, respectively, but can someone who knows confirm or correct me?

1) The extension cable for a Mac PSU. I imagine it is safe to leave this plugged into the wall because the contacts are down two long, separated sheaths. The only thing that could actually bridge both contacts is the thing it's made to plug into.

2) The other end of the Mac power supply chain: the magsafe (is that what they call it?) end that goes into the computer. This has five bare pins in a line over a half inch. Is it possible something could bridge these pins and go live? Or are some of those pins confirming to the PSU that a laptop is connected, and without one, no power will ever come out of the thing? I think to myself, "Well, what if a spider is crossing my floor and he walks across it and bridges the necessary pins and bursts into flames and burns my apartment building down?" I have absolutely no idea how ridiculous that scenario is.

3) What about my iPod connector on my desktop? Is it safe to leave this dangling when nothing is connected, or could something bridge the pins on that and fry my USB bus or something?
posted by neuromodulator to Science & Nature (8 answers total)
Nothing to worry about. During design, testing and certification appliances are checked for "can an idiot kill themselves with this?".
posted by Biru at 12:42 PM on September 26, 2010 [2 favorites]

1. I agree, this seems safe. Unless you have pets. They could bite the cable and cause a short. This is true of any cable.

2. I believe that the mag safe connector pins are spring-loaded and wont conduct unless pushed in first. The magnetic connection between the plug and mac causes the compression of the springs. It would be difficult to depress the springs manually.

3. I do this all the time and can't imagine a problem. First off, we're now talking about a lower voltage DC current. Second off, the connecting pins are well protected inside the connector.

I am, of course, not your electrician.
posted by jz at 12:46 PM on September 26, 2010

As far as safety, and assuming that your connector ends are undamaged, it's negligibly more dangerous than having it plugged into both ends and powering something. A dead cable is always safer than a live one with respect to a pet biting into the cable or something like that. Unplugging unused cords from the source is safer in that way, but that's pretty much the magnitude of danger you're looking at.

Other downsides of various magnitudes: unnecessary trip hazard, wasteful consumption of power (I'm not sure if mac power supplies are smart enough to not draw AC when not powering the load or not)
posted by ctmf at 12:56 PM on September 26, 2010

Best answer: 1) At a previous job, I did this all the time. The outlet was inaccessible, and there were several spare extension cables around the office, so I had one at home and one at work, and just had to move the power supply. Looking at the extension cable, it looks no more exposed than the outlet itself. The only issue I see is if it were placed such that it might dangle into water.

2) Contrary to jz, I believe that the spring loaded-ness of the pins is just so they make a good contact. Having just licked a MagSafe connector (in the name of science!), it tingles, but only mildly (less than licking a 9V battery). Yes, there's some voltage there, but not enough to injure you or ignite spiders. Again, don't drop it in water.

3) I don't know about this one, but I wouldn't be too concerned.
posted by JiBB at 1:11 PM on September 26, 2010 [2 favorites]

Best answer: Previously
posted by niles at 1:16 PM on September 26, 2010

@JiBB Thanks for the clarification. That makes more sense.
posted by jz at 1:49 PM on September 26, 2010

Best answer: Pretty much everything that has a wall wart is going to be safer than just an extension cord: they all step down house voltage to something that your electronics want to deal with. So an usb cable delivers the less electrocution risk as 4 alkaline batteries end to end. (the iphone cheats so it's more like 4 C cells instead of 4 AAs but either way, it's not dangerous)

The other side of the electrocution question is that mechanically, the positive and negative terminals are close together. So if the connector got wet or damaged or licked, there isn't much danger that the electrical current would pass through anything important (like your heart). So the worst case is a quarter inch of the surface of JiBB's tongue has a small amount of current going through it.

In addition to being not very high voltage, wall warts usually have some kind of short circuit protection. On big ones this could be a fuse, but most are just current-limited. So if you damage the cable by rolling over it with your office chair too many times, and it shorts out, if it doesn't blow a fuse, it will use the full power capacity the wall wart can supply, which shouldn't be a whole ton more than the device actually uses, so the wall wart will only get a tad warmer than it does when you're using it properly (e.g., charging your macbook). Instead of starting a fire and killing everyone.
posted by aubilenon at 1:56 PM on September 26, 2010 [1 favorite]

Leaving a regular extension cord (no wall wart) out is about as safe as leaving a lamp plugged in - it's safe as long as the cord isn't damaged. (assuming that anyone who was going to stick a butter knife in the extension cord outlet would just use the wall outlet for that if the extension cord weren't there)
posted by aubilenon at 2:02 PM on September 26, 2010

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